Somatic nervous system and sensory receptors

So for those of you who haven't met me on the human physiology module, it's nice to see so many of you here today. My name's Laura. I'm one of the neuroscience lecturers here. And I'm going to be continuing on from where Vera left off. And we're continuing to look at the topic of the nervous system as part of human physiology. And in particular today, we're going to be having a look at the sensory division of the nervous system. So we've got quite a lot to cover today, but we're going to start thinking about how we're actually able to detect a whole range of different sensory stimuli. And then we're going to focus specifically on the somatic sensory information, how this is transmitted and processed by the somatic nervous system. And then finally, we're going to have a little look at the transmission of special sensory information. So we're going to begin then by looking at how we're actually able to detect the sensory stimuli. And this is in the form of having some sensory receptors that's able to detect and respond to a whole range of different types of sensory stimuli. Before I get into a little bit too much detail then, I just wanted to kind of remind you about the organization of the nervous system. So broadly speaking then, the nervous system can be divided into the central nervous system and the peripheral nervous system. And the central nervous system consists of the brain and spinal cord, and its role is mainly in the receiving and the processing of information that's come from the peripheral nervous system. It interprets this information. to produce an appropriate response to that information. The peripheral nervous system then, so this can be divided again into the somatic nervous system and the autonomic nervous system. Now you've been looking at the autonomic nervous system with Vera, so I'm not going to touch on that today, but I am going to be focusing on the somatic nervous system. And the peripheral nervous system as a whole then, the role of this part of the nervous system is to relay that sensory information from... our periphery and our internal environment. into the central nervous system, but also relay motor output from the central nervous system to the target effector organs, which in this case tends to be muscles and all kinds of muscles to cause different responses. Now as well as this kind of easy division of the central and peripheral nervous system, you just want to highlight that there are also the cranial nerves, and some of these are classed as central nervous system and some of these are classed as peripheral nervous system. And we will come across these in a little bit more detail later on at the end. So these play quite an important role in the transmission of that special sensory information. So we're obviously focusing in on the peripheral nervous system then. And like I said, it's all about relaying that sensory information. So we tend to call these sensory pathways afferent pathways. And it's relaying that information into the central nervous system. I see the brain. brain and the spinal cord, and then it's going to relay out the motor output from the central nervous system to the muscles. And this motor output pathway is known as the efferent pathways. And irrespective of the division... then somatic or autonomic overall they're both bringing in sensory information and relaying out motor information but where that sensory information comes from varies and the types of motor responses that they coordinate also differs between the two so the somatic nervous system then in general it's bringing in somatic sensory information from our periphery so from our external environment Whereas the autonomic nervous system is relaying sensory information from our internal environment, from our viscera, so from our organs. For example, blood pressure that's associated with various vessels throughout the body. And then they kind of regulate and mediate these different responses then. The somatic nervous system regulates voluntary responses, and this is because it's connecting that motor output is connected to our skeletal muscle, so it's... mediating response. that we are actively choosing to do, whereas the autonomic nervous system then is all about those automatic, those involuntary responses, and that's controlling our smooth muscle, our cardiac muscles, and our glands. So there's a bit of a difference, but we're focusing specifically on the somatic nervous system today. So there's a whole variety of different senses then and sensory stimuli that are processed and transmitted by the sensory, different sensory divisions of the nervous system. So our somatic senses... then they are transmitted by the somatic nervous system and these stimuli include touch temperature pain and proprioception and proprioception refers to this kind of awareness of our position of our body within space special sense sensory information then this is more transmitted via our cranial nerves and this includes smell also known as olfaction taste hearing balance and vision and finally we've got our visceral sensors these are going to be transmitted by the sensory aspect of our autonomic nervous system and include things like blood pressure internal body temperature things that are coming from our internal environment So irrespective then of which sensory stimulus and which slight division of the nervous system, sensory division of the nervous system, is transmitting this information, there tends to be some commonalities when it comes to the sensory system. comes to the sensory pathway things that they all tend to share so firstly then we need something that is able to actually detect our sensory stimulus this is the form of our sensory receptor that information then about our particular sensory stimulus needs to be converted into a signal that can then be passed along the sensory neuron and into the central nervous system. And when it gets in the central nervous system, that information might be transmitted up to the brain, if it's for our kind of conscious perception that we need to be aware. of that sensory information that's coming in. But sometimes it kind of just remains at the level of the spinal cord if we don't necessarily need that conscious perception. So, for example, sometimes if you put your hand on something really hot, we need to just respond to that very quickly, and we need the sensory information to come into the spinal cord. and the motor information to go immediately out, so we can move our hand away very quickly. We don't want to spend time with that information coming up to the brain, us thinking about it, and then moving our hand. It's a bit too late. So it depends on the information that's coming in, and where it might go to. But there is this kind of commonality of the sensory pathway, and we'll kind of refer to this a lot as we're going through this lecture. So thinking about then the ways in which we're actually able to detect our sensory stimuli, and this involves our sensory receptors. And broadly, there are sort of three main different types. There's lots of different subtypes of each one of these, and they all look a little bit different. But broadly speaking, there are three main types that differ in their morphology. Now, the sensory receptors that detect somatic sensory stimuli, they tend to either be the form of modified or free nerve endings of our sensory neurons. So in this case, we've got example... A here, this is a free nerve ending, whereas example B, this is a modified nerve ending. So what this is then is our sensory neurons, the endings that are innervating maybe the particular area of the skin that is receiving that somatic sensory. information they either free and they're just innovating that area and they might express a particular protein that's able to check that that particular stimulus or they might be slightly modified again so that they're able to detect that specific stimulus But the actual sensory receptor is the sensory neuron itself. It is the terminals at the end that's able to detect that somatosensory information. When it comes to the special sense receptors then, they tend to take the form of an actual receptor cell. So this is more of an individual entity, a particular cell, again expressed in a particular area, that is able to detect that particular special sense. stimulus and then relays that information to the innovating sensory neuron through the release of a neurotransmitter so it's going to pop on the information to our sensory neuron so then it can be transmitted into the central nervous system so like I said then there are many different different subtypes we're going to look at some of these in a little bit more detail and they are activated by specific stimuli and they're located throughout the body but just broadly speaking these are the sort of three different types that we might see Okay, so moving on to the second part then. I've kind of introduced you to our sensory receptors and how we're actually able to detect the stimuli. Now we're going to focus specifically on the transmission and processing of those somatic sensory stimuli. And again, we're going to kind of work our way through the somatosensory pathway, all the way from detecting our somatosensory stimuli, how that information is converted into a signal that can be passed along specific sensory neurons that are involved in the somatosensory nervous system, and where it's then passed into the central nervous system. And we're then going to begin to think about how this information is then relayed on to the motor system. system and the motor neurons which then project out to the skeletal muscle. And we will pick up on this part, this efferent part of the pathway in our next lecture that's coming up next week, I think, on skeletal muscles. So we'll kind of continue it on next week as well. So beginning them thinking about those specific types of those somatosensory receptors, able to detect those particular stimuli. And the first one I want to introduce you to then are our mechanoreceptors. And these, when it comes to our somatics, sensory stimuli I found mostly in the skin and they respond to physical distortion and so when they're found in the skin and we're physically distorting our skin this refers to touch if you press a finger or whatever onto your skin you're physically distorting it and that is what we refer to as touch Now, there are many, many different subtypes. Most of them take on the form of these kind of modified nerve endings, and it allows them to detect different forms of touch. For example, we've got someone's Merkel's disc that responds to static touch, and we've got Piscinian corpuscles that respond to vibration. So the different subtypes bring in different touch information. They also differ on whether they're... expressed in non-hairy and hairy tissue. And they also differ in their threshold. So this is the size of the stimulus that they will respond to. So you can see here we go from low threshold all the way up to very high touch threshold. So the higher the threshold, the more physical distortion that we're needing to apply in order to actually activate these particular receptors. Generally speaking, then, the deeper into the skin these receptors are found, the higher the threshold. This makes sense. The more pressure that you apply to your skin, you know, the harder that particular stimulus is, and therefore we need to be activating receptors that are further down. One other point that's quite important then is that the ones that tend to have the very high thresholds tend to take the form of free nerve endings. And this will crop up again a little bit later on. But those that have the highest thresholds are not seen as modified. They tend to be the free nerve ending versions of these receptors. Now, the mechanoreceptors, then, they also differ in their ability to adapt. And what we mean by this is essentially the ability of the receptors to stop responding in the presence of that specific stimulus. So, for example then, Meissner corpuscle and Piscinian corpuscle, we describe these as being rapidly adapting. So, in this diagram then, we are recording from our sensory neuron. So, we're recording essentially... the action potentials each one of these lines is an action potential and here is when we are applying the stimulus so applying the touch stimulus so when the the line goes down it means we're applying the stimulus keeping the stimulus on and then removing again And you can see then that these two, they respond when we initially apply the stimulus, but then in that constant presence of the stimulus, they stop responding. They stop sending the signal. Their axons stop firing. And then they then fire again on that offset. remove this when we remove the stimulus The other two then, Merkel's disc and Ruffin's end, these are different then. We describe these as being slowly adapting. So you can see that they continue to respond. So we still get these action potentials firing, even in the presence of the stimulus. Now, there's no one way is better than the other. This is just different ways in which information about a specific stimulus is transmitted. And they differ in their ability to adapt. So the second type of sensory receptor then are our thermoreceptors. Again, these are mostly present in skin, and these are responding to temperature. And they tend to take the form of free nerve endings, and they... a particular protein that allows them to detect the specific temperature. And there are a whole range of different proteins that allow you to detect the various different parts of temperature. all the way up from our extreme hot to our stream end, our hot end and our cold end, and everything in between. But a couple of notes that I just kind of wanted to highlight to you, a couple of channels, the TRIPv1 channels, these detect hot temperatures, whereas the TRIPM8 channels, these respond to cold temperatures. So we have these free nerve endings that express the different channels that allow them to detect different temperatures that might be applying. to the skin. So the third type then are our nociceptors, again mostly found in the skin, and these respond to stimuli that have the potential to cause tissue damage, and we call this noxious stimuli, or essentially anything that is painful. And there are, again, a whole variety of different nociceptors that respond to slightly different somatosensory stimuli. but at the extreme ends, at the painful end of those particular stimuli. So for example then, we've got our mechanical nociceptors. We've just come across the mechanoreceptors. These are the mechanical nociceptors. So these ones are going to be responding to those extreme ends of touch, things that feel painful when it refers to touch. And we've got the thermal nociceptors. Again, at the extreme ends of our heart. hot and cold and there's also examples of chemical nociceptors and the nociceptors then these tend to take the form of free nerve endings i mentioned right at the beginning that free nerve endings tend to be the ones with the highest threshold and that is because they tend to be nociceptors so they're able to detect those highest threshold particular stimuli Finally then, we've got our pro-preceptors, and these ones are a little bit different in that these are actually expressed internally, and these are found in muscles, tendons, ligaments, and joints, and these are able to detect... a whole range of stimuli that's able to provide information about the position of the body in space. And again, there are different subtypes. We're going to look at these in a little bit more detail in our skeletal muscle lecture next week. But just to kind of introduce you, we've got our muscle spindles. These are found within the actual muscle itself, and these respond to stretch in the muscle. We've got our Golgi tendon organs. So these are found in the tendons that attach the skeletal muscle to the bone. And these respond to the contraction of the muscle. And then we've also got propyreceptors that are found in the joints itself. And these provide information about the angle, direction, and the velocity of movement in the joint. Okay, so we've seen that there are a variety of different receptors that are able to detect this sensory information. But how is that sensory information, that sensory stimuli, then converted into a signal that can be transmitted along the brain? our axons of our sensory neurons and hopefully you've got from with with Vera that our neurons of our nervous system they are transmitting information in form of electrical signals known as action potentials so we need to get our sensory stimulus, that feeling of touch, into an action potential. How does that happen? And this process is known as sensory transduction. Essentially, again, the slightly different versions of the different receptors will have kind of unique ways of doing this. But broadly speaking then, when we activate our particular sensory receptor, it's going to cause a change, an opening or closing of ion channels. And these ion channels are then going to allow the movement of current, which is then going to change the membrane potential of our particular sensory receptor. So we need our stimulus to be of a specific threshold, so that minimum required stimulus in order to actually activate these particular receptors. But once we reach that particular stimulus threshold, if we apply it, we're going to get a change in the membrane potential of our particular sensory receptor. And this change in the membrane potential of our sensory receptor, we call these receptor potentials. And they are graded. in that if we increase the size of our stimulus, we're going to increase the change in membrane potential. And essentially, if this change in membrane potential at our receptor, this receptor potential, if that change in membrane potential is sufficient enough to then reach the threshold for an action potential, we're then going to get action potentials firing along our sensory neurons. So you can see in this example then, if we apply our particular stimulus, it's at the threshold to cause a change in the membrane potential of our receptor, but it's not sufficient enough to reach the threshold for an action potential, so we don't get any action potentials going along our sensory neurons. We're not transmitting that information into the central nervous system. But as we increase the size of the stimulus, we increase our receptor potential. and that triggers the firing of action potentials. That sensory information is being transmitted into our central nervous system. And as we increase it a bit further, again, the receptor potential gets bigger, but the action potentials don't get bigger, and instead we just get this increased firing frequency. So action potentials don't get any bigger. If we increase the stimulus, we increase the frequency of firing. So we've seen how they kind of convert the sensory stimulus then into our electrical signal, but what about where these actual receptors are able to detect the stimulus? So the area which a specific sensory receptor is actually able to detect its stimulus is called the receptive field. And a really nice way of kind of demonstrating this is by thinking about our mechanoreceptors, so our receptors that are responding to touch, and their ability to essentially distinguish between a stimulus that has two points to it, two sharp points. So here we've got an example of basically using a protractor, and we've got two sharp points to it. And this test is known as the two-point discrimination test. And the reason why this is good at kind of determining the size of a receptive field is because essentially the size of the receptive field and the density of the receptive fields can influence your ability to discriminate between a stimulus and essentially how sensitive you are to that specific stimulus. And thinking about touch is a really good one. So for example then, if we had an area of the skin that was innervated by these mechanoreceptors that had a really large receptive field, so they were covering a large area of the skin, and we applied our stimulus for two points, it is likely that we're actually only going to hit one of our sensory receptors. So that means that those two points will actually appear as one stimulus. So you're likely to say, I can only feel one point. But if we were to apply it to a section of the skin that was actually very highly innervated, by lots of these sensory receptors that had very small receptive fields, and we applied the same stimulus at the same distance apart, it's then more likely that we're going to hit maybe two or more than one of these sensory receptors. And therefore, you're going to get two different signals going into the central nervous system, and you're then going to be able to discriminate between these two points, and you're likely to say, yes, I can feel two points to that. And different areas of the body, particularly when it comes to touch, differ in their ability to discriminate between touch stimuli. So, for example, then, the fingers and the lips, these are very highly discriminative. So they have lots of innovation by sensory receptors that have small receptive fields. Compared to... Our back, our legs, our arms, these are much less discriminative. And this makes total sense because the way that we go about exploring the world is with our hands, and particularly when we're young as babies, is with our lips and our mouths. You'll see babies putting things into their mouths all the time. So we really need our fingers to be really highly sensitive so that we are able to explore our world a lot more effectively. Whereas we don't really go around exploring our world and our environment with our back or our leg or our arm. So we don't need those to be as highly innovated, and we don't need them to be as discriminative. Okay, so we've had a think about our sensory receptors, and we've seen how we go from our stimuli to our electrical signal, and we get this action potential that travels along the sensory neurons. So now we're going to have a little bit more of a look at our sensory neurons. And I said to you then, when it comes to sensory pathways, we call these our afferent neurons. And these first sensory neurons that are transmitting that sensory information from our periphery into the central nervous system We call these the primary afferent neurons. And for the ones that are transmitting our somatosensory information, we call them the primary somatosensory neurons. So like I said then, they're transmitting that information from our... periphery into the central nervous system. So they have their cell bodies just outside the spinal cord, and then they have this really long axon that projects from the area in the periphery, so a particular part of the skin, and it projects all the way into the spinal cord. And it enters via the dorsal root and synapses here in the dorsal horn. So important point, the dorsal side the central nervous system tends to be the sensory side so lots of sensory information comes in via the dorsal side so this this kind of diagram then it's a bit misleading we don't usually just have one sensory neuron and they'll usually be lots of these sensory neurons that are entering at different levels of the spinal cord and so don't just think about it as one single neuron But there are lots of different subtypes of these primary afferent neurons that differ both in their morphology but also in their function. So broadly speaking then, we can divide these primary afferent neurons into A-fibers and the C-fibers. Now A-fibers tend to be much larger in diameter and they're myelinated, whereas the C-fibers, these are much smaller in diameter and they are unmyelinated. So the A fibers then, we can further subdivide these again. So beginning with our A alpha fibers, these are the largest diameter, they're the most myelinated, and they also have the fastest conduction velocity. transmit that information the quickest. And in particular, these are transmitting information about the position of our body in space. So these are transmitting information from the proprioceptors of our skeletal muscle into the central nerve. nervous system our slightly smaller diameter a fibers then are known as the a beta fibers so slightly smaller in diameter slightly less myelin and therefore slightly slower conduction velocity and these are transmitting information about touch from the skin into the central nervous system and finally then our smallest diameter least myelinated slowest conduction a fibers are known as as the a Delta fibers and these play an important role in transmitting information about pain and so any of our noxious our information from our nociceptors and information about temperature as well Finally then, the C fibers. So I said these were the smallest diameter and the non-myelinated ones. So these are also very, very slowly conducting fibers. And in fact, they often get called the pain fibers because they play a very important role in transmitting information about pain, but they also transmit information about temperature as well. So that sensory information then enters into the spinal cord and then can be transmitted up to the brain so that we're able to consciously perceive and process that sensory information. And this occurs via ascending tracks. Ascending is going up the way, so from the spinal cord up to the brain. Now, there are two main tracks that are involved in the somatosensory nervous system, the sensory division of it. and they differ in what information is being transmitted, but there are some commonalities between them. So firstly, they tend to take on a three-neuron pathway. And so we have our first neurons, the one we looked at. They're the primary neurons that transmit that information into the spinal cord. They then synapse with a second-order neuron and then a third-order neuron. Both of them also cross over, so irrespective of the pathway, sensory information that comes from the left side of the body goes to the right-hand side of the brain and vice versa. So they both cross over, just at different points. They also both pass through this part of the brain called the thalamus. This is usually where our third-order neuron is found. And this is an important part of the brain that's involved in the relaying of sensory information. And finally then, the pathway, the sensory pathway, always terminates in a part of the brain that is able to process and interpret that specific sensory information. So when it comes to the somatosensory nervous system then, it's going to terminate in the primary somatosensory cortex. That's where all our somatic stimuli is going to be processed and interpreted. So looking at the first pathway then, so we've got our dorsal column medial lymniscus. pathway. And this is transmitting information about touch and proprioception. So we've got our primary sensory neuron that transmits information up the spinal cord, up to the medulla. Now this is part of the brainstem. And here it synapses with our second-order neuron, and it's here where the crossing over happens. So the axons of our second-order neuron crosses over to the other the side of the brain stem, continues up to the thalamus, where it synapses the third order neuron, all the way up to the primary somatosensory cortex. So the crossing over happens at the level of the brain stem. Our second pathway, then, is known as the spinothalamic pathway, and this is transmitting information about temperature and pain. Now this time, the crossing over occurs much sooner, and so occurs actually at the level at the level of the spinal cord, where our primary afferent neuron synapses with our second-order neuron. And then these second-order neurons have these really long axons that project all the way from the spinal cord up to the thalamus, to the third-order neuron, which then goes to the primary somatosensory cortex. So here, then, that crossing over is happening at the level of the spinal cord. So once that information then gets up to the brain, like I said, it's going to project to a specific area of the brain where that information can be interpreted and processed. And this is known as the somatosensory cortex. Now, this information then, from different parts of the body, actually projects to different parts of the cortex, of the somatosensory cortex. This means we can create a bit of a map... on our cortex, on our primary somatosensory cortex. So here is a section then of our somatosensory cortex. This is just one side of the brain. Remember, we're going to have the opposite side as well, which will have a mirror image of the map. And you can see that we've got different parts of the body kind of mapped onto it. So this is where the information from the lips is going to go. It's going to go to this specific part of the somatosensory cortex. But there are a couple of unique features about this map. So... Firstly, the map is upside down. See, this is the top of the brain, and this is moving our way down the side of the brain towards the bottom of it. You might think then that the top of the body would project to the top of the brain, but it's not. It's the other way around. So information from lower parts of the body go to the top of the somatosensory cortex. The other unique feature is that the area of the cortex that is dedicated to the area of the body, is not equal. And it's actually dependent on the level of innovation that different areas of the body have. So coming back to those receptive fields and our ability to be discriminative and how sensitive we are. So areas of the body that are much more highly sensitive, they are much more highly innovated, get a bigger area of the cortex. And that allows us to create these maps called a homunculus. And this is basically a representation of the the area of the cortex that is dedicated to the different areas of the body. And you can see then again that the lips and the hands have the biggest areas of the cortex. cortex, they are most highly innovated for that sensory innovation. So this makes total sense. Okay, so we've looked at the sensory part then, and I mentioned that that sensory information processed by the central nervous system and then is relayed on to the motor output, the efferent neurons. And we're going to look at this in more detail in our next lecture. but just to kind of highlight and kind of make the link for you. So our sensory neurons then that are found in the spinal cord that are going to be transmitting that motor information, that information about movement. Out from our central nervous system to the muscle to causal contraction, these motor neurons are also called alpha motor neurons, or in the spinal cord, they're called lower motor neurons. So they have their cell bodies in the ventral horn of the spinal cord. So I told you before that dorsal is sensory, so this time ventral is motor. So they have their cell bodies in the ventral horn, and then their axons. project out of the spinal cord, out all the way to the specific muscle that they'll innervate and cause the contraction. And when it comes to the somatic nervous system, the particular muscle that these neurons are innervating are the skeletal muscle. So it's all about voluntary movement, us choosing to produce a particular movement. And that's all about the skeletal muscle. And some really nice examples then of kind of putting that whole pathway together and thinking about the somatic nervous system working as a whole includes some reflexes. And I'm not going to go into these in a huge amount of detail right now, because we'll look at them again in our next. next lecture, but just to kind of make these links. So some really nice examples include the myotactic reflex. This is all about a kind of stretch reflex in the muscle that causes a contraction and the leg to lift. And we've also got the Golgi tendon organ reflex, all about that sensory information from our Golgi tendon organ, one of those proprioceptors, and how that then regulates muscle contraction as a result of receiving that sensory information. Like I said, we can go through this in more detail in the next lecture, but just to make that link. Okay, so it's a bit of a summary. I've kind of added on some of the key bits to this pathway that we first started looking at. So I'll let you look at that through in your own time, but it's a bit of a summary of all the bits that we've spoken about. So before moving on to the final part, I thought as it's kind of a five o'clock lecture on a Tuesday, I thought I'd add in some of my quiz questions a little bit earlier. So for those of you who have not had me before, I do love a quiz question. And I just do it simple. It's all hands up. So I'll just read out the question, read out the answers, give you a second, and then we'll go through. Anybody think it's A? Hands up. Yes, no, that sort of thing. So question one, based on the morphology in this image here, what broad category of sensory receptor is this? So we've got modified nerve ending, free nerve ending, receptor cell, or sensory cell. So hands up for A, B. C and D. Yeah, perfect. So free nerve endings is the sensory neuron itself, and it's these terminals here that are going to express some kind of protein that detects a particular stimulus. So a sensory receptor that detects inoxious, so this is not harmful, not painful, touch, is known as mechanoreceptor, thermoreceptor, nociceptor, or proproceptor. Hands up for A, B, C, and D. Oh, I feel like I was a bit more hesitant on this one. So anything that is not harmful, we know it's not a nociceptor. Nociceptors are getting information about harmful, about painful stimuli. And we spoke about it being touch. This is a mechanical stimulus, so that means it's going to be a mechanoreceptor. Okay, so moving on to the final part of the lecture then, it's just kind of to introduce you more of a sort of a comparison about this kind of the pathway that's involved in transmitting and processing special sense information. We're going to cover four of them. And again, there is this commonality as to the somatic nervous system sensory division. You've got a sensory receptor. It needs to be transmitted or converted into an electrical signal. Then it needs to be passed to the brain. brain this time through the cranial nerve so there's a little bit different and there's kind of the that whole part of it passing via the thalamus to the specific area brain that is then going to process that information. This time, though, it's not the somatosensory nervous system, because this is special sense information, so it will go to different areas of the brain. There are some exceptions to this that we'll kind of talk about as we go through. So quickly beginning with taste then. So we're able to detect the taste of certain foods that we're taking in because the tastants in the different foods that we eat, they dissolve in the saliva of our tongue and then essentially interact with these taste receptor cells. So remember... when it comes to sensory receptors for the special sensors, they tend to take the form of these receptor cells, these individual entities that are able to detect the specific stimuli. So these taste sensors interact with our particular taste receptor cells, and that interaction ultimately leads to the release of a neurotransmitter. In this case, it's serotonin, which then is able to bind to our... primary gustatory neurons. So these are kind of like equivalent to our primary somatosensory neurons. But these are specific for taste information. And that binding of our neurotransmitter is then going to trigger action potentials, which then can travel along these axons. And the axons of our primary gustatory neurons, then, they make up three different types of cranial neurons. nerves. So we've got the facial nerve, we've got the glossopharyngeal nerve, and the vagus nerve. So that information is going to be transmitted along these axons of the nerves, this time straight into the brainstems. There's no information... going to the spinal cord when it comes to the special sensors, because the cranial nerves essentially extend from the brain and the brain stem, so we miss out the spinal cord. So that information is passed to the spinal cord. It's going to synapse with our second-order neuron, which then transmits information up to the thalamus, that key sensory relay center, where it synapses with our third-order neuron that's going to terminate in the gustatory cord. cortex, this particular part of the brain that allows us to interpret and process information about taste. And these receptor cells then, they all respond to slightly different tastants. So that's how we're able to determine the different tastes associated with food. So that's taste. So the second one we're looking at is hearing. So able to detect auditory information, essentially in the form of sound. waves and these enter the inner ear via the ear canal and again these are detected by a particular sensory receptor cell and these are our hair cells and these hair cells express these little cilia on the top and when the sound waves come in causes the bending of these cilia and the bending of the cilia again essentially causes the opening of iron channels these P2X channels, which again allows current to flow in, ions to flow in. We get a change in the membrane potential that results in the release of a neurotransmitter. This time it's glutamate, which then combines those primary auditory neurons, again triggering an action potential that can be sent along the pathway. And these primary auditory neurons then, the axons of these, make up the eighth cranial nerve. No. known as the cochlear nerve, or sometimes the vestibular cochlear nerve. And again, that information is transmitted via the brainstem to the thalamus. It's the specific area of the thalamus, but it's still the thalamus. all the way to the auditory cortex, which is found in the temporal lobes. It's kind of the part of the brain that sits quite close to the ears. And again, this allows us to kind of interpret and process that auditory information. So the third type then is vision. So this time when we're receiving visual information, this is in the form of light, of photons of light. And that information is transmitted through the pupil of the eye and hits the retina. at the back of the eye, and it's the particular sensory receptor cells that are able to detect this particular light known as the photoreceptors. And the photoreceptors, they express a particular protein that is essentially activated by specific wavelengths of light, which again causes a change in the membrane potential, getting a movement of ions, and then that allows A change in essentially the neurotransmitter that's passed on to the next cell. It's a little bit more complicated when it comes to vision in that our photoreceptors are not directly innervated by the primary sensory neuron. The information has to be passed through a few more neurons before it reaches the retinal ganglion cells. And the axons of these retinal ganglion cells make up our second cranial nerve, known as the optic nerve. And again, like the other pathways, this information is passed along via the optic nerve, via the optic tract, again to the thalamus, and then to our third-order neuron that terminates in the visual cortex. This is a part right at the back of the brain known as the occipital lobe. So again, some similarities and differences. Now the final one I want to introduce you to then is olfaction. This one is a little bit different to the rest of them, or the rest of the special senses. So in this case then, this is all about smells and different odorants. in our environment, they dissolve in the olfactory epithelium that essentially lines the nasal cavity. And the different odorants are detected by receptors that are expressed on the old... olfactory sensory neurons themselves. So these are essentially free nerve endings, so they're not the receptor cells like we saw for the other special sensors. These are free nerve endings that express these particular receptors that allow you to detect the different odorants. And again, causes a change in the membrane potential, and the axons of our olfactory sensory neurons form part of our first cranial nerve, known as the olfactory. nerve which essentially forms part of the old factory bulb and again we have a look at this image over here that sensory information then it's going to go from our nose via the old factory tract this time it's going to bypass the thalamus so that information isn't past the thalamus and it's projected to the piriform cortex or the old factory cortex and for processing of that information so slight differences this one we don't have those sensory receptor cells as such and it bypasses the thalamus okay so it's a bit of a summary then i've kind of put together this sort of more compare and contrast um thinking about the the pathways that are involved and the different aspects when it comes to the smash sensory sensory processing and our special sensory process Certainly some similarities and differences, but the old factory has some unique properties to it. So a couple more questions then. So question three, which area of the brain is common to most sensory pathways? Was it the pons, medulla, thalamus, or primary somatosensory cortex? Hands up for A. B. C. And D. Yeah, perfect. So the thalamus is common to most of them. That relay center, apart from when it comes to old factory, kind of bypasses that. Question four. Which of these is not a special sense receptor? Is it the hair cell, the taste receptor cell, the photoreceptor, or the auditory receptor cell? Hands up for A. B, C, and D. Oh, so the answer to which one is not a special sense receptor is D, the auditory receptor cell. These are our hair cells. These are the ones with the little cilia, the bend, when we get the sound waves coming through the ear canal. So thank you very much, and I think I'll see you next week.

And in particular today, we're going to be having a look at the sensory division of the nervous system. So we've got quite a lot to cover today, but we're going to start thinking about how we're actually able to detect a whole range of different sensory stimuli. And then we're going to focus specifically on the somatic sensory information, how this is transmitted and processed by the somatic nervous system. And then finally, we're going to have a little look at the transmission of special sensory information.

So we're going to begin then by looking at how we're actually able to detect the sensory stimuli. And this is in the form of having some sensory receptors that's able to detect and respond to a whole range of different types of sensory stimuli. Before I get into a little bit too much detail then, I just wanted to kind of remind you about the organization of the nervous system.

So broadly speaking then, the nervous system can be divided into the central nervous system and the peripheral nervous system. And the central nervous system consists of the brain and spinal cord, and its role is mainly in the receiving and the processing of information that's come from the peripheral nervous system. It interprets this information.

to produce an appropriate response to that information. The peripheral nervous system then, so this can be divided again into the somatic nervous system and the autonomic nervous system. Now you've been looking at the autonomic nervous system with Vera, so I'm not going to touch on that today, but I am going to be focusing on the somatic nervous system. And the peripheral nervous system as a whole then, the role of this part of the nervous system is to relay that sensory information from... our periphery and our internal environment.

into the central nervous system, but also relay motor output from the central nervous system to the target effector organs, which in this case tends to be muscles and all kinds of muscles to cause different responses. Now as well as this kind of easy division of the central and peripheral nervous system, you just want to highlight that there are also the cranial nerves, and some of these are classed as central nervous system and some of these are classed as peripheral nervous system. And we will come across these in a little bit more detail later on at the end. So these play quite an important role in the transmission of that special sensory information.

So we're obviously focusing in on the peripheral nervous system then. And like I said, it's all about relaying that sensory information. So we tend to call these sensory pathways afferent pathways.

And it's relaying that information into the central nervous system. I see the brain. brain and the spinal cord, and then it's going to relay out the motor output from the central nervous system to the muscles. And this motor output pathway is known as the efferent pathways.

And irrespective of the division... then somatic or autonomic overall they're both bringing in sensory information and relaying out motor information but where that sensory information comes from varies and the types of motor responses that they coordinate also differs between the two so the somatic nervous system then in general it's bringing in somatic sensory information from our periphery so from our external environment Whereas the autonomic nervous system is relaying sensory information from our internal environment, from our viscera, so from our organs. For example, blood pressure that's associated with various vessels throughout the body. And then they kind of regulate and mediate these different responses then. The somatic nervous system regulates voluntary responses, and this is because it's connecting that motor output is connected to our skeletal muscle, so it's...

mediating response. that we are actively choosing to do, whereas the autonomic nervous system then is all about those automatic, those involuntary responses, and that's controlling our smooth muscle, our cardiac muscles, and our glands. So there's a bit of a difference, but we're focusing specifically on the somatic nervous system today. So there's a whole variety of different senses then and sensory stimuli that are processed and transmitted by the sensory, different sensory divisions of the nervous system. So our somatic senses...

then they are transmitted by the somatic nervous system and these stimuli include touch temperature pain and proprioception and proprioception refers to this kind of awareness of our position of our body within space special sense sensory information then this is more transmitted via our cranial nerves and this includes smell also known as olfaction taste hearing balance and vision and finally we've got our visceral sensors these are going to be transmitted by the sensory aspect of our autonomic nervous system and include things like blood pressure internal body temperature things that are coming from our internal environment So irrespective then of which sensory stimulus and which slight division of the nervous system, sensory division of the nervous system, is transmitting this information, there tends to be some commonalities when it comes to the sensory system. comes to the sensory pathway things that they all tend to share so firstly then we need something that is able to actually detect our sensory stimulus this is the form of our sensory receptor that information then about our particular sensory stimulus needs to be converted into a signal that can then be passed along the sensory neuron and into the central nervous system. And when it gets in the central nervous system, that information might be transmitted up to the brain, if it's for our kind of conscious perception that we need to be aware. of that sensory information that's coming in.

But sometimes it kind of just remains at the level of the spinal cord if we don't necessarily need that conscious perception. So, for example, sometimes if you put your hand on something really hot, we need to just respond to that very quickly, and we need the sensory information to come into the spinal cord. and the motor information to go immediately out, so we can move our hand away very quickly.

We don't want to spend time with that information coming up to the brain, us thinking about it, and then moving our hand. It's a bit too late. So it depends on the information that's coming in, and where it might go to. But there is this kind of commonality of the sensory pathway, and we'll kind of refer to this a lot as we're going through this lecture. So thinking about then the ways in which we're actually able to detect our sensory stimuli, and this involves our sensory receptors.

And broadly, there are sort of three main different types. There's lots of different subtypes of each one of these, and they all look a little bit different. But broadly speaking, there are three main types that differ in their morphology.

Now, the sensory receptors that detect somatic sensory stimuli, they tend to either be the form of modified or free nerve endings of our sensory neurons. So in this case, we've got example... A here, this is a free nerve ending, whereas example B, this is a modified nerve ending.

So what this is then is our sensory neurons, the endings that are innervating maybe the particular area of the skin that is receiving that somatic sensory. information they either free and they're just innovating that area and they might express a particular protein that's able to check that that particular stimulus or they might be slightly modified again so that they're able to detect that specific stimulus But the actual sensory receptor is the sensory neuron itself. It is the terminals at the end that's able to detect that somatosensory information.

When it comes to the special sense receptors then, they tend to take the form of an actual receptor cell. So this is more of an individual entity, a particular cell, again expressed in a particular area, that is able to detect that particular special sense. stimulus and then relays that information to the innovating sensory neuron through the release of a neurotransmitter so it's going to pop on the information to our sensory neuron so then it can be transmitted into the central nervous system so like I said then there are many different different subtypes we're going to look at some of these in a little bit more detail and they are activated by specific stimuli and they're located throughout the body but just broadly speaking these are the sort of three different types that we might see Okay, so moving on to the second part then. I've kind of introduced you to our sensory receptors and how we're actually able to detect the stimuli.

Now we're going to focus specifically on the transmission and processing of those somatic sensory stimuli. And again, we're going to kind of work our way through the somatosensory pathway, all the way from detecting our somatosensory stimuli, how that information is converted into a signal that can be passed along specific sensory neurons that are involved in the somatosensory nervous system, and where it's then passed into the central nervous system. And we're then going to begin to think about how this information is then relayed on to the motor system. system and the motor neurons which then project out to the skeletal muscle. And we will pick up on this part, this efferent part of the pathway in our next lecture that's coming up next week, I think, on skeletal muscles.

So we'll kind of continue it on next week as well. So beginning them thinking about those specific types of those somatosensory receptors, able to detect those particular stimuli. And the first one I want to introduce you to then are our mechanoreceptors. And these, when it comes to our somatics, sensory stimuli I found mostly in the skin and they respond to physical distortion and so when they're found in the skin and we're physically distorting our skin this refers to touch if you press a finger or whatever onto your skin you're physically distorting it and that is what we refer to as touch Now, there are many, many different subtypes.

Most of them take on the form of these kind of modified nerve endings, and it allows them to detect different forms of touch. For example, we've got someone's Merkel's disc that responds to static touch, and we've got Piscinian corpuscles that respond to vibration. So the different subtypes bring in different touch information. They also differ on whether they're... expressed in non-hairy and hairy tissue.

And they also differ in their threshold. So this is the size of the stimulus that they will respond to. So you can see here we go from low threshold all the way up to very high touch threshold.

So the higher the threshold, the more physical distortion that we're needing to apply in order to actually activate these particular receptors. Generally speaking, then, the deeper into the skin these receptors are found, the higher the threshold. This makes sense.

The more pressure that you apply to your skin, you know, the harder that particular stimulus is, and therefore we need to be activating receptors that are further down. One other point that's quite important then is that the ones that tend to have the very high thresholds tend to take the form of free nerve endings. And this will crop up again a little bit later on.

But those that have the highest thresholds are not seen as modified. They tend to be the free nerve ending versions of these receptors. Now, the mechanoreceptors, then, they also differ in their ability to adapt. And what we mean by this is essentially the ability of the receptors to stop responding in the presence of that specific stimulus.

So, for example then, Meissner corpuscle and Piscinian corpuscle, we describe these as being rapidly adapting. So, in this diagram then, we are recording from our sensory neuron. So, we're recording essentially... the action potentials each one of these lines is an action potential and here is when we are applying the stimulus so applying the touch stimulus so when the the line goes down it means we're applying the stimulus keeping the stimulus on and then removing again And you can see then that these two, they respond when we initially apply the stimulus, but then in that constant presence of the stimulus, they stop responding.

They stop sending the signal. Their axons stop firing. And then they then fire again on that offset. remove this when we remove the stimulus The other two then, Merkel's disc and Ruffin's end, these are different then.

We describe these as being slowly adapting. So you can see that they continue to respond. So we still get these action potentials firing, even in the presence of the stimulus.

Now, there's no one way is better than the other. This is just different ways in which information about a specific stimulus is transmitted. And they differ in their ability to adapt.

So the second type of sensory receptor then are our thermoreceptors. Again, these are mostly present in skin, and these are responding to temperature. And they tend to take the form of free nerve endings, and they...

a particular protein that allows them to detect the specific temperature. And there are a whole range of different proteins that allow you to detect the various different parts of temperature. all the way up from our extreme hot to our stream end, our hot end and our cold end, and everything in between.

But a couple of notes that I just kind of wanted to highlight to you, a couple of channels, the TRIPv1 channels, these detect hot temperatures, whereas the TRIPM8 channels, these respond to cold temperatures. So we have these free nerve endings that express the different channels that allow them to detect different temperatures that might be applying. to the skin.

So the third type then are our nociceptors, again mostly found in the skin, and these respond to stimuli that have the potential to cause tissue damage, and we call this noxious stimuli, or essentially anything that is painful. And there are, again, a whole variety of different nociceptors that respond to slightly different somatosensory stimuli. but at the extreme ends, at the painful end of those particular stimuli.

So for example then, we've got our mechanical nociceptors. We've just come across the mechanoreceptors. These are the mechanical nociceptors.

So these ones are going to be responding to those extreme ends of touch, things that feel painful when it refers to touch. And we've got the thermal nociceptors. Again, at the extreme ends of our heart.

hot and cold and there's also examples of chemical nociceptors and the nociceptors then these tend to take the form of free nerve endings i mentioned right at the beginning that free nerve endings tend to be the ones with the highest threshold and that is because they tend to be nociceptors so they're able to detect those highest threshold particular stimuli Finally then, we've got our pro-preceptors, and these ones are a little bit different in that these are actually expressed internally, and these are found in muscles, tendons, ligaments, and joints, and these are able to detect... a whole range of stimuli that's able to provide information about the position of the body in space. And again, there are different subtypes. We're going to look at these in a little bit more detail in our skeletal muscle lecture next week.

But just to kind of introduce you, we've got our muscle spindles. These are found within the actual muscle itself, and these respond to stretch in the muscle. We've got our Golgi tendon organs. So these are found in the tendons that attach the skeletal muscle to the bone. And these respond to the contraction of the muscle.

And then we've also got propyreceptors that are found in the joints itself. And these provide information about the angle, direction, and the velocity of movement in the joint. Okay, so we've seen that there are a variety of different receptors that are able to detect this sensory information.

But how is that sensory information, that sensory stimuli, then converted into a signal that can be transmitted along the brain? our axons of our sensory neurons and hopefully you've got from with with Vera that our neurons of our nervous system they are transmitting information in form of electrical signals known as action potentials so we need to get our sensory stimulus, that feeling of touch, into an action potential. How does that happen? And this process is known as sensory transduction.

Essentially, again, the slightly different versions of the different receptors will have kind of unique ways of doing this. But broadly speaking then, when we activate our particular sensory receptor, it's going to cause a change, an opening or closing of ion channels. And these ion channels are then going to allow the movement of current, which is then going to change the membrane potential of our particular sensory receptor.

So we need our stimulus to be of a specific threshold, so that minimum required stimulus in order to actually activate these particular receptors. But once we reach that particular stimulus threshold, if we apply it, we're going to get a change in the membrane potential of our particular sensory receptor. And this change in the membrane potential of our sensory receptor, we call these receptor potentials.

And they are graded. in that if we increase the size of our stimulus, we're going to increase the change in membrane potential. And essentially, if this change in membrane potential at our receptor, this receptor potential, if that change in membrane potential is sufficient enough to then reach the threshold for an action potential, we're then going to get action potentials firing along our sensory neurons. So you can see in this example then, if we apply our particular stimulus, it's at the threshold to cause a change in the membrane potential of our receptor, but it's not sufficient enough to reach the threshold for an action potential, so we don't get any action potentials going along our sensory neurons.

We're not transmitting that information into the central nervous system. But as we increase the size of the stimulus, we increase our receptor potential. and that triggers the firing of action potentials. That sensory information is being transmitted into our central nervous system.

And as we increase it a bit further, again, the receptor potential gets bigger, but the action potentials don't get bigger, and instead we just get this increased firing frequency. So action potentials don't get any bigger. If we increase the stimulus, we increase the frequency of firing.

So we've seen how they kind of convert the sensory stimulus then into our electrical signal, but what about where these actual receptors are able to detect the stimulus? So the area which a specific sensory receptor is actually able to detect its stimulus is called the receptive field. And a really nice way of kind of demonstrating this is by thinking about our mechanoreceptors, so our receptors that are responding to touch, and their ability to essentially distinguish between a stimulus that has two points to it, two sharp points. So here we've got an example of basically using a protractor, and we've got two sharp points to it. And this test is known as the two-point discrimination test.

And the reason why this is good at kind of determining the size of a receptive field is because essentially the size of the receptive field and the density of the receptive fields can influence your ability to discriminate between a stimulus and essentially how sensitive you are to that specific stimulus. And thinking about touch is a really good one. So for example then, if we had an area of the skin that was innervated by these mechanoreceptors that had a really large receptive field, so they were covering a large area of the skin, and we applied our stimulus for two points, it is likely that we're actually only going to hit one of our sensory receptors.

So that means that those two points will actually appear as one stimulus. So you're likely to say, I can only feel one point. But if we were to apply it to a section of the skin that was actually very highly innervated, by lots of these sensory receptors that had very small receptive fields, and we applied the same stimulus at the same distance apart, it's then more likely that we're going to hit maybe two or more than one of these sensory receptors. And therefore, you're going to get two different signals going into the central nervous system, and you're then going to be able to discriminate between these two points, and you're likely to say, yes, I can feel two points to that. And different areas of the body, particularly when it comes to touch, differ in their ability to discriminate between touch stimuli.

So, for example, then, the fingers and the lips, these are very highly discriminative. So they have lots of innovation by sensory receptors that have small receptive fields. Compared to...

Our back, our legs, our arms, these are much less discriminative. And this makes total sense because the way that we go about exploring the world is with our hands, and particularly when we're young as babies, is with our lips and our mouths. You'll see babies putting things into their mouths all the time.

So we really need our fingers to be really highly sensitive so that we are able to explore our world a lot more effectively. Whereas we don't really go around exploring our world and our environment with our back or our leg or our arm. So we don't need those to be as highly innovated, and we don't need them to be as discriminative.

Okay, so we've had a think about our sensory receptors, and we've seen how we go from our stimuli to our electrical signal, and we get this action potential that travels along the sensory neurons. So now we're going to have a little bit more of a look at our sensory neurons. And I said to you then, when it comes to sensory pathways, we call these our afferent neurons. And these first sensory neurons that are transmitting that sensory information from our periphery into the central nervous system We call these the primary afferent neurons. And for the ones that are transmitting our somatosensory information, we call them the primary somatosensory neurons.

So like I said then, they're transmitting that information from our... periphery into the central nervous system. So they have their cell bodies just outside the spinal cord, and then they have this really long axon that projects from the area in the periphery, so a particular part of the skin, and it projects all the way into the spinal cord.

And it enters via the dorsal root and synapses here in the dorsal horn. So important point, the dorsal side the central nervous system tends to be the sensory side so lots of sensory information comes in via the dorsal side so this this kind of diagram then it's a bit misleading we don't usually just have one sensory neuron and they'll usually be lots of these sensory neurons that are entering at different levels of the spinal cord and so don't just think about it as one single neuron But there are lots of different subtypes of these primary afferent neurons that differ both in their morphology but also in their function. So broadly speaking then, we can divide these primary afferent neurons into A-fibers and the C-fibers. Now A-fibers tend to be much larger in diameter and they're myelinated, whereas the C-fibers, these are much smaller in diameter and they are unmyelinated. So the A fibers then, we can further subdivide these again.

So beginning with our A alpha fibers, these are the largest diameter, they're the most myelinated, and they also have the fastest conduction velocity. transmit that information the quickest. And in particular, these are transmitting information about the position of our body in space.

So these are transmitting information from the proprioceptors of our skeletal muscle into the central nerve. nervous system our slightly smaller diameter a fibers then are known as the a beta fibers so slightly smaller in diameter slightly less myelin and therefore slightly slower conduction velocity and these are transmitting information about touch from the skin into the central nervous system and finally then our smallest diameter least myelinated slowest conduction a fibers are known as as the a Delta fibers and these play an important role in transmitting information about pain and so any of our noxious our information from our nociceptors and information about temperature as well Finally then, the C fibers. So I said these were the smallest diameter and the non-myelinated ones. So these are also very, very slowly conducting fibers. And in fact, they often get called the pain fibers because they play a very important role in transmitting information about pain, but they also transmit information about temperature as well.

So that sensory information then enters into the spinal cord and then can be transmitted up to the brain so that we're able to consciously perceive and process that sensory information. And this occurs via ascending tracks. Ascending is going up the way, so from the spinal cord up to the brain.

Now, there are two main tracks that are involved in the somatosensory nervous system, the sensory division of it. and they differ in what information is being transmitted, but there are some commonalities between them. So firstly, they tend to take on a three-neuron pathway. And so we have our first neurons, the one we looked at.

They're the primary neurons that transmit that information into the spinal cord. They then synapse with a second-order neuron and then a third-order neuron. Both of them also cross over, so irrespective of the pathway, sensory information that comes from the left side of the body goes to the right-hand side of the brain and vice versa. So they both cross over, just at different points. They also both pass through this part of the brain called the thalamus.

This is usually where our third-order neuron is found. And this is an important part of the brain that's involved in the relaying of sensory information. And finally then, the pathway, the sensory pathway, always terminates in a part of the brain that is able to process and interpret that specific sensory information. So when it comes to the somatosensory nervous system then, it's going to terminate in the primary somatosensory cortex.

That's where all our somatic stimuli is going to be processed and interpreted. So looking at the first pathway then, so we've got our dorsal column medial lymniscus. pathway. And this is transmitting information about touch and proprioception.

So we've got our primary sensory neuron that transmits information up the spinal cord, up to the medulla. Now this is part of the brainstem. And here it synapses with our second-order neuron, and it's here where the crossing over happens. So the axons of our second-order neuron crosses over to the other the side of the brain stem, continues up to the thalamus, where it synapses the third order neuron, all the way up to the primary somatosensory cortex. So the crossing over happens at the level of the brain stem.

Our second pathway, then, is known as the spinothalamic pathway, and this is transmitting information about temperature and pain. Now this time, the crossing over occurs much sooner, and so occurs actually at the level at the level of the spinal cord, where our primary afferent neuron synapses with our second-order neuron. And then these second-order neurons have these really long axons that project all the way from the spinal cord up to the thalamus, to the third-order neuron, which then goes to the primary somatosensory cortex. So here, then, that crossing over is happening at the level of the spinal cord.

So once that information then gets up to the brain, like I said, it's going to project to a specific area of the brain where that information can be interpreted and processed. And this is known as the somatosensory cortex. Now, this information then, from different parts of the body, actually projects to different parts of the cortex, of the somatosensory cortex.

This means we can create a bit of a map... on our cortex, on our primary somatosensory cortex. So here is a section then of our somatosensory cortex. This is just one side of the brain. Remember, we're going to have the opposite side as well, which will have a mirror image of the map.

And you can see that we've got different parts of the body kind of mapped onto it. So this is where the information from the lips is going to go. It's going to go to this specific part of the somatosensory cortex.

But there are a couple of unique features about this map. So... Firstly, the map is upside down. See, this is the top of the brain, and this is moving our way down the side of the brain towards the bottom of it.

You might think then that the top of the body would project to the top of the brain, but it's not. It's the other way around. So information from lower parts of the body go to the top of the somatosensory cortex.

The other unique feature is that the area of the cortex that is dedicated to the area of the body, is not equal. And it's actually dependent on the level of innovation that different areas of the body have. So coming back to those receptive fields and our ability to be discriminative and how sensitive we are.

So areas of the body that are much more highly sensitive, they are much more highly innovated, get a bigger area of the cortex. And that allows us to create these maps called a homunculus. And this is basically a representation of the the area of the cortex that is dedicated to the different areas of the body. And you can see then again that the lips and the hands have the biggest areas of the cortex. cortex, they are most highly innovated for that sensory innovation.

So this makes total sense. Okay, so we've looked at the sensory part then, and I mentioned that that sensory information processed by the central nervous system and then is relayed on to the motor output, the efferent neurons. And we're going to look at this in more detail in our next lecture.

but just to kind of highlight and kind of make the link for you. So our sensory neurons then that are found in the spinal cord that are going to be transmitting that motor information, that information about movement. Out from our central nervous system to the muscle to causal contraction, these motor neurons are also called alpha motor neurons, or in the spinal cord, they're called lower motor neurons. So they have their cell bodies in the ventral horn of the spinal cord. So I told you before that dorsal is sensory, so this time ventral is motor.

So they have their cell bodies in the ventral horn, and then their axons. project out of the spinal cord, out all the way to the specific muscle that they'll innervate and cause the contraction. And when it comes to the somatic nervous system, the particular muscle that these neurons are innervating are the skeletal muscle. So it's all about voluntary movement, us choosing to produce a particular movement.

And that's all about the skeletal muscle. And some really nice examples then of kind of putting that whole pathway together and thinking about the somatic nervous system working as a whole includes some reflexes. And I'm not going to go into these in a huge amount of detail right now, because we'll look at them again in our next.

next lecture, but just to kind of make these links. So some really nice examples include the myotactic reflex. This is all about a kind of stretch reflex in the muscle that causes a contraction and the leg to lift. And we've also got the Golgi tendon organ reflex, all about that sensory information from our Golgi tendon organ, one of those proprioceptors, and how that then regulates muscle contraction as a result of receiving that sensory information. Like I said, we can go through this in more detail in the next lecture, but just to make that link.

Okay, so it's a bit of a summary. I've kind of added on some of the key bits to this pathway that we first started looking at. So I'll let you look at that through in your own time, but it's a bit of a summary of all the bits that we've spoken about.

So before moving on to the final part, I thought as it's kind of a five o'clock lecture on a Tuesday, I thought I'd add in some of my quiz questions a little bit earlier. So for those of you who have not had me before, I do love a quiz question. And I just do it simple.

It's all hands up. So I'll just read out the question, read out the answers, give you a second, and then we'll go through. Anybody think it's A? Hands up. Yes, no, that sort of thing.

So question one, based on the morphology in this image here, what broad category of sensory receptor is this? So we've got modified nerve ending, free nerve ending, receptor cell, or sensory cell. So hands up for A, B. C and D. Yeah, perfect.

So free nerve endings is the sensory neuron itself, and it's these terminals here that are going to express some kind of protein that detects a particular stimulus. So a sensory receptor that detects inoxious, so this is not harmful, not painful, touch, is known as mechanoreceptor, thermoreceptor, nociceptor, or proproceptor. Hands up for A, B, C, and D. Oh, I feel like I was a bit more hesitant on this one. So anything that is not harmful, we know it's not a nociceptor.

Nociceptors are getting information about harmful, about painful stimuli. And we spoke about it being touch. This is a mechanical stimulus, so that means it's going to be a mechanoreceptor. Okay, so moving on to the final part of the lecture then, it's just kind of to introduce you more of a sort of a comparison about this kind of the pathway that's involved in transmitting and processing special sense information. We're going to cover four of them.

And again, there is this commonality as to the somatic nervous system sensory division. You've got a sensory receptor. It needs to be transmitted or converted into an electrical signal. Then it needs to be passed to the brain. brain this time through the cranial nerve so there's a little bit different and there's kind of the that whole part of it passing via the thalamus to the specific area brain that is then going to process that information.

This time, though, it's not the somatosensory nervous system, because this is special sense information, so it will go to different areas of the brain. There are some exceptions to this that we'll kind of talk about as we go through. So quickly beginning with taste then. So we're able to detect the taste of certain foods that we're taking in because the tastants in the different foods that we eat, they dissolve in the saliva of our tongue and then essentially interact with these taste receptor cells.

So remember... when it comes to sensory receptors for the special sensors, they tend to take the form of these receptor cells, these individual entities that are able to detect the specific stimuli. So these taste sensors interact with our particular taste receptor cells, and that interaction ultimately leads to the release of a neurotransmitter.

In this case, it's serotonin, which then is able to bind to our... primary gustatory neurons. So these are kind of like equivalent to our primary somatosensory neurons.

But these are specific for taste information. And that binding of our neurotransmitter is then going to trigger action potentials, which then can travel along these axons. And the axons of our primary gustatory neurons, then, they make up three different types of cranial neurons. nerves. So we've got the facial nerve, we've got the glossopharyngeal nerve, and the vagus nerve.

So that information is going to be transmitted along these axons of the nerves, this time straight into the brainstems. There's no information... going to the spinal cord when it comes to the special sensors, because the cranial nerves essentially extend from the brain and the brain stem, so we miss out the spinal cord.

So that information is passed to the spinal cord. It's going to synapse with our second-order neuron, which then transmits information up to the thalamus, that key sensory relay center, where it synapses with our third-order neuron that's going to terminate in the gustatory cord. cortex, this particular part of the brain that allows us to interpret and process information about taste. And these receptor cells then, they all respond to slightly different tastants.

So that's how we're able to determine the different tastes associated with food. So that's taste. So the second one we're looking at is hearing.

So able to detect auditory information, essentially in the form of sound. waves and these enter the inner ear via the ear canal and again these are detected by a particular sensory receptor cell and these are our hair cells and these hair cells express these little cilia on the top and when the sound waves come in causes the bending of these cilia and the bending of the cilia again essentially causes the opening of iron channels these P2X channels, which again allows current to flow in, ions to flow in. We get a change in the membrane potential that results in the release of a neurotransmitter.

This time it's glutamate, which then combines those primary auditory neurons, again triggering an action potential that can be sent along the pathway. And these primary auditory neurons then, the axons of these, make up the eighth cranial nerve. No. known as the cochlear nerve, or sometimes the vestibular cochlear nerve. And again, that information is transmitted via the brainstem to the thalamus. It's the specific area of the thalamus, but it's still the thalamus.

all the way to the auditory cortex, which is found in the temporal lobes. It's kind of the part of the brain that sits quite close to the ears. And again, this allows us to kind of interpret and process that auditory information.

So the third type then is vision. So this time when we're receiving visual information, this is in the form of light, of photons of light. And that information is transmitted through the pupil of the eye and hits the retina.

at the back of the eye, and it's the particular sensory receptor cells that are able to detect this particular light known as the photoreceptors. And the photoreceptors, they express a particular protein that is essentially activated by specific wavelengths of light, which again causes a change in the membrane potential, getting a movement of ions, and then that allows A change in essentially the neurotransmitter that's passed on to the next cell. It's a little bit more complicated when it comes to vision in that our photoreceptors are not directly innervated by the primary sensory neuron.

The information has to be passed through a few more neurons before it reaches the retinal ganglion cells. And the axons of these retinal ganglion cells make up our second cranial nerve, known as the optic nerve. And again, like the other pathways, this information is passed along via the optic nerve, via the optic tract, again to the thalamus, and then to our third-order neuron that terminates in the visual cortex. This is a part right at the back of the brain known as the occipital lobe. So again, some similarities and differences.

Now the final one I want to introduce you to then is olfaction. This one is a little bit different to the rest of them, or the rest of the special senses. So in this case then, this is all about smells and different odorants.

in our environment, they dissolve in the olfactory epithelium that essentially lines the nasal cavity. And the different odorants are detected by receptors that are expressed on the old... olfactory sensory neurons themselves. So these are essentially free nerve endings, so they're not the receptor cells like we saw for the other special sensors. These are free nerve endings that express these particular receptors that allow you to detect the different odorants.

And again, causes a change in the membrane potential, and the axons of our olfactory sensory neurons form part of our first cranial nerve, known as the olfactory. nerve which essentially forms part of the old factory bulb and again we have a look at this image over here that sensory information then it's going to go from our nose via the old factory tract this time it's going to bypass the thalamus so that information isn't past the thalamus and it's projected to the piriform cortex or the old factory cortex and for processing of that information so slight differences this one we don't have those sensory receptor cells as such and it bypasses the thalamus okay so it's a bit of a summary then i've kind of put together this sort of more compare and contrast um thinking about the the pathways that are involved and the different aspects when it comes to the smash sensory sensory processing and our special sensory process Certainly some similarities and differences, but the old factory has some unique properties to it. So a couple more questions then.

So question three, which area of the brain is common to most sensory pathways? Was it the pons, medulla, thalamus, or primary somatosensory cortex? Hands up for A.

B. C. And D. Yeah, perfect. So the thalamus is common to most of them. That relay center, apart from when it comes to old factory, kind of bypasses that.

Question four. Which of these is not a special sense receptor? Is it the hair cell, the taste receptor cell, the photoreceptor, or the auditory receptor cell? Hands up for A. B, C, and D.

Oh, so the answer to which one is not a special sense receptor is D, the auditory receptor cell. These are our hair cells. These are the ones with the little cilia, the bend, when we get the sound waves coming through the ear canal. So thank you very much, and I think I'll see you next week.

Transcript for:Somatic nervous system and sensory receptors

Transcript for:
Somatic nervous system and sensory receptors