Overview of the Peripheral Nervous System

In this video, I am going to introduce the peripheral nervous system and discuss the different features of the peripheral nervous system, the receptors, the different types of sensors that you typically see within any of your peripheral organs. We're going to discuss circuit levels like an ascending track carrying sensory information. We will talk about motor endings that you typically see in the peripheral organs as well. So the first part is going to be a classification of different receptors. But first, let's orient ourselves and kind of review where the peripheral nervous system is organized in relation to the entire nervous system. So let's look at this schematic, which you should be familiar with already. So we're referring to all of the different neural structures that you typically see outside of the brain, the spinal cord. This could be sensory endings or sensory receptors within the peripheral nervous system. The entire transfer of information, sensory information to the brain and motor information out of the brain, all associated with the PNS, this is all carried through a set of nerves. So these are all peripheral nerves. Depending on the origin of the nerves, you would classify them as either cranial nerves if they originated from the brain or... spinal nerves if they originated from the spinal cord and all of the different associated ganglia related to those nerves and then within the peripheral nervous system or within the peripheral organs a description of the different motor endings or the efferent motor endings so here's where the pns is in relation to the entire organization of the nervous system remember the cns that we just finished discussing in the previous chapter The CNS, the central nervous system, consists of two main organs, the brain and the spinal cord. So everything other than the brain and the spinal cord would constitute the peripheral nervous system or the PNS, which really consists of two main divisions. The blue part that you see showing information, sensory information being carried through the peripheral organs to the CNS. That's your sensory division. Now, these sensory inputs could be from anywhere in the body, like the skin. It could be the skeletal muscles. So it could be also the cardiac muscle, smooth muscle, and any sensory information. So really, this could be visceral sensory or somatic sensory. So it could be related to the skeletal muscle, or it could be other inputs that you get from other autonomic targets. So pretty much any sensory information is carried through this division. towards the brain. Okay. So this is always sensory is always afferent with an A meaning carrying information towards the brain and the spinal cord. All of the red part that you see here, which is what we're going to discuss to a certain extent would be the motor output originating from the CNS and leaving the CNS. So this would be efferent with an E, efferent motor output via the peripheral nervous system. So that's the motor division. The motor division can be split into two basic subdivisions. Somatic here refers to anything that is controlled, a voluntary effector. Typically we refer to all of the skeletal muscles as being part of the somatic motor division. The autonomic motor division consists of involuntary targets such as the cardiac muscle, smooth muscle that you see in any hollow organ. And of course, any glands. So these are all involuntary targets in the case of the autonomic nervous system, but voluntary targets in the case of the somatic nervous system. You can then break down or further classify the ANS, the autonomic nervous system, into two main divisions, the sympathetic and the parasympathetic, which we will discuss in the next chapter. So really what I want to do here today is within this discussion of the peripheral nervous system, I'm going to kind of... highlight a few things related to the autonomic nervous system, but that's a chapter in itself. Really, my focus is going to be more of the somatic nervous system. And then of course, we've got to talk about the sensory division. So what I want to talk about here is break this down into what are the different sensory receptors. So what are the different sensors or receptors within the peripheral organs that pick up information related to sensory stimuli? Now, this could be general senses like touch. stretch or pain, temperature, any of that. Or it could be special senses as well, like information related to light. So vision and then, of course, auditory information, equilibrium information, all of that would be your special senses. So that's the sensory information. So we're going to talk about different classifications of sensory receptors. And then we're talking about all of this information going to the brain and the spinal cord. cord and then the motor division which carries information out of the brain the spinal cord to then target different effectors and these effectors could be like i said somatic meaning voluntary effectors like the skeletal muscle or involuntary targets such as these which would be related to the ans which is your cardiac muscle and your smooth muscle for examples now information either sensory towards the brain or motor out of the brain you is all carried through a set of nerves. So we're going to talk about peripheral nerves. Okay, so we'll discuss mostly spinal nerves, but just generally we'll discuss an ascending track, which we've already talked about in the previous chapter. So ascending tracks would carry sensory information towards the brain and a descending track would carry motor information out of the brain. And then once this motor information reaches whatever this target is, so skeletal or... any involuntary target. Then we're going to talk about motor endings that you see within these effectors. So let's break this down. Let's first start with sensory, talk about the different classification of receptors, and then we will kind of discuss the conduit level where you, or rather the circuit level where you talk about an ascending or descending track. And then of course, tie this all in with motor endings. All right, so let's start with sensory receptors. So these are sensors that are located anywhere in your body other than the brain and the spinal cord. So any peripheral organ, any peripheral effector or target that has these receptors, which are sensors that respond to changes in the external environment. So these changes are called stimuli or a stimulus. It's a trigger that is sensed or detected by the sensory receptors. When a sensory receptor responds to a stimulus like pain or temperature or touch or whatever different type of stimuli, these sensory receptors will then get activated, which then results in the creation of a greater potential. So this is really important in terms of perception of this stimulus. The stimulus has to be converted into a greater potential. Sensation is awareness of that stimulus and the sensation or being aware is really carried out by these sensory receptors. And then converting it to a greater potential is important for perception of that stimulus. And typically this perception occurs in higher brain regions. So you can classify sensory receptors based on three different categories listed here. Okay, the first category is what type of stimulus? does this receptor respond to? What does it detect? Second could be, okay, where exactly, what is the location of these receptors inside the body? And the third category is more structural, referring to whether they are just modified dendritic endings, so very modified neurons or free nerve endings, or is this something that has more complexity to it? So does it have a very specific organization and a structure? and which you would typically see in sensory receptors associated with special senses, like, for example, within the retina of the eye, which is important for the special sense of vision. Now, the retina is more than just a modified neuron. It has a very complicated structure. It has a very organized structure, so there's a lot of detail to it. Likewise, a different special sense, such as hearing, is carried out by a different receptor located within the cochlea within the inner ear region. Now the cochlea also has a very very specific structure so really structural complexity would be a category where you're classifying your receptors based on a very basic simplified kind of a structure or something that has more complexity. So let's classify them first by stimulus type. Okay, so if a receptor responds to touch, pressure, vibration, or stretch, these are mechanical stimuli like touch, pressure. Therefore, these are called mechanoreceptors. Any receptor that responds to changes in temperature would be a thermoreceptor. Receptors, especially in the retina of the eye, that respond to light energy would be a photoreceptor. If a receptor responds to chemicals in the body, especially related to smell and taste, changes in chemicals related to blood chemistry, increases in carbon dioxide, things like that, hydrogen ions, all of those would be chemoreceptors because they're responding to chemicals. And this last category are those that respond or are sensitive to pain-causing stimuli. So pain-related receptors are called nociceptors. So... This could be extreme heat or cold, excessive pressure, or it could be an inflammatory chemical. So depending on the type of stimulus that the receptor responds to, you can classify them as these as shown here in blue. So that's the first classification of a receptor based on stimulus type. The second classification is based on location. Where exactly are these receptors located within the body? Okay. and what type of stimulus does it respond to? Where does the stimulus arise from? Okay, so if the stimulus is coming from outside of the body and these would be exteroceptors. So normally this is related to the integument. So you see exteroceptors mostly in the integument on the skin. And of course, these are receptors that respond to touch, pressure, pain or temperature. Of course, all of your special sense organs for the most part are exteroceptors because they're receiving light information from the outside or pitch and frequency and amplitude information related to sound waves from the outside. So those would be examples of exteroceptors. Okay, if the stimulus is originating from inside of the body, then those would be an interoceptor. So these receptors would respond to stimuli. within internal viscera. This could be, let's say, a distended stomach wall in response to food entering into the lumen of the stomach. So that could be basically an interoceptor. It could be in the wall of the blood vessels, which obviously detect changes in blood pressure. So all of these interoceptors are responsive to stretch, as in the case of blood pressure, in the case of blood vessels, temperature receptors, or other chemical changes in the body. So those will be interoceptors. Proprioceptors are specific. These are mechanoreceptors that are responding to stretch in skeletal muscles, or it could be within the tendons or the joints. So keep this in mind. We will talk about certain examples of proprioceptors towards the end of this discussion when we talk about the muscle spindle and the... the tendon organs. Those are two examples of proprioceptors. So this gives you information and it's carrying information related to a person's stretch of skeletal muscles, which is kind of an indication of a movement. And that's the information that's being carried to the brain. So that's the classification based on location, which then brings me to this next classification, which is by receptor structure. You can broadly classify this as general sense receptors or special sense receptors. General senses are located throughout your body. These are very simplistic receptors that respond to different sensations like touch pressure, stretch, vibration, pain, temperature. All of these are general or visceral senses. You can classify these general sense receptors as either kind of simplistic, called non-encapsulated. And so these could just be free nerve endings. Your nociceptors, your thermoreceptors, your light touch receptors, these are all hair follicle receptors example, would be non-encapsulated. So nothing too complicated about their structure. Or they can be something way more complex where they have a capsule surrounding that receptor. We've already discussed some of these examples when we... discuss the integument chapter where we talked about a special, I'm sorry, a light touch sensor called the Meissner's or the tactile corpuscle that you typically saw. between the dermis and the epidermis in the dermal papillae. And then we also talked about these guys, the Pacinian corpuscles that are located deeper inside of the dermis, almost closer to the hypodermis. And these are responsible for deep pressure. We will talk about muscle spindles, which pick up information or sense information related to the stretch or the length of a muscle. We will also discuss tendon organs, which is an example of an encapsulated receptor, which you will see within joint cavities, which is going to give information related to muscle tension or the contractile force or the amount of contraction that is being experienced by the muscle. So all of these are examples of general sense receptors. And then based on the receptor structure, you can classify them as either non-encapsulated, so they're kind of... free without a surrounding capsule, or they could be encapsulated where they have more of an outer capsule-like structure. And then, of course, you've got your receptors for special senses, and the special senses are related to vision, hearing, equilibrium, smell, and taste. And this is all covered in a separate chapter. So we're not going to be focusing on special sense receptors here. Here, we're going to mostly focus on general sense receptors, okay? Okay. So we've already talked about the very first part of the peripheral nervous system, which is how do you change, how do you detect changes in stimuli or these different sensations in the body like touch and pressure and stretch and pain, temperature, so on and so forth. Those are all detected by these receptors, which we just classified based on location, based on structure, based on the stimulus that it responds to and so on and so forth. Now what I want to do is kind of show you the connection between the peripheral nervous system and the central nervous system. So let's start with the bottom here and work our way towards the top, which of course you can identify there as the brain and here's the spinal cord. And this is information from the brain and the spinal cord communicating with all of the different peripheral organs. Example shown here is skeletal muscle. So there's a very specific receptor here within the... skeletal muscle called a muscle spindle that detects changes in the length of this muscle. It detects whether that muscle is in a state of contraction and so on and so forth. So that would be either shortening of the sarcomere resulting in a shortening of the length of the muscle. That's detected by the muscle spindle. You have joint kinesthetic receptors that are typically seen within ligaments or within the joint cavities. You also have shown here the integument. Those are different sensory receptors located to detect changes in pain, in pressure, in touch, and so on and so forth. So here's all of the peripheral organs, or at least an example of a few of your peripheral organs, and different receptors. So that's why this is called, the very first level is called the receptor level, which is important for sensory reception. Okay, now what do you do with this information? When, say, your muscle spindle or your Meissner's corpuscle or your Pacinian corpuscle or any of your modified free endings of your neurons or whatever, when they pick up information related to some kind of a stimulus, some kind of a sensory information, well, you're going to have to take it, carry it through pathways, through these tracks. That's why you talk about the circuit level next. So all of this information, sensory information, is carried upwards through the spinal cord. And this is the RAS right there showing you the region that spans the brainstem, the midbrain, the pons, and the medullar vanguards. And then from here through the thalamus and from the thalamus into the parietal lobe where you will see. located on the postcentral gyrus. This region is functionally important as the somatosensory cortex that we discussed in the CNS chapter. So obviously, these are all general sense information, sensory information like stretch and pain and pressure and touch. These are general senses. So it's all going to be carried through different tracks like an ascending track. And this is the circuit level. I'm going to carry it up. to this specific region, the somatosensory cortex, located in the postcentral gyrus, dotted line representing the central celsus. So right behind it should be the postcentral gyrus. that is located within the parietal lobe. And this is really where perception occurs. Perception of what? Perception of all of this sensory information that's being carried into this specific region. And this could be perception of pain signals, could be perception of touch, being able to discriminate between something like... velvet or like a soft feather across your skin or something that's sharp and pointed or something that's causing pain. All of that sensory perception is carried out here in the postcentral gyrus within the somatosensory cortex. So to summarize, when you think about sensory integration, it consists of three different levels. First, you have to sense, you have to detect. this stimulus, this sensory information, all carried out by the receptors within the peripheral organs. You have to carry this information, this sensory information as basically action potentials. Okay. You got to carry it through ascending tracks or networks of neurons ultimately. So that's, that's called the second level is the circuit level. And then you're going to carry it up here into the somatosensory cortex region within the, within the brain. cerebral cortex where perceptions are occurring. So three different levels, receptor to circuit to perceptual levels. Okay. What I want to discuss next is this concept of somatotopy. So let me first show you what's going on here on this slide. So you're looking at, say, the superior view of the brain. That should be the anterior part of the brain. Here's the posterior part of the brain. So all of this, this lobe here in the front should be the frontal lobe. You're seeing the central sulcus right there. That should kind of be the partition between the frontal lobe and the front. Then you're kind of seeing the red part that's associated with the frontal lobe and the portion there at the back, which is associated with the parietal lobe. And of course here, right through the center, dividing out the left and the right cerebral hemispheres, that should be your longitudinal fissure. So if you were to look at, say, if you remember, the central sulcus is the boundary between the frontal lobe and the parietal lobe, right? And this kind of raised fold, upward fold in the front, in front of the central sulcus was the pre-central gyrus. And this red part here is... kind of showing you the precentral gyrus on both the hemispheres. The precentral gyrus is important because that's where you see the primary somatic motor cortex region. Behind it, shown in blue, that's on the parietal lobe. This should be the postcentral gyrus. This region is important as a somatosensory cortex region. So regardless of whether you are mapping, the motor cortex in the frontal lobe or the somatosensory cortex shown in blue in the parietal lobe. So motor cortex shown here on the left, that's on the frontal lobe. And the blue part here, this is the somatosensory cortex shown from the parietal lobe shown here on the right, regardless of what you're mapping. Okay, so I just kind of want to orient you. So if you take a cross section, as you can see there, a slice right through the somatosensory cortex, kind of map it over here. um and then you take a slice through the motor cortex the red portion here on the left and kind of map it on the left here again the center the longitudinal fissure is towards the middle aspect so the median aspect is right here okay towards the middle and towards the sides the left and the right side is what you're going to see here okay does that make sense um so let's say let me back up here right quick so say information um from the integument, of course, integument is going to cover your entire body. So whatever sensory receptors within the integument picking up changes like, say, touch or pressure, whatever, depending on where these sensory receptors are originating from. So where am I carrying this information from? This sensory information is taken to the somatosensory cortex, but where exactly? Every... I'm sorry, stimuli or sensory information from every part of your body is mapped precisely to a specific region within the somatosensory cortex. And that's what I'm showing you over here. Okay. So say you were... touching velvet or a soft object okay so that touch information that is picked up by sensory receptors in your in the in the fingers in the integument associated with the fingers all of that information is carried through ascending tracks all the way through the brainstem and the thalamus and then it's going to get to the somatosensory cortex region to make sense of it right so what exactly are you feeling is this uh a sharp object or is it a soft object? What is it, right? So this perception is occurring in the somatosensory cortex, but where exactly? And that's what you're seeing here. Anything related to the hand, the fingers, the forearm and all of that is right there. So kind of somewhere over there. Does that make sense? The median part should be over here. And then as you move away, the lateral aspect should be here towards the end. Okay. So everything related to the upper extremities, the upper limbs, all of that is mapped in this region. If you're picking up information, say you step on a sharp object, like so something, a nail pierces your foot. OK, also from the leg region or from the foot region. Notice the foot region and that sensory information is picked up or it is mapped to this specific region. of the somatosensory cortex. So everything related to the face is over there. And then of course the rest of your organs. So it's all very precisely mapped. And this is the same even in your motor cortex region. So if I were to send motor information from the frontal lobe, from the primary motor cortex region, and I wanted to control facial muscles, then basically all of those outputs would originate from this specific region of the... of the motor cortex and it's going to carry all of those neurons are going to carry that information towards your face to control whatever muscles of your face. So this is somatotopy and this is basically sensory or motor information being precisely mapped within very very specific regions of either the motor cortex or the sensory cortex. So let's talk about processing. Okay, so when a receptor, which is located within a peripheral organ, when a receptor responds to some kind of a stimulus, again, pressure, touch, or whatever, this stimulus has to be converted. This is transduction. It has to be converted to a graded potential. So these graded potentials that occur in general sense. receptors is called a generator potential. So this is the progression of what's happening in terms of processing at the receptor level. So a general sense receptor picks up information related to some kind of a stimulus. So let's say this is the Pacinian corpuscle located deep in the dermis of the integument that is responding to a specific stimulus like a pressure signal. Okay. It's then going to convert that sensory information. pressure into a greater potential called a generator potential. And then that then gets converted into an action potential and you're going to carry it through a set of afferent neurons towards the brain and specifically taken to the postcentral gyrus within the somatosensory cortex area where you're going to make sense of that sensory information. So that's in the case of a general sense receptor. It's more or less the same pathway in a special sense organ, special sense organs like within the retina of the eye there are receptors for vision for for light information called uh this um the well the retina would would serve as as the special sense organ within within the eye to pick up information related to vision or in the case of hearing within the cochlea which is located within the inner ear there is a specialized receptor called the spiral organ you that will detect changes related to pitch or amplitude of sound waves. So when those stimuli like light information or frequency of sound waves and amplitude of sound waves, when all of that is detected by these special sense organs like the retina or the spiral organ of the cochlea, those stimuli get... converted into some kind of a greater potential. Now instead of it being called, let me back up, instead of it being called a graded potential, which is the case in a general sense receptor, it's now called a receptor potential in the case of a special sense organ. It's the same principle. The stimulus has to get converted into a greater potential. Now, this receptive potential, it's a little more detailed in the case of your special sense organs, because typically this great potential will cause the release of some kind of a neurotransmitter from your... receptors. Let's say, for example, within the retina, you have your photoreceptor cells, like your rod or your cone, that will develop a greater potential called a receptor potential. And then in response to it, it's going to release certain neurotransmitters, which then causes activation of other neurons and therefore basically generates greater potentials in sensory neurons. In the case of the retina, these sensory neurons would ultimately be your ganglion cells, which carry your action potentials all the way to the brain, specifically within the occipital lobe, where that information is received by the primary visual cortex. Okay, so we talked about processing that needs to occur at the receptor level. Let's talk about processing at the circuit level. How do I carry information from the PNS to the CNS? So this is all carrying sensory information towards the CNS has to occur with the help of an ascending track, which we've already discussed in the previous chapter, consists of three levels of neurons, first order, second order, and third order. So I'm not going to go into that again, since we've already talked about it with respect to the CNS. Once you carry this sensory information to the brain, then perception has to occur. within the somatosensory cortex and the association area associated with that cortex region. Here are the different aspects of sensory perception. So perceptual detection, which is ability to detect a stimulus. So typically this requires an addition or a cumulative effect of what we call a summation of impulses. Magnitude estimation. So the higher the number of frequency of impulses, the higher the frequency of action potentials, That is a reflection of a higher magnitude signal or a more intense signal. So a greater level of pain or a greater level of pressure, okay, would be coded for by a greater frequency of action potentials. Spatial discrimination, that's the next aspect. Okay, so this is being basically identifying patterns, so pattern recognition and so on and so forth. Quality discrimination, this is... within a specific sensation, like say, for example, taste. Is this a sweet taste or a sour taste? Or is it bitter? Or is it salty? Things like that. That would be quality discrimination. And of course, pattern recognition, which is again, this could be facial recognition or it could be pattern recognition within, say, music. So all of that's carried out. That's the last aspect of perception. So all of these need to occur. where within the brain, within the association area related to the somatosensory cortex. Okay. Okay. So let's talk about the circuit level. This is all carried out through a set of nerves. So all of this information, sensory information being carried to the CNS or motor information being carried out of the CNS towards the PNS, it's all carried out through nerves. two sets of nerves or two types of nerves. Cranial nerves if they originate from the brain and spinal nerves if they originate from the spinal cord. Depending on the information that these nerves are carrying, you can functionally categorize them as sensory motor or mixed. Okay, so sensory nerves only carry sensory inputs towards the brain and the spinal cord, towards the CNS. And some examples of cranial nerves that carried sensory inputs would be, well, your olfactory nerve carrying information related to smell or your optic nerve carrying information related to vision. And so these are purely sensory nerves carrying all of that information towards the brain and the spinal cord. If it's a motor nerve, then this would carry motor information out of the CNS towards a peripheral organ. Cranial nerve number three, ocular motor, would be an example of a motor nerve because this is going to carry motor information to control specific eye muscles, like your extrinsic eye muscles, which allows you to move your eyes up and down or whatever in different directions. So it helps you control the skeletal muscles, which make up your extrinsic eye muscles. controlling eye movements. So that would be an example of a motor nerve. It can also control autonomic targets within the eye, allowing for you to control like the ciliary muscle, which allows you to change the shape of the lens, or it can be controlling the iris muscles, bringing about changes in pupil diameter. So again, those would be examples of motor nerves. Now, what's a mixed nerve? A mixed nerve is one that contains both sensory and motor information. All of your spinal nerves are considered mixed nerves because there's always sensory information that's being carried through the dorsal root, which makes contact and synapses in the dorsal greyhound area. And then you also know that, like, say, for example, the... Mortar information out of the spinal cord is carried from, say, the ventral grey horn area through the ventral root, carrying information that would then target skeletal muscles and bring about muscle contraction, for example. So therefore, a sensory nerve is one that only carries sensory inputs towards the brain and spinal cord. Mortar nerves carry motor outputs out of the CNS, and a mixed nerve carries both sensory and motor outputs. Okay, so. You can further classify your sensory and motor as either somatic or visceral. So typically somatic is related to skeletal. Visceral is any other organ. So you can say somatic sensory or somatic afferent. Always afferent is related to sensory and efferent is related to motor. So somatic sensory would mean carrying sensory information from the skeletal muscle towards the brain and the spinal cord. Visceral sensory means carrying information from any other organ in the body, like the blood vessels that are detecting sensory information like stretch because of high blood pressure. And then, of course, chemoreceptors within the blood vessels that are detecting changes in relation to elevated CO2 levels, for example. So those are all visceral. sensory information carried from different organs towards the brain and the spinal cord. And likewise, you can designate somatic and visceral in relation to motor nerves as well. Okay. So now that we are leading towards the motor aspects or the motor functions that need to be carried out in the peripheral organs, let's talk about what are some of the motor endings and how do you classify the levels of control? of motor activity. Okay. So motor endings, these are the distal endings of motor neurons that basically activate whatever that target cell, that effector is. That effector could be a somatic target like skeletal muscles, or it could be an autonomic target like cardiac muscles, smooth muscle, glands. And what it does is that these motor endings will release neurotransmitters or chemicals, right, onto these effectors. and bring about contraction or it can bring about relaxation if you're talking about a muscle or it can bring about in the case of glands it could bring about increased secretion of glandular secretions or can inhibit those glands okay when it comes to movements when it comes to controlling skeletal muscles And the cerebellum and the basal nuclei within the brain are very, very important in coordinating activity with the primary motor cortex region to basically control smooth, coordinated, complex movements in the body. Okay, so this is important for you to know, I guess, the organization or the hierarchy of motor control. So what you see here on the bottom is everything related to the PNS. These are... peripheral organs. And as you go higher up, obviously you're seeing the spinal cord and all of the higher brain regions. All of this is related to the CNS. So I'm asking you, how do you control movement of information? How do you control both from the sensory end of things and the motor end of things? What is the cooperation or what is the control between the CNS and the PNS? Always remember information being carried out of the PNS towards the CNS is sensory, which you see by these blue arrows here on the left. And information out of the CNS towards the PNS is motor, which you see with the red arrows. OK, so there's three different levels of control. Your lowest level is called the segmental level, which is all at the level of the spinal cord. And we will talk about CPGs on a subsequent slide. So this is the lowest level. The spinal cord kind of works by itself, mostly related to reflex activity, which I'll talk about here in just a little bit. One level higher than this is called the projection level. This is associated with the brainstem region, the motor cortex. I should have highlighted that as well. the motor cortex and the brainstem region. This can provide some input, as you can see, it can control the spinal cord at the segmental level as well. And the highest level is way up here. This is called the pre-command level. This is consisting of the cerebellum and the basal nuclei. So the cerebellum and the basal nuclei are the ones that actually come up with the instructions, especially in relation to the motor output. It comes up with the instructions. It comes up with the blueprint required to determine what those motor output instructions need to be. And then it's going to communicate this, especially like, say, the cerebellum along with the basal nuclei. It's going to communicate that blueprint to the motor cortex, the somatic motor cortex within the cerebral cortex in association with the brainstem. And then that motor cortex sends that information through the spinal cord. to bring about motor activity like contraction and relaxation and so on and so forth off your targets within the peripheral nervous system. Okay, so let's go in a few more details. Before I leave the slide, I want to recap one more time. Segmental is the lowest level consisting of the spinal cord. This is mostly reflex activity. Projection level is in the middle consisting of the motor cortex and the brainstem regions and the highest level of control of motor activity is pre-command. which is cerebellum and the basal nuclei. Okay, so what you're seeing here is, let's see, segmental level, that's the spinal cord, that's the lowest level. Projection level, which consists of your cerebral cortex, specifically your primary motor cortex region and your brainstem region right there. So all of that's going to be your projection level in the middle and your highest level, the pre-command level. which is really the cerebellum and basal nuclei that you see within the interior region of the brain. All of that's going to be your highest pre-command level. So what do these three levels do? Okay. Like we said, segmental is the lowest. This is spinal cord, mostly reflexes and automatic movements. CPGs, central pattern generators, these are like a network of neurons specifically. Now, remember we're talking about the spinal cord. So the ventral gray horn of the spinal cord is going to be important for motor neurons that control skeletal muscles. So basically your central pattern generators are circuits or networks of these ventral gray horn neurons originating from here so that you send specific outputs to different types of skeletal muscles. Okay. So that's your skeletal level. Sorry, that's your segmental level. Your projection level consists of the cerebral cortex. Remember, your descending tract consists of your upper motor neurons, like, for example, your pyramidal neurons or your pyramidal cells from the cerebral motor cortex region and other brainstem regions. So this is going to be the second level or the middle level. Highest level, precomat, consists of cerebellum and the basal nuclei. This is controlling activity. activity at all these levels. So again, just generally, it's going to regulate all motor activity of your PNS targets, precisely starting and stopping movements, coordination of skilled movements so that it's not too jerky, so that it is more smooth, a smooth transition from one movement to the next, monitoring muscle tone, and of course, unconscious planning of movements as well. It's all controlled by the pre-command level. So those are your... three levels, okay, starting with the lowest, the least control, moving upwards towards the highest level, which is the most control, okay? Okay, so now that we've talked about the PNS in general, we've talked about sensory receptors, the circuit level, the different nerves, we talked about that. I mean, then we discussed motor control. I think we've mostly laid out the main aspects of the... peripheral nervous system. So what I want to do next is discuss a few more topics here, like reflexes. This is important. And then we're going to talk about two important reflexes, the stretch reflex and the tendon reflex. Okay. So there are two main types of reflexes. Some are called intrinsic reflexes. This is what you're born with. So being able to maintain posture, avoid pain, all of that. You're not learning these activities. These are just intrinsic, inborn reflexes. And then you've got others that you acquire over your lifetime. These are learned reflexes. They come from practice and repetition, like driving skills. So if you are an experienced driver and know what you're doing. on the road, you are not really thinking about your driving. It doesn't take that much effort once you are skilled enough at that function. So if you're driving down the highway and you see, you know, red lights in front of you, like, you know, cars in front of you and you can see their taillights and you see those red lights. Okay. So, well, this is an acquired reflex. You don't even think about it this is all kind of controlled by your spinal cord what do you do you hit the brakes right that's that is a natural reflex that you're not it's not requiring too much processing you don't have to think think it through it doesn't require too much interpretation this is a natural reflex um which you have learned um or if you touch a hot object while cooking well you might you might just uh drop that pan or that skillet right so that would be basically an acquired reflex as well okay any reflex or a reflex arc consists of five main components that are listed over here make sure you know this i'm going to use this schematic on this slide to discuss those same five components okay so you're seeing here by the way that's a cross-section of the spinal cord you should recognize that by now central canal and you'll remember the different gray horns dorsal gray horn, the lateral gray horn, and the ventral gray horn. So a reflex arc consists of a receptor. That's a receptor in any peripheral organ. So what you see here is skin. Okay, so this could be a nerve ending. It could be a corpuscle. It could be a modified dendrite. Doesn't matter. These receptors located here and shown here in the skin would pick up information related to touch pressure or pain. or temperature and things like that. So this sensory information is the stimulus is picked up or detected by a receptor. Then the receptor will generate a greater potential, right? Which then basically kind of converts it into an action potential. And you're gonna take that information, that sensory information is carried along this blue neuron. This blue neuron is called a sensory neuron. This is an afferent. neuron carrying sensory information towards a CNS organ. Pictured here is one such CNS organ, namely the spinal cord. So which brings me to number three. Number three is my integration center, which is a CNS target, like this could be the brain or the spinal cord. So integration is really where the spinal cord is receiving the sensory information and then it has to make sense or interpret this sensory information. determine what the right course of action needs to be and sends a set of outputs. So that's why this is called the integration center. By the way, the stimulus kind of shown here was pain because of a sharp object penetrating the skin. So in response to that stimulus, like I said, there are receptors that are nociceptors, right? That detect information related to that pain, carrying that sensory information through the sensory neuron to my, um, integration center, the spinal cord. Now, oftentimes you will see the shorter green neurons. These are interneurons, which helps to connect a sensory neuron to a motor neuron. Motor neuron is shown in red here. What's happening in the spinal cord is I am perceiving the stimulus. I am making sense of that stimulus, namely pain. So what the spinal cord should say or decide to do is basically send appropriate motor output through this. motor neuron, which is part four, to a select group of skeletal muscles, depending on which region was experiencing this painful stimulus. And it's going to basically cause contraction of your skeletal muscles, which allows you to say, if you say this sharp object was penetrating the integument in your lower extremity. So like if you stepped on a sharp object, so then this would be, this motor output would be controlling skeletal muscles in the lower limbs, which helps you to bring about flexion and remove your foot from that. painful stimulus. So this is a reflex arc. So there's a stimulus and your receptor will detect and respond to the stimulus by sending sensory information through a sensory neuron to an integration center, which makes sense of it or interprets that stimulus and then generates a set of outputs, which then travels through this motor neuron right there. And the motor neuron will then target. the appropriate effector, in this case, a skeletal muscle, bringing about some desired response. So this is a reflex arc. And of course, everything related to reflex activity of skeletal muscles is controlled by the spinal cord. And that's why you see it pictured here as the integration center. So let's talk about different reflexes. So if you were activating skeletal muscle, this would be a somatic reflex. If you have a different visceral effector like smoother cardiac muscle or a gland, it would be an autonomic reflex. For this chapter, we're only focusing on somatic reflexes. So we're talking about how do you bring about certain reflex activity or changes in the skeletal muscle in response to certain stimuli. And the two stimuli we're going to discuss would be stretch and tension. And. How do you bring about a specific response in the skeletal muscle via integration occurring at the level of the spinal cord? That would be your somatic reflex. So remember in the case of a spinal somatic reflex, the integration center is always a spinal cord. It does not involve any of your higher brain regions. And once the spinal cord makes sense of what that stimulus is and sends a set of output information. that output information is going to be sent to your effectors. Effectors in this case, since it's somatic, it would be skeletal muscle, okay? So this is really important for us to understand because anywhere along that pathway of the reflex arc, like at the level of the receptor or the sensory neuron or the integration center or the motor neuron, whatever, there could be abnormalities or there could be damage to any of those regions. So knowing what your normal reflex is, is important because then you can clinically assess if there's a problem with any of the components in the reflex arc, especially if it is nerve damage or if it is damaged to a certain level of the spinal cord or degeneration of nerves or malfunctioning of the receptor. All of that can be detected or at least your... clinician has a better understanding of what is wrong with the pathway. If you know what normal is, then you would see if you don't have that normal reflex response, then that could be either a problem at any different level of the reflex arc. So two such reflexes that I want to describe here are the stretch reflex and the tendon reflex. So the first part is the stretch reflex. So this is going to detect changes in the length of the muscle. The special receptor that detects these changes in the length of the muscle is called a muscle spindle. So this is a muscle spindle that responds to stretch. So stretch would be an increase in the length of the muscle. There's a different receptor called the tendon organ that detects different changes in tension. associated with the muscle. Tension would be the amount of contractile force. So if a muscle is in a state of contraction, the force generated is tension in that muscle. This is detected by a tendon organ. So obviously you don't want the muscle to wear out and to have exaggerated tension or a buildup of contractile force. I'm very quickly going to describe the stretch reflex first. What you see here is skeletal muscle and that's a joint okay so so within the skeletal muscle fibers a group of these muscle fibers get modified and get encapsulated where it forms this outer structure encapsulating this these muscle fibers and this must these muscle fibers encapsulated serves as your special receptor within the skeletal muscles called a muscle spindle okay what does a muscle spindle do it detects changes in the muscle length that this muscle spindle is situated in. So when you see stretch of the muscle, and that can only happen if you saw lengthening of the sarcomere, right? If you saw stretch of the muscle, that stimulus is picked up or detected by your muscle spindle, which is then going to carry that stretch information via your blue neuron, which is your sensory neuron. It's of course going to carry this information to the spinal cord, the integration center, where the spinal cord should basically interpret this information and say, okay, the muscle is stretched out too much. So therefore, it should generate a set of outputs that is sent by your red neuron, which is the motor neuron. And then it's going to target this effector, which is the skeletal muscle that is stretched out to begin with. And it's going to basically bring about a relaxation of the muscle, which brings about a decrease in the amount of stretch. So let's go ahead and explain this a little bit more. Before I leave the slide, let's talk about this right here. This is a different type of receptor. This is called a tendon organ that is associated with a joint. And this would detect a different stimulus instead of it detecting stretch. um in the muscle in the skeletal muscle it's going to detect the state of contraction of this muscle so it's going to detect how much tension or contractile force is being generated by this skeletal muscle so let me let me show you a schematic to um put this into better perspective so we're going to talk about the muscle spindle first and then the tendon organ so the stretch reflex this is related to uh the muscle spindle receptor so the muscle spindle detects when there's a change in the skeletal muscle length, which in other words, in terms of stretch. So kind of shown here on the bottom, the stimulus here is stretch of the muscle, which is obviously an increase in the length of the overall muscle. So the sarcomere length being longer. In response to the stretch, the stimulus, the receptor, namely the muscle spindle, will carry this information to the spinal cord, which then sends a motor output back to the stretched muscle and says, contract now, okay? Because what does contraction do? Contraction is going to reduce the length of the muscle. So the problem or the stimulus was an increase in length of the muscle. And the response would be to bring about contraction of the stretched muscle, which would decrease the length of the muscle. So that's how... the stretch reflex works. Okay. I'm going to explain these terms monosynaptic, ipsilateral in just a little bit. And I also want to talk about reciprocal inhibition, but I think this will make sense if I actually give you an example of a reflex pathway like right here. Okay. Now, if you assume, well, normally muscles kind of work in pairs where they're antagonistic to each other. So if you assume like, say these are muscles within the upper arm region, so that That could maybe be the biceps on the top there. And this could be the triceps here on the bottom. Or if you were focusing on the lower leg, lower limb region, this could be the quadriceps in the front and the hamstrings behind it. Okay, so let's go with biceps and triceps. Okay, so let's say this muscle here, the biceps is stretched out. So here's your initial stimulus. Stimulus is stretch of this muscle, the biceps. Okay. So what happens is you see a change in length, stretch is going to increase the length of the muscle fibers of this biceps group. So the muscle spindle here is going to pick up this stimulus. It's going to detect that stimulus, which is a stretch in these muscle fibers. I'm going to carry all of that information through my sensory neuron to the spinal cord where you're going to make sense of it. And then notice there are two. sets of neurons here. These are both motor neurons because they're both leaving the spinal cord through the ventral root. So these are both motor neurons. Let's focus on the red neuron first. This red motor neuron, notice that's a plus sign, a positive sign. I'll tell you what that means. This motor neuron is going to carry this information back to my stretched muscle, right? My muscle of interest. And it's going to bring about a positive effect. And the positive effect in the case of a muscle. when you when your part when it has a positive effect this is a stimulation effect it always brings about contraction so plus always translate to translates to contraction whereas negative inhibition would translate to relaxation so it began with an initial stimulus which is muscle stretch so the muscle fibers are stretched out that you're seeing an increase in the length then the motor neuron from the spinal cord is going to bring about a stimulation signal which is going to bring about contraction or it's going to reduce the length of those fibers so the problem was an increase in length and the response from the spinal cord through this reflex arc would be right the opposite contraction which is going to bring about a decrease in the muscle fiber length okay so that's that's what's going on with the biceps Normally, you've got an antagonistic muscle. And again, this would be the triceps in this case, if you're considering the biceps on the top. The triceps will go through right the opposite. The motor neuron from the spinal cord to the antagonistic muscle here on the bottom would have the opposite effect, would have a negative effect. So if the stretched muscle is getting a positive stimulation effect, causing it to contract, at the same time... the spinal cord is going to send a negative inhibition effect to the antagonistic muscle, bringing about relaxation. And that's important because when the biceps are contracting, this is going to bring about flexion, right? Well, if the biceps are flexing or going through the process of flexion, then the antagonistic muscle, the triceps, cannot flex as well at the same time. it has to do right the opposite. It has to relax, bringing about basically kind of like an extension. Okay. So this is how the stretch reflex works. I want to point out a few things here. Let me go back here. I want to point out what's monosynaptic, ipsilateral, and reciprocal inhibition. So let's get back here. Okay. So notice that's my blue sensory neuron connecting or synapsing directly with the red motor neuron. This is called a monosynaptic connection or a monosynaptic terminal in this case, because you see the sensory directly synapsing with the motor, meaning it's just one synapse between these two neurons. But on the right here, you're looking at a sensory connecting to an interneuron, a short interneuron, which then connects to the motor neuron. So this is one synapse. there and a second synapse down here. So therefore, this would be an example of a polysynaptic pathway. So monosynaptic is when you only have sensory directly connecting with motor neuron and polysynaptic when there's multiple neurons in play. Now, there could be multiple interneurons as well. It doesn't have to be just one interneuron. What's ipsilateral? Okay, so say this is the stimulus, a stretch signal being transferred or being sensed in the left upper extremity in in your left arm okay so in your left upper limb that's carried to the spinal cord and notice the motor information is also originating from the left side of the spinal cord so the spinal cord is controlling uh the left side of the spinal cord is controlling the left side of the body and therefore this would be an example of ipsilateral if this kind of crossed over so stimulus from the left side of the body is then connecting to the right side of the spinal cord and then the motor output is originating from the right side of the spinal cord then that would be an example of a contralateral type of response but in this case as you can see this is all controlled on the same side of the spinal cord so the sensory and the motor information all of those are occurring on the same side of the spinal cord so therefore this is ipsilateral but if the sensory and the motor are on the opposite side of the spinal cord, it would be contralateral. Okay. And then coming back to our concept here of agonist and antagonistic muscles. Let me back up. What is reciprocal inhibition? Okay. Reciprocal inhibition is related to what's going on in the antagonistic muscle. Let me just back up a little bit. The motor output. to the stretched muscle is positive, namely that is stimulation. But the motor output to the antagonistic muscle is negative, namely inhibition. So that's why this is called reciprocal inhibition. So reciprocal is basically examining what type of response is being brought about in the antagonistic muscle. In this case, it's a negative response. So therefore, this is reciprocal inhibition. I'll show you in the next scenario. where it's going to actually be a positive signal to the antagonistic muscle. In that case, it would be an example of basically a reciprocal stimulation or cooperation. Okay, so I think we've wrapped up the stretch reflex. Oh, here's another example of the stretch reflex before we move on. This is a very important example called the knee jerk reflex or the patella reflex. Okay, so what you're seeing pictured here is your lower limb. Okay, so that's the muscle in the thigh region. That's the quadriceps femoris muscle group and the antagonistic muscle behind it, which is your hamstrings. So what happens when the quadriceps muscle is stretched out? So the stimulus is an increase in stretch of the quadriceps muscle. This is sensed by the muscle spindle in this muscle. It's going to carry that sensory information, namely increase in stretch, which is an increase in the muscle length. I'm going to carry that sensory information through the sensory neuron to the spinal cord. And then from here, my motor neuron generated. from the spinal cord is going to send a positive signal, red neuron, positive signal to the stretched muscle, in this case, the stretched quadriceps group, which is going to bring about, since it's positive, positive is always a stimulation and excitation signal, which would bring about contraction in this muscle. Contraction of this quadriceps is going to reduce the stretch, which was the initial problem, okay? Now the antagonistic muscle, namely the hamstrings, through a different motor neuron from the spinal cord, is getting the opposite signal. So positive for the quadriceps, but negative for the hamstrings, which is the antagonistic muscle. And what happens here with the hamstrings? Because this is negative, this is inhibition. Therefore, it's going to bring about relaxation of the hamstrings. Because this is a negative signal, being received by the hamstrings, the antagonistic muscle. This is an example of reciprocal inhibition. Let's look back here. On the left, again, I'm showing you the sensory neuron making a direct synapse with the motor neuron. So therefore, this is a monosynaptic pathway, whereas on the right, sensory neuron to interneuron, which then connects with the motor neuron. So that's a polysynaptic pathway. Okay. That basically is an example of forgot to mention the response i'm sorry um when uh when the motor neuron sends a positive signal to the quadriceps it brings about contraction of the quadriceps and the main function of the quadriceps when it contracts would be what um basically you're going to kick your leg uh outwards which is basically uh extension occurring at the knee okay that this is the normal knee jerk reflex so when you um use a reflex hammer to basically excite the patellar ligament. When you kind of tap on the patellar ligament, it's going to bring about this whole reflex pathway, which is going to cause basically stretching the quadriceps muscle. And then you have this entire reflex pathway that's carried out. And if all of these different components, the muscle spindle, the sensory neuron, the spinal cord, the motor neuron, if all of these components were working fine, then the expected response of this knee jerk reflex would be extension at the knee where you kick your leg outwards. And any abnormality or pathology related to any of these components could cause zero response where you don't see any extension, or you may see a distorted response, a slightly distorted response like like an exaggerated response where you kick your foot forward in a more exaggerated manner. Okay. So that's a stretch reflex, which then brings me to this last part here, which is the tendon reflex. Okay. So this is also related. This is a somatic reflex as well related to the skeletal muscles. This is right the opposite of a stretch reflex. Okay. So this is going to kind of shown here that the stimulus here is going to be an increase in tension. So you When you have a muscle that is experiencing a greater amount of tension, which is basically a greater amount of contractile force, meaning this muscle is in a contracted state, okay? You do not want to damage that muscle by extending out that state of contraction over a long period of time because that would not be good. So therefore, your tendon reflex is going to prevent damage by okay, what happens here in response to the stimulus, which is an increase in tension, the receptor, which is your tendon organ, receives this information, sends it to the spinal cord. And the response in this case is. Remember, the muscle was contracted. So the response then by negative feedback should be relaxation of the muscle, which is going to reduce your contractile force and reduce your tension. Okay. So that's in essence what's going on in the case of the tendon reflex. And I'm going to talk about reciprocal activation or reciprocal stimulation, however you want to call it. We talked about reciprocal inhibition with respect to the stretch reflex, right? The opposites happen. here in the case of the tendon reflex so let's use this example right here so a same set of muscles here you've got your quadriceps in the front and then towards the posterior aspect you've got the antagonistic hamstring group so here's my tendon organ in the joint cavity this is going to basically oh in the joint I meant this is going to pick up information related to the stimulus of alpha excessive tension or increased increased force of contraction so when this muscle is in a state of contraction that tension that is developed or that contractile force that's developed is sensed by this receptor the tendon organ same story i'm going to carry the sensory information through the sensory neuron which is my blue neuron to the spinal cord which is my integration center and i'm going to send two outputs as always okay well the red motor output goes back to my muscle of interest The blue motor output goes to the antagonistic muscle. So this muscle is in a state of contraction. So what's going to happen now is I'm going to send a negative signal to that contracted muscle. Negative means inhibition. Inhibition would bring about relaxation of this contracted muscle, which makes perfect sense, right? You don't want to damage the muscle by making it stay in a prolonged state of contraction. Now, okay, so my hamstring, I'm sorry, my quadriceps received a negative inhibition signal, which brought about relaxation, which would then mean that the antagonistic muscle group, the hamstrings, would get a positive signal. This is stimulation or excitation, which is going to bring about contraction of the hamstring group. Okay, I want to point out in this case, because your antagonistic muscle is receiving a positive stimulation signal, This is an example of reciprocal activation or reciprocal stimulation. Okay. And then in here, in the spinal cord, these are both examples of, on both sides, they are examples of a polysynaptic reflex or a polysynaptic pathway. And it's still ipsilateral because sensory information is going in through on the left side of the spinal cord. The motor information is also being generated on the left side of the spinal cord. So that. is a description of reflexes where we talked about two different types of reflexes, the stretch reflex and the tendon reflex. So the peripheral nervous system chapter is pretty straightforward. I just want you to remember the classification of different receptors, make sure you know what's going on on the sensory end of things, what's going on with the circuit level. And then motor control using those three different levels of control. And then we moved on to our description of reflexes. So that wraps up my description of the peripheral nervous system. I hope this made sense to you. This is important for you to kind of integrate what we've already discussed with the central nervous system. Because we're talking about... the connection or the communication between the CNS and the PNS, and which is really important for you to kind of make that connection between what's going on in the brain and the spinal cord and all of the different peripheral organs in terms of sensory inputs towards the brain and, of course, motor outputs out of the brain, but towards the peripheral nervous system. Hope this made sense to you. Thank you.

So the first part is going to be a classification of different receptors. But first, let's orient ourselves and kind of review where the peripheral nervous system is organized in relation to the entire nervous system. So let's look at this schematic, which you should be familiar with already. So we're referring to all of the different neural structures that you typically see outside of the brain, the spinal cord.

This could be sensory endings or sensory receptors within the peripheral nervous system. The entire transfer of information, sensory information to the brain and motor information out of the brain, all associated with the PNS, this is all carried through a set of nerves. So these are all peripheral nerves.

Depending on the origin of the nerves, you would classify them as either cranial nerves if they originated from the brain or... spinal nerves if they originated from the spinal cord and all of the different associated ganglia related to those nerves and then within the peripheral nervous system or within the peripheral organs a description of the different motor endings or the efferent motor endings so here's where the pns is in relation to the entire organization of the nervous system remember the cns that we just finished discussing in the previous chapter The CNS, the central nervous system, consists of two main organs, the brain and the spinal cord. So everything other than the brain and the spinal cord would constitute the peripheral nervous system or the PNS, which really consists of two main divisions. The blue part that you see showing information, sensory information being carried through the peripheral organs to the CNS.

That's your sensory division. Now, these sensory inputs could be from anywhere in the body, like the skin. It could be the skeletal muscles.

So it could be also the cardiac muscle, smooth muscle, and any sensory information. So really, this could be visceral sensory or somatic sensory. So it could be related to the skeletal muscle, or it could be other inputs that you get from other autonomic targets.

So pretty much any sensory information is carried through this division. towards the brain. Okay.

So this is always sensory is always afferent with an A meaning carrying information towards the brain and the spinal cord. All of the red part that you see here, which is what we're going to discuss to a certain extent would be the motor output originating from the CNS and leaving the CNS. So this would be efferent with an E, efferent motor output via the peripheral nervous system.

So that's the motor division. The motor division can be split into two basic subdivisions. Somatic here refers to anything that is controlled, a voluntary effector. Typically we refer to all of the skeletal muscles as being part of the somatic motor division.

The autonomic motor division consists of involuntary targets such as the cardiac muscle, smooth muscle that you see in any hollow organ. And of course, any glands. So these are all involuntary targets in the case of the autonomic nervous system, but voluntary targets in the case of the somatic nervous system. You can then break down or further classify the ANS, the autonomic nervous system, into two main divisions, the sympathetic and the parasympathetic, which we will discuss in the next chapter.

So really what I want to do here today is within this discussion of the peripheral nervous system, I'm going to kind of... highlight a few things related to the autonomic nervous system, but that's a chapter in itself. Really, my focus is going to be more of the somatic nervous system. And then of course, we've got to talk about the sensory division.

So what I want to talk about here is break this down into what are the different sensory receptors. So what are the different sensors or receptors within the peripheral organs that pick up information related to sensory stimuli? Now, this could be general senses like touch.

stretch or pain, temperature, any of that. Or it could be special senses as well, like information related to light. So vision and then, of course, auditory information, equilibrium information, all of that would be your special senses. So that's the sensory information. So we're going to talk about different classifications of sensory receptors.

And then we're talking about all of this information going to the brain and the spinal cord. cord and then the motor division which carries information out of the brain the spinal cord to then target different effectors and these effectors could be like i said somatic meaning voluntary effectors like the skeletal muscle or involuntary targets such as these which would be related to the ans which is your cardiac muscle and your smooth muscle for examples now information either sensory towards the brain or motor out of the brain you is all carried through a set of nerves. So we're going to talk about peripheral nerves.

Okay, so we'll discuss mostly spinal nerves, but just generally we'll discuss an ascending track, which we've already talked about in the previous chapter. So ascending tracks would carry sensory information towards the brain and a descending track would carry motor information out of the brain. And then once this motor information reaches whatever this target is, so skeletal or... any involuntary target.

Then we're going to talk about motor endings that you see within these effectors. So let's break this down. Let's first start with sensory, talk about the different classification of receptors, and then we will kind of discuss the conduit level where you, or rather the circuit level where you talk about an ascending or descending track. And then of course, tie this all in with motor endings.

All right, so let's start with sensory receptors. So these are sensors that are located anywhere in your body other than the brain and the spinal cord. So any peripheral organ, any peripheral effector or target that has these receptors, which are sensors that respond to changes in the external environment. So these changes are called stimuli or a stimulus.

It's a trigger that is sensed or detected by the sensory receptors. When a sensory receptor responds to a stimulus like pain or temperature or touch or whatever different type of stimuli, these sensory receptors will then get activated, which then results in the creation of a greater potential. So this is really important in terms of perception of this stimulus.

The stimulus has to be converted into a greater potential. Sensation is awareness of that stimulus and the sensation or being aware is really carried out by these sensory receptors. And then converting it to a greater potential is important for perception of that stimulus.

And typically this perception occurs in higher brain regions. So you can classify sensory receptors based on three different categories listed here. Okay, the first category is what type of stimulus? does this receptor respond to?

What does it detect? Second could be, okay, where exactly, what is the location of these receptors inside the body? And the third category is more structural, referring to whether they are just modified dendritic endings, so very modified neurons or free nerve endings, or is this something that has more complexity to it? So does it have a very specific organization and a structure?

and which you would typically see in sensory receptors associated with special senses, like, for example, within the retina of the eye, which is important for the special sense of vision. Now, the retina is more than just a modified neuron. It has a very complicated structure.

It has a very organized structure, so there's a lot of detail to it. Likewise, a different special sense, such as hearing, is carried out by a different receptor located within the cochlea within the inner ear region. Now the cochlea also has a very very specific structure so really structural complexity would be a category where you're classifying your receptors based on a very basic simplified kind of a structure or something that has more complexity.

So let's classify them first by stimulus type. Okay, so if a receptor responds to touch, pressure, vibration, or stretch, these are mechanical stimuli like touch, pressure. Therefore, these are called mechanoreceptors. Any receptor that responds to changes in temperature would be a thermoreceptor.

Receptors, especially in the retina of the eye, that respond to light energy would be a photoreceptor. If a receptor responds to chemicals in the body, especially related to smell and taste, changes in chemicals related to blood chemistry, increases in carbon dioxide, things like that, hydrogen ions, all of those would be chemoreceptors because they're responding to chemicals. And this last category are those that respond or are sensitive to pain-causing stimuli. So pain-related receptors are called nociceptors. So...

This could be extreme heat or cold, excessive pressure, or it could be an inflammatory chemical. So depending on the type of stimulus that the receptor responds to, you can classify them as these as shown here in blue. So that's the first classification of a receptor based on stimulus type.

The second classification is based on location. Where exactly are these receptors located within the body? Okay. and what type of stimulus does it respond to?

Where does the stimulus arise from? Okay, so if the stimulus is coming from outside of the body and these would be exteroceptors. So normally this is related to the integument.

So you see exteroceptors mostly in the integument on the skin. And of course, these are receptors that respond to touch, pressure, pain or temperature. Of course, all of your special sense organs for the most part are exteroceptors because they're receiving light information from the outside or pitch and frequency and amplitude information related to sound waves from the outside. So those would be examples of exteroceptors. Okay, if the stimulus is originating from inside of the body, then those would be an interoceptor.

So these receptors would respond to stimuli. within internal viscera. This could be, let's say, a distended stomach wall in response to food entering into the lumen of the stomach.

So that could be basically an interoceptor. It could be in the wall of the blood vessels, which obviously detect changes in blood pressure. So all of these interoceptors are responsive to stretch, as in the case of blood pressure, in the case of blood vessels, temperature receptors, or other chemical changes in the body. So those will be interoceptors. Proprioceptors are specific.

These are mechanoreceptors that are responding to stretch in skeletal muscles, or it could be within the tendons or the joints. So keep this in mind. We will talk about certain examples of proprioceptors towards the end of this discussion when we talk about the muscle spindle and the...

the tendon organs. Those are two examples of proprioceptors. So this gives you information and it's carrying information related to a person's stretch of skeletal muscles, which is kind of an indication of a movement.

And that's the information that's being carried to the brain. So that's the classification based on location, which then brings me to this next classification, which is by receptor structure. You can broadly classify this as general sense receptors or special sense receptors.

General senses are located throughout your body. These are very simplistic receptors that respond to different sensations like touch pressure, stretch, vibration, pain, temperature. All of these are general or visceral senses.

You can classify these general sense receptors as either kind of simplistic, called non-encapsulated. And so these could just be free nerve endings. Your nociceptors, your thermoreceptors, your light touch receptors, these are all hair follicle receptors example, would be non-encapsulated. So nothing too complicated about their structure. Or they can be something way more complex where they have a capsule surrounding that receptor.

We've already discussed some of these examples when we... discuss the integument chapter where we talked about a special, I'm sorry, a light touch sensor called the Meissner's or the tactile corpuscle that you typically saw. between the dermis and the epidermis in the dermal papillae.

And then we also talked about these guys, the Pacinian corpuscles that are located deeper inside of the dermis, almost closer to the hypodermis. And these are responsible for deep pressure. We will talk about muscle spindles, which pick up information or sense information related to the stretch or the length of a muscle.

We will also discuss tendon organs, which is an example of an encapsulated receptor, which you will see within joint cavities, which is going to give information related to muscle tension or the contractile force or the amount of contraction that is being experienced by the muscle. So all of these are examples of general sense receptors. And then based on the receptor structure, you can classify them as either non-encapsulated, so they're kind of... free without a surrounding capsule, or they could be encapsulated where they have more of an outer capsule-like structure. And then, of course, you've got your receptors for special senses, and the special senses are related to vision, hearing, equilibrium, smell, and taste.

And this is all covered in a separate chapter. So we're not going to be focusing on special sense receptors here. Here, we're going to mostly focus on general sense receptors, okay? Okay.

So we've already talked about the very first part of the peripheral nervous system, which is how do you change, how do you detect changes in stimuli or these different sensations in the body like touch and pressure and stretch and pain, temperature, so on and so forth. Those are all detected by these receptors, which we just classified based on location, based on structure, based on the stimulus that it responds to and so on and so forth. Now what I want to do is kind of show you the connection between the peripheral nervous system and the central nervous system.

So let's start with the bottom here and work our way towards the top, which of course you can identify there as the brain and here's the spinal cord. And this is information from the brain and the spinal cord communicating with all of the different peripheral organs. Example shown here is skeletal muscle. So there's a very specific receptor here within the... skeletal muscle called a muscle spindle that detects changes in the length of this muscle.

It detects whether that muscle is in a state of contraction and so on and so forth. So that would be either shortening of the sarcomere resulting in a shortening of the length of the muscle. That's detected by the muscle spindle. You have joint kinesthetic receptors that are typically seen within ligaments or within the joint cavities. You also have shown here the integument.

Those are different sensory receptors located to detect changes in pain, in pressure, in touch, and so on and so forth. So here's all of the peripheral organs, or at least an example of a few of your peripheral organs, and different receptors. So that's why this is called, the very first level is called the receptor level, which is important for sensory reception. Okay, now what do you do with this information?

When, say, your muscle spindle or your Meissner's corpuscle or your Pacinian corpuscle or any of your modified free endings of your neurons or whatever, when they pick up information related to some kind of a stimulus, some kind of a sensory information, well, you're going to have to take it, carry it through pathways, through these tracks. That's why you talk about the circuit level next. So all of this information, sensory information, is carried upwards through the spinal cord.

And this is the RAS right there showing you the region that spans the brainstem, the midbrain, the pons, and the medullar vanguards. And then from here through the thalamus and from the thalamus into the parietal lobe where you will see. located on the postcentral gyrus. This region is functionally important as the somatosensory cortex that we discussed in the CNS chapter. So obviously, these are all general sense information, sensory information like stretch and pain and pressure and touch.

These are general senses. So it's all going to be carried through different tracks like an ascending track. And this is the circuit level.

I'm going to carry it up. to this specific region, the somatosensory cortex, located in the postcentral gyrus, dotted line representing the central celsus. So right behind it should be the postcentral gyrus.

that is located within the parietal lobe. And this is really where perception occurs. Perception of what? Perception of all of this sensory information that's being carried into this specific region.

And this could be perception of pain signals, could be perception of touch, being able to discriminate between something like... velvet or like a soft feather across your skin or something that's sharp and pointed or something that's causing pain. All of that sensory perception is carried out here in the postcentral gyrus within the somatosensory cortex.

So to summarize, when you think about sensory integration, it consists of three different levels. First, you have to sense, you have to detect. this stimulus, this sensory information, all carried out by the receptors within the peripheral organs. You have to carry this information, this sensory information as basically action potentials.

Okay. You got to carry it through ascending tracks or networks of neurons ultimately. So that's, that's called the second level is the circuit level.

And then you're going to carry it up here into the somatosensory cortex region within the, within the brain. cerebral cortex where perceptions are occurring. So three different levels, receptor to circuit to perceptual levels.

Okay. What I want to discuss next is this concept of somatotopy. So let me first show you what's going on here on this slide.

So you're looking at, say, the superior view of the brain. That should be the anterior part of the brain. Here's the posterior part of the brain.

So all of this, this lobe here in the front should be the frontal lobe. You're seeing the central sulcus right there. That should kind of be the partition between the frontal lobe and the front.

Then you're kind of seeing the red part that's associated with the frontal lobe and the portion there at the back, which is associated with the parietal lobe. And of course here, right through the center, dividing out the left and the right cerebral hemispheres, that should be your longitudinal fissure. So if you were to look at, say, if you remember, the central sulcus is the boundary between the frontal lobe and the parietal lobe, right?

And this kind of raised fold, upward fold in the front, in front of the central sulcus was the pre-central gyrus. And this red part here is... kind of showing you the precentral gyrus on both the hemispheres.

The precentral gyrus is important because that's where you see the primary somatic motor cortex region. Behind it, shown in blue, that's on the parietal lobe. This should be the postcentral gyrus.

This region is important as a somatosensory cortex region. So regardless of whether you are mapping, the motor cortex in the frontal lobe or the somatosensory cortex shown in blue in the parietal lobe. So motor cortex shown here on the left, that's on the frontal lobe.

And the blue part here, this is the somatosensory cortex shown from the parietal lobe shown here on the right, regardless of what you're mapping. Okay, so I just kind of want to orient you. So if you take a cross section, as you can see there, a slice right through the somatosensory cortex, kind of map it over here. um and then you take a slice through the motor cortex the red portion here on the left and kind of map it on the left here again the center the longitudinal fissure is towards the middle aspect so the median aspect is right here okay towards the middle and towards the sides the left and the right side is what you're going to see here okay does that make sense um so let's say let me back up here right quick so say information um from the integument, of course, integument is going to cover your entire body. So whatever sensory receptors within the integument picking up changes like, say, touch or pressure, whatever, depending on where these sensory receptors are originating from.

So where am I carrying this information from? This sensory information is taken to the somatosensory cortex, but where exactly? Every... I'm sorry, stimuli or sensory information from every part of your body is mapped precisely to a specific region within the somatosensory cortex.

And that's what I'm showing you over here. Okay. So say you were... touching velvet or a soft object okay so that touch information that is picked up by sensory receptors in your in the in the fingers in the integument associated with the fingers all of that information is carried through ascending tracks all the way through the brainstem and the thalamus and then it's going to get to the somatosensory cortex region to make sense of it right so what exactly are you feeling is this uh a sharp object or is it a soft object?

What is it, right? So this perception is occurring in the somatosensory cortex, but where exactly? And that's what you're seeing here.

Anything related to the hand, the fingers, the forearm and all of that is right there. So kind of somewhere over there. Does that make sense?

The median part should be over here. And then as you move away, the lateral aspect should be here towards the end. Okay.

So everything related to the upper extremities, the upper limbs, all of that is mapped in this region. If you're picking up information, say you step on a sharp object, like so something, a nail pierces your foot. OK, also from the leg region or from the foot region.

Notice the foot region and that sensory information is picked up or it is mapped to this specific region. of the somatosensory cortex. So everything related to the face is over there. And then of course the rest of your organs.

So it's all very precisely mapped. And this is the same even in your motor cortex region. So if I were to send motor information from the frontal lobe, from the primary motor cortex region, and I wanted to control facial muscles, then basically all of those outputs would originate from this specific region of the...

of the motor cortex and it's going to carry all of those neurons are going to carry that information towards your face to control whatever muscles of your face. So this is somatotopy and this is basically sensory or motor information being precisely mapped within very very specific regions of either the motor cortex or the sensory cortex. So let's talk about processing.

Okay, so when a receptor, which is located within a peripheral organ, when a receptor responds to some kind of a stimulus, again, pressure, touch, or whatever, this stimulus has to be converted. This is transduction. It has to be converted to a graded potential. So these graded potentials that occur in general sense.

receptors is called a generator potential. So this is the progression of what's happening in terms of processing at the receptor level. So a general sense receptor picks up information related to some kind of a stimulus. So let's say this is the Pacinian corpuscle located deep in the dermis of the integument that is responding to a specific stimulus like a pressure signal.

Okay. It's then going to convert that sensory information. pressure into a greater potential called a generator potential. And then that then gets converted into an action potential and you're going to carry it through a set of afferent neurons towards the brain and specifically taken to the postcentral gyrus within the somatosensory cortex area where you're going to make sense of that sensory information. So that's in the case of a general sense receptor.

It's more or less the same pathway in a special sense organ, special sense organs like within the retina of the eye there are receptors for vision for for light information called uh this um the well the retina would would serve as as the special sense organ within within the eye to pick up information related to vision or in the case of hearing within the cochlea which is located within the inner ear there is a specialized receptor called the spiral organ you that will detect changes related to pitch or amplitude of sound waves. So when those stimuli like light information or frequency of sound waves and amplitude of sound waves, when all of that is detected by these special sense organs like the retina or the spiral organ of the cochlea, those stimuli get... converted into some kind of a greater potential.

Now instead of it being called, let me back up, instead of it being called a graded potential, which is the case in a general sense receptor, it's now called a receptor potential in the case of a special sense organ. It's the same principle. The stimulus has to get converted into a greater potential. Now, this receptive potential, it's a little more detailed in the case of your special sense organs, because typically this great potential will cause the release of some kind of a neurotransmitter from your...

receptors. Let's say, for example, within the retina, you have your photoreceptor cells, like your rod or your cone, that will develop a greater potential called a receptor potential. And then in response to it, it's going to release certain neurotransmitters, which then causes activation of other neurons and therefore basically generates greater potentials in sensory neurons. In the case of the retina, these sensory neurons would ultimately be your ganglion cells, which carry your action potentials all the way to the brain, specifically within the occipital lobe, where that information is received by the primary visual cortex.

Okay, so we talked about processing that needs to occur at the receptor level. Let's talk about processing at the circuit level. How do I carry information from the PNS to the CNS?

So this is all carrying sensory information towards the CNS has to occur with the help of an ascending track, which we've already discussed in the previous chapter, consists of three levels of neurons, first order, second order, and third order. So I'm not going to go into that again, since we've already talked about it with respect to the CNS. Once you carry this sensory information to the brain, then perception has to occur.

within the somatosensory cortex and the association area associated with that cortex region. Here are the different aspects of sensory perception. So perceptual detection, which is ability to detect a stimulus.

So typically this requires an addition or a cumulative effect of what we call a summation of impulses. Magnitude estimation. So the higher the number of frequency of impulses, the higher the frequency of action potentials, That is a reflection of a higher magnitude signal or a more intense signal. So a greater level of pain or a greater level of pressure, okay, would be coded for by a greater frequency of action potentials.

Spatial discrimination, that's the next aspect. Okay, so this is being basically identifying patterns, so pattern recognition and so on and so forth. Quality discrimination, this is... within a specific sensation, like say, for example, taste.

Is this a sweet taste or a sour taste? Or is it bitter? Or is it salty?

Things like that. That would be quality discrimination. And of course, pattern recognition, which is again, this could be facial recognition or it could be pattern recognition within, say, music. So all of that's carried out. That's the last aspect of perception.

So all of these need to occur. where within the brain, within the association area related to the somatosensory cortex. Okay.

Okay. So let's talk about the circuit level. This is all carried out through a set of nerves. So all of this information, sensory information being carried to the CNS or motor information being carried out of the CNS towards the PNS, it's all carried out through nerves.

two sets of nerves or two types of nerves. Cranial nerves if they originate from the brain and spinal nerves if they originate from the spinal cord. Depending on the information that these nerves are carrying, you can functionally categorize them as sensory motor or mixed.

Okay, so sensory nerves only carry sensory inputs towards the brain and the spinal cord, towards the CNS. And some examples of cranial nerves that carried sensory inputs would be, well, your olfactory nerve carrying information related to smell or your optic nerve carrying information related to vision. And so these are purely sensory nerves carrying all of that information towards the brain and the spinal cord.

If it's a motor nerve, then this would carry motor information out of the CNS towards a peripheral organ. Cranial nerve number three, ocular motor, would be an example of a motor nerve because this is going to carry motor information to control specific eye muscles, like your extrinsic eye muscles, which allows you to move your eyes up and down or whatever in different directions. So it helps you control the skeletal muscles, which make up your extrinsic eye muscles. controlling eye movements.

So that would be an example of a motor nerve. It can also control autonomic targets within the eye, allowing for you to control like the ciliary muscle, which allows you to change the shape of the lens, or it can be controlling the iris muscles, bringing about changes in pupil diameter. So again, those would be examples of motor nerves. Now, what's a mixed nerve?

A mixed nerve is one that contains both sensory and motor information. All of your spinal nerves are considered mixed nerves because there's always sensory information that's being carried through the dorsal root, which makes contact and synapses in the dorsal greyhound area. And then you also know that, like, say, for example, the... Mortar information out of the spinal cord is carried from, say, the ventral grey horn area through the ventral root, carrying information that would then target skeletal muscles and bring about muscle contraction, for example.

So therefore, a sensory nerve is one that only carries sensory inputs towards the brain and spinal cord. Mortar nerves carry motor outputs out of the CNS, and a mixed nerve carries both sensory and motor outputs. Okay, so. You can further classify your sensory and motor as either somatic or visceral.

So typically somatic is related to skeletal. Visceral is any other organ. So you can say somatic sensory or somatic afferent.

Always afferent is related to sensory and efferent is related to motor. So somatic sensory would mean carrying sensory information from the skeletal muscle towards the brain and the spinal cord. Visceral sensory means carrying information from any other organ in the body, like the blood vessels that are detecting sensory information like stretch because of high blood pressure.

And then, of course, chemoreceptors within the blood vessels that are detecting changes in relation to elevated CO2 levels, for example. So those are all visceral. sensory information carried from different organs towards the brain and the spinal cord.

And likewise, you can designate somatic and visceral in relation to motor nerves as well. Okay. So now that we are leading towards the motor aspects or the motor functions that need to be carried out in the peripheral organs, let's talk about what are some of the motor endings and how do you classify the levels of control? of motor activity. Okay.

So motor endings, these are the distal endings of motor neurons that basically activate whatever that target cell, that effector is. That effector could be a somatic target like skeletal muscles, or it could be an autonomic target like cardiac muscles, smooth muscle, glands. And what it does is that these motor endings will release neurotransmitters or chemicals, right, onto these effectors.

and bring about contraction or it can bring about relaxation if you're talking about a muscle or it can bring about in the case of glands it could bring about increased secretion of glandular secretions or can inhibit those glands okay when it comes to movements when it comes to controlling skeletal muscles And the cerebellum and the basal nuclei within the brain are very, very important in coordinating activity with the primary motor cortex region to basically control smooth, coordinated, complex movements in the body. Okay, so this is important for you to know, I guess, the organization or the hierarchy of motor control. So what you see here on the bottom is everything related to the PNS. These are...

peripheral organs. And as you go higher up, obviously you're seeing the spinal cord and all of the higher brain regions. All of this is related to the CNS. So I'm asking you, how do you control movement of information? How do you control both from the sensory end of things and the motor end of things?

What is the cooperation or what is the control between the CNS and the PNS? Always remember information being carried out of the PNS towards the CNS is sensory, which you see by these blue arrows here on the left. And information out of the CNS towards the PNS is motor, which you see with the red arrows. OK, so there's three different levels of control. Your lowest level is called the segmental level, which is all at the level of the spinal cord.

And we will talk about CPGs on a subsequent slide. So this is the lowest level. The spinal cord kind of works by itself, mostly related to reflex activity, which I'll talk about here in just a little bit.

One level higher than this is called the projection level. This is associated with the brainstem region, the motor cortex. I should have highlighted that as well. the motor cortex and the brainstem region. This can provide some input, as you can see, it can control the spinal cord at the segmental level as well.

And the highest level is way up here. This is called the pre-command level. This is consisting of the cerebellum and the basal nuclei.

So the cerebellum and the basal nuclei are the ones that actually come up with the instructions, especially in relation to the motor output. It comes up with the instructions. It comes up with the blueprint required to determine what those motor output instructions need to be. And then it's going to communicate this, especially like, say, the cerebellum along with the basal nuclei.

It's going to communicate that blueprint to the motor cortex, the somatic motor cortex within the cerebral cortex in association with the brainstem. And then that motor cortex sends that information through the spinal cord. to bring about motor activity like contraction and relaxation and so on and so forth off your targets within the peripheral nervous system.

Okay, so let's go in a few more details. Before I leave the slide, I want to recap one more time. Segmental is the lowest level consisting of the spinal cord.

This is mostly reflex activity. Projection level is in the middle consisting of the motor cortex and the brainstem regions and the highest level of control of motor activity is pre-command. which is cerebellum and the basal nuclei. Okay, so what you're seeing here is, let's see, segmental level, that's the spinal cord, that's the lowest level. Projection level, which consists of your cerebral cortex, specifically your primary motor cortex region and your brainstem region right there.

So all of that's going to be your projection level in the middle and your highest level, the pre-command level. which is really the cerebellum and basal nuclei that you see within the interior region of the brain. All of that's going to be your highest pre-command level.

So what do these three levels do? Okay. Like we said, segmental is the lowest. This is spinal cord, mostly reflexes and automatic movements.

CPGs, central pattern generators, these are like a network of neurons specifically. Now, remember we're talking about the spinal cord. So the ventral gray horn of the spinal cord is going to be important for motor neurons that control skeletal muscles. So basically your central pattern generators are circuits or networks of these ventral gray horn neurons originating from here so that you send specific outputs to different types of skeletal muscles.

Okay. So that's your skeletal level. Sorry, that's your segmental level. Your projection level consists of the cerebral cortex.

Remember, your descending tract consists of your upper motor neurons, like, for example, your pyramidal neurons or your pyramidal cells from the cerebral motor cortex region and other brainstem regions. So this is going to be the second level or the middle level. Highest level, precomat, consists of cerebellum and the basal nuclei. This is controlling activity. activity at all these levels.

So again, just generally, it's going to regulate all motor activity of your PNS targets, precisely starting and stopping movements, coordination of skilled movements so that it's not too jerky, so that it is more smooth, a smooth transition from one movement to the next, monitoring muscle tone, and of course, unconscious planning of movements as well. It's all controlled by the pre-command level. So those are your...

three levels, okay, starting with the lowest, the least control, moving upwards towards the highest level, which is the most control, okay? Okay, so now that we've talked about the PNS in general, we've talked about sensory receptors, the circuit level, the different nerves, we talked about that. I mean, then we discussed motor control.

I think we've mostly laid out the main aspects of the... peripheral nervous system. So what I want to do next is discuss a few more topics here, like reflexes.

This is important. And then we're going to talk about two important reflexes, the stretch reflex and the tendon reflex. Okay. So there are two main types of reflexes.

Some are called intrinsic reflexes. This is what you're born with. So being able to maintain posture, avoid pain, all of that. You're not learning these activities. These are just intrinsic, inborn reflexes.

And then you've got others that you acquire over your lifetime. These are learned reflexes. They come from practice and repetition, like driving skills.

So if you are an experienced driver and know what you're doing. on the road, you are not really thinking about your driving. It doesn't take that much effort once you are skilled enough at that function.

So if you're driving down the highway and you see, you know, red lights in front of you, like, you know, cars in front of you and you can see their taillights and you see those red lights. Okay. So, well, this is an acquired reflex. You don't even think about it this is all kind of controlled by your spinal cord what do you do you hit the brakes right that's that is a natural reflex that you're not it's not requiring too much processing you don't have to think think it through it doesn't require too much interpretation this is a natural reflex um which you have learned um or if you touch a hot object while cooking well you might you might just uh drop that pan or that skillet right so that would be basically an acquired reflex as well okay any reflex or a reflex arc consists of five main components that are listed over here make sure you know this i'm going to use this schematic on this slide to discuss those same five components okay so you're seeing here by the way that's a cross-section of the spinal cord you should recognize that by now central canal and you'll remember the different gray horns dorsal gray horn, the lateral gray horn, and the ventral gray horn.

So a reflex arc consists of a receptor. That's a receptor in any peripheral organ. So what you see here is skin. Okay, so this could be a nerve ending. It could be a corpuscle.

It could be a modified dendrite. Doesn't matter. These receptors located here and shown here in the skin would pick up information related to touch pressure or pain. or temperature and things like that.

So this sensory information is the stimulus is picked up or detected by a receptor. Then the receptor will generate a greater potential, right? Which then basically kind of converts it into an action potential. And you're gonna take that information, that sensory information is carried along this blue neuron. This blue neuron is called a sensory neuron.

This is an afferent. neuron carrying sensory information towards a CNS organ. Pictured here is one such CNS organ, namely the spinal cord. So which brings me to number three. Number three is my integration center, which is a CNS target, like this could be the brain or the spinal cord.

So integration is really where the spinal cord is receiving the sensory information and then it has to make sense or interpret this sensory information. determine what the right course of action needs to be and sends a set of outputs. So that's why this is called the integration center. By the way, the stimulus kind of shown here was pain because of a sharp object penetrating the skin. So in response to that stimulus, like I said, there are receptors that are nociceptors, right?

That detect information related to that pain, carrying that sensory information through the sensory neuron to my, um, integration center, the spinal cord. Now, oftentimes you will see the shorter green neurons. These are interneurons, which helps to connect a sensory neuron to a motor neuron. Motor neuron is shown in red here.

What's happening in the spinal cord is I am perceiving the stimulus. I am making sense of that stimulus, namely pain. So what the spinal cord should say or decide to do is basically send appropriate motor output through this. motor neuron, which is part four, to a select group of skeletal muscles, depending on which region was experiencing this painful stimulus.

And it's going to basically cause contraction of your skeletal muscles, which allows you to say, if you say this sharp object was penetrating the integument in your lower extremity. So like if you stepped on a sharp object, so then this would be, this motor output would be controlling skeletal muscles in the lower limbs, which helps you to bring about flexion and remove your foot from that. painful stimulus.

So this is a reflex arc. So there's a stimulus and your receptor will detect and respond to the stimulus by sending sensory information through a sensory neuron to an integration center, which makes sense of it or interprets that stimulus and then generates a set of outputs, which then travels through this motor neuron right there. And the motor neuron will then target. the appropriate effector, in this case, a skeletal muscle, bringing about some desired response. So this is a reflex arc.

And of course, everything related to reflex activity of skeletal muscles is controlled by the spinal cord. And that's why you see it pictured here as the integration center. So let's talk about different reflexes. So if you were activating skeletal muscle, this would be a somatic reflex.

If you have a different visceral effector like smoother cardiac muscle or a gland, it would be an autonomic reflex. For this chapter, we're only focusing on somatic reflexes. So we're talking about how do you bring about certain reflex activity or changes in the skeletal muscle in response to certain stimuli.

And the two stimuli we're going to discuss would be stretch and tension. And. How do you bring about a specific response in the skeletal muscle via integration occurring at the level of the spinal cord? That would be your somatic reflex. So remember in the case of a spinal somatic reflex, the integration center is always a spinal cord.

It does not involve any of your higher brain regions. And once the spinal cord makes sense of what that stimulus is and sends a set of output information. that output information is going to be sent to your effectors.

Effectors in this case, since it's somatic, it would be skeletal muscle, okay? So this is really important for us to understand because anywhere along that pathway of the reflex arc, like at the level of the receptor or the sensory neuron or the integration center or the motor neuron, whatever, there could be abnormalities or there could be damage to any of those regions. So knowing what your normal reflex is, is important because then you can clinically assess if there's a problem with any of the components in the reflex arc, especially if it is nerve damage or if it is damaged to a certain level of the spinal cord or degeneration of nerves or malfunctioning of the receptor.

All of that can be detected or at least your... clinician has a better understanding of what is wrong with the pathway. If you know what normal is, then you would see if you don't have that normal reflex response, then that could be either a problem at any different level of the reflex arc.

So two such reflexes that I want to describe here are the stretch reflex and the tendon reflex. So the first part is the stretch reflex. So this is going to detect changes in the length of the muscle.

The special receptor that detects these changes in the length of the muscle is called a muscle spindle. So this is a muscle spindle that responds to stretch. So stretch would be an increase in the length of the muscle.

There's a different receptor called the tendon organ that detects different changes in tension. associated with the muscle. Tension would be the amount of contractile force.

So if a muscle is in a state of contraction, the force generated is tension in that muscle. This is detected by a tendon organ. So obviously you don't want the muscle to wear out and to have exaggerated tension or a buildup of contractile force.

I'm very quickly going to describe the stretch reflex first. What you see here is skeletal muscle and that's a joint okay so so within the skeletal muscle fibers a group of these muscle fibers get modified and get encapsulated where it forms this outer structure encapsulating this these muscle fibers and this must these muscle fibers encapsulated serves as your special receptor within the skeletal muscles called a muscle spindle okay what does a muscle spindle do it detects changes in the muscle length that this muscle spindle is situated in. So when you see stretch of the muscle, and that can only happen if you saw lengthening of the sarcomere, right?

If you saw stretch of the muscle, that stimulus is picked up or detected by your muscle spindle, which is then going to carry that stretch information via your blue neuron, which is your sensory neuron. It's of course going to carry this information to the spinal cord, the integration center, where the spinal cord should basically interpret this information and say, okay, the muscle is stretched out too much. So therefore, it should generate a set of outputs that is sent by your red neuron, which is the motor neuron. And then it's going to target this effector, which is the skeletal muscle that is stretched out to begin with. And it's going to basically bring about a relaxation of the muscle, which brings about a decrease in the amount of stretch.

So let's go ahead and explain this a little bit more. Before I leave the slide, let's talk about this right here. This is a different type of receptor. This is called a tendon organ that is associated with a joint. And this would detect a different stimulus instead of it detecting stretch.

um in the muscle in the skeletal muscle it's going to detect the state of contraction of this muscle so it's going to detect how much tension or contractile force is being generated by this skeletal muscle so let me let me show you a schematic to um put this into better perspective so we're going to talk about the muscle spindle first and then the tendon organ so the stretch reflex this is related to uh the muscle spindle receptor so the muscle spindle detects when there's a change in the skeletal muscle length, which in other words, in terms of stretch. So kind of shown here on the bottom, the stimulus here is stretch of the muscle, which is obviously an increase in the length of the overall muscle. So the sarcomere length being longer. In response to the stretch, the stimulus, the receptor, namely the muscle spindle, will carry this information to the spinal cord, which then sends a motor output back to the stretched muscle and says, contract now, okay? Because what does contraction do?

Contraction is going to reduce the length of the muscle. So the problem or the stimulus was an increase in length of the muscle. And the response would be to bring about contraction of the stretched muscle, which would decrease the length of the muscle.

So that's how... the stretch reflex works. Okay.

I'm going to explain these terms monosynaptic, ipsilateral in just a little bit. And I also want to talk about reciprocal inhibition, but I think this will make sense if I actually give you an example of a reflex pathway like right here. Okay. Now, if you assume, well, normally muscles kind of work in pairs where they're antagonistic to each other. So if you assume like, say these are muscles within the upper arm region, so that That could maybe be the biceps on the top there.

And this could be the triceps here on the bottom. Or if you were focusing on the lower leg, lower limb region, this could be the quadriceps in the front and the hamstrings behind it. Okay, so let's go with biceps and triceps.

Okay, so let's say this muscle here, the biceps is stretched out. So here's your initial stimulus. Stimulus is stretch of this muscle, the biceps.

Okay. So what happens is you see a change in length, stretch is going to increase the length of the muscle fibers of this biceps group. So the muscle spindle here is going to pick up this stimulus.

It's going to detect that stimulus, which is a stretch in these muscle fibers. I'm going to carry all of that information through my sensory neuron to the spinal cord where you're going to make sense of it. And then notice there are two.

sets of neurons here. These are both motor neurons because they're both leaving the spinal cord through the ventral root. So these are both motor neurons. Let's focus on the red neuron first.

This red motor neuron, notice that's a plus sign, a positive sign. I'll tell you what that means. This motor neuron is going to carry this information back to my stretched muscle, right?

My muscle of interest. And it's going to bring about a positive effect. And the positive effect in the case of a muscle. when you when your part when it has a positive effect this is a stimulation effect it always brings about contraction so plus always translate to translates to contraction whereas negative inhibition would translate to relaxation so it began with an initial stimulus which is muscle stretch so the muscle fibers are stretched out that you're seeing an increase in the length then the motor neuron from the spinal cord is going to bring about a stimulation signal which is going to bring about contraction or it's going to reduce the length of those fibers so the problem was an increase in length and the response from the spinal cord through this reflex arc would be right the opposite contraction which is going to bring about a decrease in the muscle fiber length okay so that's that's what's going on with the biceps Normally, you've got an antagonistic muscle.

And again, this would be the triceps in this case, if you're considering the biceps on the top. The triceps will go through right the opposite. The motor neuron from the spinal cord to the antagonistic muscle here on the bottom would have the opposite effect, would have a negative effect.

So if the stretched muscle is getting a positive stimulation effect, causing it to contract, at the same time... the spinal cord is going to send a negative inhibition effect to the antagonistic muscle, bringing about relaxation. And that's important because when the biceps are contracting, this is going to bring about flexion, right? Well, if the biceps are flexing or going through the process of flexion, then the antagonistic muscle, the triceps, cannot flex as well at the same time. it has to do right the opposite.

It has to relax, bringing about basically kind of like an extension. Okay. So this is how the stretch reflex works.

I want to point out a few things here. Let me go back here. I want to point out what's monosynaptic, ipsilateral, and reciprocal inhibition.

So let's get back here. Okay. So notice that's my blue sensory neuron connecting or synapsing directly with the red motor neuron. This is called a monosynaptic connection or a monosynaptic terminal in this case, because you see the sensory directly synapsing with the motor, meaning it's just one synapse between these two neurons. But on the right here, you're looking at a sensory connecting to an interneuron, a short interneuron, which then connects to the motor neuron.

So this is one synapse. there and a second synapse down here. So therefore, this would be an example of a polysynaptic pathway. So monosynaptic is when you only have sensory directly connecting with motor neuron and polysynaptic when there's multiple neurons in play.

Now, there could be multiple interneurons as well. It doesn't have to be just one interneuron. What's ipsilateral? Okay, so say this is the stimulus, a stretch signal being transferred or being sensed in the left upper extremity in in your left arm okay so in your left upper limb that's carried to the spinal cord and notice the motor information is also originating from the left side of the spinal cord so the spinal cord is controlling uh the left side of the spinal cord is controlling the left side of the body and therefore this would be an example of ipsilateral if this kind of crossed over so stimulus from the left side of the body is then connecting to the right side of the spinal cord and then the motor output is originating from the right side of the spinal cord then that would be an example of a contralateral type of response but in this case as you can see this is all controlled on the same side of the spinal cord so the sensory and the motor information all of those are occurring on the same side of the spinal cord so therefore this is ipsilateral but if the sensory and the motor are on the opposite side of the spinal cord, it would be contralateral. Okay.

And then coming back to our concept here of agonist and antagonistic muscles. Let me back up. What is reciprocal inhibition? Okay.

Reciprocal inhibition is related to what's going on in the antagonistic muscle. Let me just back up a little bit. The motor output.

to the stretched muscle is positive, namely that is stimulation. But the motor output to the antagonistic muscle is negative, namely inhibition. So that's why this is called reciprocal inhibition. So reciprocal is basically examining what type of response is being brought about in the antagonistic muscle.

In this case, it's a negative response. So therefore, this is reciprocal inhibition. I'll show you in the next scenario. where it's going to actually be a positive signal to the antagonistic muscle.

In that case, it would be an example of basically a reciprocal stimulation or cooperation. Okay, so I think we've wrapped up the stretch reflex. Oh, here's another example of the stretch reflex before we move on. This is a very important example called the knee jerk reflex or the patella reflex.

Okay, so what you're seeing pictured here is your lower limb. Okay, so that's the muscle in the thigh region. That's the quadriceps femoris muscle group and the antagonistic muscle behind it, which is your hamstrings.

So what happens when the quadriceps muscle is stretched out? So the stimulus is an increase in stretch of the quadriceps muscle. This is sensed by the muscle spindle in this muscle. It's going to carry that sensory information, namely increase in stretch, which is an increase in the muscle length.

I'm going to carry that sensory information through the sensory neuron to the spinal cord. And then from here, my motor neuron generated. from the spinal cord is going to send a positive signal, red neuron, positive signal to the stretched muscle, in this case, the stretched quadriceps group, which is going to bring about, since it's positive, positive is always a stimulation and excitation signal, which would bring about contraction in this muscle. Contraction of this quadriceps is going to reduce the stretch, which was the initial problem, okay? Now the antagonistic muscle, namely the hamstrings, through a different motor neuron from the spinal cord, is getting the opposite signal.

So positive for the quadriceps, but negative for the hamstrings, which is the antagonistic muscle. And what happens here with the hamstrings? Because this is negative, this is inhibition.

Therefore, it's going to bring about relaxation of the hamstrings. Because this is a negative signal, being received by the hamstrings, the antagonistic muscle. This is an example of reciprocal inhibition.

Let's look back here. On the left, again, I'm showing you the sensory neuron making a direct synapse with the motor neuron. So therefore, this is a monosynaptic pathway, whereas on the right, sensory neuron to interneuron, which then connects with the motor neuron. So that's a polysynaptic pathway.

Okay. That basically is an example of forgot to mention the response i'm sorry um when uh when the motor neuron sends a positive signal to the quadriceps it brings about contraction of the quadriceps and the main function of the quadriceps when it contracts would be what um basically you're going to kick your leg uh outwards which is basically uh extension occurring at the knee okay that this is the normal knee jerk reflex so when you um use a reflex hammer to basically excite the patellar ligament. When you kind of tap on the patellar ligament, it's going to bring about this whole reflex pathway, which is going to cause basically stretching the quadriceps muscle.

And then you have this entire reflex pathway that's carried out. And if all of these different components, the muscle spindle, the sensory neuron, the spinal cord, the motor neuron, if all of these components were working fine, then the expected response of this knee jerk reflex would be extension at the knee where you kick your leg outwards. And any abnormality or pathology related to any of these components could cause zero response where you don't see any extension, or you may see a distorted response, a slightly distorted response like like an exaggerated response where you kick your foot forward in a more exaggerated manner.

Okay. So that's a stretch reflex, which then brings me to this last part here, which is the tendon reflex. Okay.

So this is also related. This is a somatic reflex as well related to the skeletal muscles. This is right the opposite of a stretch reflex.

Okay. So this is going to kind of shown here that the stimulus here is going to be an increase in tension. So you When you have a muscle that is experiencing a greater amount of tension, which is basically a greater amount of contractile force, meaning this muscle is in a contracted state, okay?

You do not want to damage that muscle by extending out that state of contraction over a long period of time because that would not be good. So therefore, your tendon reflex is going to prevent damage by okay, what happens here in response to the stimulus, which is an increase in tension, the receptor, which is your tendon organ, receives this information, sends it to the spinal cord. And the response in this case is. Remember, the muscle was contracted.

So the response then by negative feedback should be relaxation of the muscle, which is going to reduce your contractile force and reduce your tension. Okay. So that's in essence what's going on in the case of the tendon reflex. And I'm going to talk about reciprocal activation or reciprocal stimulation, however you want to call it. We talked about reciprocal inhibition with respect to the stretch reflex, right?

The opposites happen. here in the case of the tendon reflex so let's use this example right here so a same set of muscles here you've got your quadriceps in the front and then towards the posterior aspect you've got the antagonistic hamstring group so here's my tendon organ in the joint cavity this is going to basically oh in the joint I meant this is going to pick up information related to the stimulus of alpha excessive tension or increased increased force of contraction so when this muscle is in a state of contraction that tension that is developed or that contractile force that's developed is sensed by this receptor the tendon organ same story i'm going to carry the sensory information through the sensory neuron which is my blue neuron to the spinal cord which is my integration center and i'm going to send two outputs as always okay well the red motor output goes back to my muscle of interest The blue motor output goes to the antagonistic muscle. So this muscle is in a state of contraction.

So what's going to happen now is I'm going to send a negative signal to that contracted muscle. Negative means inhibition. Inhibition would bring about relaxation of this contracted muscle, which makes perfect sense, right? You don't want to damage the muscle by making it stay in a prolonged state of contraction.

Now, okay, so my hamstring, I'm sorry, my quadriceps received a negative inhibition signal, which brought about relaxation, which would then mean that the antagonistic muscle group, the hamstrings, would get a positive signal. This is stimulation or excitation, which is going to bring about contraction of the hamstring group. Okay, I want to point out in this case, because your antagonistic muscle is receiving a positive stimulation signal, This is an example of reciprocal activation or reciprocal stimulation. Okay. And then in here, in the spinal cord, these are both examples of, on both sides, they are examples of a polysynaptic reflex or a polysynaptic pathway.

And it's still ipsilateral because sensory information is going in through on the left side of the spinal cord. The motor information is also being generated on the left side of the spinal cord. So that.

is a description of reflexes where we talked about two different types of reflexes, the stretch reflex and the tendon reflex. So the peripheral nervous system chapter is pretty straightforward. I just want you to remember the classification of different receptors, make sure you know what's going on on the sensory end of things, what's going on with the circuit level.

And then motor control using those three different levels of control. And then we moved on to our description of reflexes. So that wraps up my description of the peripheral nervous system.

I hope this made sense to you. This is important for you to kind of integrate what we've already discussed with the central nervous system. Because we're talking about...

the connection or the communication between the CNS and the PNS, and which is really important for you to kind of make that connection between what's going on in the brain and the spinal cord and all of the different peripheral organs in terms of sensory inputs towards the brain and, of course, motor outputs out of the brain, but towards the peripheral nervous system. Hope this made sense to you. Thank you.

Transcript for:Overview of the Peripheral Nervous System

Transcript for:
Overview of the Peripheral Nervous System