Overview of Nervous System Fundamentals

Chapter 11, Fundamentals of the Nervous System and Nervous Tissue. Today we're going to be discussing the divisions of the nervous system. So here we have what's known as the nervous system tree, which is essentially a branching diagram that allows us to classify the nervous system. We're first going to classify the nervous system structurally. So these are the two. structural divisions of the nervous system. The central nervous system is the division comprised of the centrally located brain and spinal cord, which are situated centrally or along the midline. The other half of the structural division is known as the peripheral nervous system, which is exclusively comprised of nerves. Nerves are situated beyond or outside of the spinal cord, hence the term peripheral. If you consider the concept of peripheral vision, it's what you can see along the edges of your vision. So the peripheral nervous system is comprised of nerves that exist or are located beyond or outside of the brain and spinal cord. So here, We have two sets of nerves. One is known as the cranial nerves. They emerge from the base of their brain, so they're physically situated beyond or outside the brain. You have a second set of nerves known as the spinal nerves, and those emerge from either side of the spinal cord, from left and right sides of the spinal cord, and they too exist beyond or outside of the spinal cord. Hence, they are both part of the peripheral nervous system because they're situated outside of the brain and spinal cord. We want to consider that the nerves are a two-way delivery system for electrical messages called nerve impulses, right? Nerves are a two-way highway or delivery system, okay? They conduct impulses towards the brain and spinal cord. along the incoming direction, but they also conduct impulses away from the brain and spinal cord along the outgoing direction as well. So we can classify our peripheral nervous system into two broad systems based on function. So nerves that function in conducting impulses towards the brain and spinal cord along the incoming direction we refer to that as the sensory division of your PNS. Nerves that are responsible for conducting messages away from the brain and spinal cord along the outgoing direction is known as the motor division of the PNS. So your sensory division is alternatively known as your afferent division. So afferent. which translates to incoming, right? Think of arriving, A for arriving. So this is in a nutshell, our sensory division is our input system, right? That provides us with the means to acquire information. This is how we acquire information from our environment. And this input system also allows us to interpret and process that incoming sensory information so we can make sense of it. Okay, so your sensory division is what's known as your senses, right? So think of your special senses, sense of sight, smell, taste. Now your motor division, alternatively known as your efferent division, so efferent in this context. translates to outgoing or efferent e for exiting. Motor division provides us with a means to respond to that incoming sensory info. We can respond to incoming sensory info through movement. Right. Through in the form of muscle contractions. Right. So, for instance, if the stimuli were heat or pain or pressure, we can respond by moving a body part essentially away from the source of the pain and the object inflicting the pain. Right. So if we sustained, let's say, a prick or thumbtack to the skin. We can respond by ultimately flinching our hand away, jerking our hand back and contracting the muscles in our limbs. So this is the part of your motor division that's controlled by your somatic nervous system. Skeletal muscle manipulation is coordinated by a branch of your motor division called the somatic nervous system. Okay, so soma actually translates to body. So if you think about the division of your nervous system that moves the body, right, through skeletal muscle manipulation, that's all. coordinated by a branch of your nervous system called the somatic nervous system. So the target organ or the target effector of the somatic nervous system will always be skeletal muscle. Skeletal muscle is the only muscle type under voluntary control. Okay, we do want to consider. In the context of reflexes, okay, where skeletal muscle is involved, for instance, the knee jerk reflex, or the reflex of again, jerking your hand back when you're when you're exposed to heat, for instance, or pain. This is by and large involuntary, right? So there is an exception to the rule. And so the exception is that skeletal muscle can be involuntary when it comes to reflexes. But outside of reflexes, yes, skeletal muscle is under your complete voluntary control where you have to will something to happen. You have to consciously will it to happen. That is true. But skeletal muscle can be involuntary. with respect to reflexes. So when we're flinching back or jerking our hand back from pain stimuli or pressure stimuli, that is completely under involuntary control. We did not have time to react and think about it and consciously will it to happen. So that's something to consider as well. That yes, skeletal muscle is largely under our voluntary control, but skeletal muscle can be involuntary when it comes to reflexes. So in this particular example, let's talk about how we can respond to incoming sensory info through voluntary and involuntary skeletal muscle contraction. Okay, so So, um, The incoming sensory stimuli could be, for instance, an object that you see, let's say a glass of water, right? We can respond through voluntary means, right, through voluntary muscle manipulation by consciously picking up the glass of water, right? Contracting our skeletal muscles in the arm to pick up the glass of water, right? So that would be an example of your somatic nervous system coordinating a response. to sensory stimuli that is completely voluntary. You willed it to happen, right? So you process and detected a glass of water, you're responding, right? That's your sensory stimuli, that's your sensory info, and you're responding. voluntarily by moving your arm to pick up the glass of water. Now, if the incoming stimuli were, let's say, a hot surface, you would involuntarily respond, again, by flinching your hand back away from the hot surface. And again, that was entirely under your involuntary control. So that's an example. that's an example scenario coordinated by your somatic nervous system. Okay, so your somatic nervous system also coordinates your reflexes. There are also an entirely different set of involuntary response to stimuli, okay, and that's coordinated by what's known as your autonomic nervous system, okay. So this is entirely and completely under involuntary control, okay? So when you respond to sensory information through completely involuntary means, that is all coordinated by your autonomic nervous system. This is the division of your nervous system that operates on autopilot, okay? And it's self-regulating, self-coordinating, right? So auto, again, translates to self, right? So This is the autonomic nervous system, abbreviated as the ANS. This is the branch of your motor division that is entirely under involuntary control. And the autonomic nervous system oversees the activity of your heart, cardiac muscle, smooth... muscle, which dominates hollow organs, such as blood vessels, the digestive tract, among other things, and also glands. For instance, sweat glands, salivary glands, adrenal glands, cardiac muscles, smooth muscle, and glands. These are the target organs or effectors under the influence or the control of the autonomic. nervous system. So for instance, if we respond by adjusting the rate of heart muscle contractions, or the force of heart muscle contractions, right, that's coordinated by the autonomic nervous system, right? If we respond to sensory info by Activating our sweat glands, right? That's, again, coordinated by the autonomic nervous system, right? So the autonomic nervous system is absolutely essential for our survival. It's the branch of the nervous system that is constantly monitoring your dynamic state of equilibrium, okay? This is constantly, constantly maintaining and seeking to control what's known as your homeostasis. Okay, so it's continually monitoring and surveying your environment, right? and helping us respond to any internal and external changes, right? Controlling and monitoring countless variables, right? In order to ensure our survival and that we maintain that balanced steady state of equilibrium known as homeostasis. And these are the two subdivisions of the sympathetic, known as the sympathetic and the parasympathetic. They actually... antagonize or counteract each other's effects in order to help us maintain the steady state known as homeostasis. So there's a constant interplay where the sympathetic and the parasympathetic are constantly counteracting or offsetting one another to maintain this balance. Let's talk a little bit about nervous tissue. So this is a brief kind of refresher on nervous tissue. So consider that the... The building block or the structural and functional unit of your nervous system is what's known as the neuron or the nerve cell, right? Without which the nervous system would cease to exist structurally, right? And it would cease to exist functionally. It would cease to function, right? So we refer to the neuron or the nerve cell as really the... structural and functional unit of the nervous system. In a nutshell, the building block of the nervous system. The neurons are cells that are specialized for transmitting electrical signals, which we refer to as nerve impulses. And in terms of their main structural features, they're comprised of a nucleated cell body. Right. And. cytoplasm that radiates extends into long processes called dendrites. Okay, so dendrites are really just extensions of the cytoplasm. Okay, so they're cytoplasmic extensions. Okay, so they almost kind of resemble microscopic fingers, right? They're kind of finger-like microscopic projections. In this figure, you can see that there are many, many radiating dendrites, right, compared to the singular axon. The axon is essentially the neuron's singular cytoplasmic extension, okay. that acts as an output channel. So in terms of their overall function, we rely on dendrites to conduct nerve impulses to the cell body. So think of the dendrites as sort of the input channels. They receive incoming nerve impulses and convey them to the cell body. So think of dendrites as sort of the input channel to receive information from various receptors in the body, and they conduct those nerve impulses to the cell body. The axon, on the other hand, is the, as we see, the long branch here, right, that extends from the cell body as its output channel, right, to conduct impulses to other neurons or other cells away from the cell body. So the output channel is the axon that conducts impulses away from the cell body towards other cells or other neurons. Dendrites allow the cell body to receive. information and the axon allows the cell body to send information away. We will discuss the supportive cells a little bit later. These are anatomically referred to as neuroglia, okay, glia, which translates to glue, because these are the support cells that hold the nervous tissue together by providing support to the neuron. Okay, so these are sort of the background players, behind the scenes players, the supportive cells called neuroglia. Literally, these cells are the glue that hold the neurons together. Okay, they help the neuron function as well. And so they're always much smaller in size compared to the neurons, right? So... these smaller surrounding cells kind of scattered around the neuron. These are supportive cells called neuroglia, which we'll discuss more a little bit later. And this is a tissue slide, a photo micrograph of nervous tissue. And you can see that the you can see the nucleated cell body, the nucleus in the middle of the cell body. And this is kind of the general appearance of neurons. Sometimes they can appear kind of almost pyramidal. kind of almost triangular underneath the tissue slide. So this is the appearance of neurons under microscopy. And they're kind of pyramidal at times. And it is difficult to distinguish between dendrites and axons within a tissue smear, unless it's very prominent like so here. So you can, we can... we can interpret that this is the singular axon. Okay. There is an area of the cell body that tapers into the long axon. Kind of see this triangular tapered area, right? There's a part of the cytoplasm that tapers into the axon, right? tapers into the axon, and this is the part of the cytoplasm called the axon hillock, okay? So the axon hillock is just a tapered portion of the neuron's cytoplasm, okay, that basically transitions into the axon, okay? That's actually where nerve impulses are actually generated by the by neurons in the axon helic. So this particular portion right here is the axon helic. Here's another kind of figure of a neuron here. You can make out the finger-like dendrites, nucleated cell body. Here's the axon helic that tapers into the singular axon. Now what's downstream here? All the way at the end, we often refer to them as nerve endings. Okay, but these are anatomically known as axon terminals. They're the endings of the axon. This is the part of the neuron that connects to target cells or fellow neurons. This is where the neuron communicates with other neurons or other cells. Now we can classify neurons functionally. So that's what we're going to discuss here. The three different types of neurons based on function. So there are are neurons that function exclusively in conducting impulses from receptors such as pain, heat, temperature, all types of receptors in the environment towards your central nervous system, towards your brain and spinal cord. Okay, so they conduct impulses in the along the incoming direction. Okay, so those are known as sensory neurons, okay, that carry nerve impulses from receptors, right, in the body, in your, in your, there's all kinds of receptors, right, there's mechanical pain receptors, there's temperature receptors. And they conduct impulses from those receptors towards the CNS. You have a set of neurons known as motor or efferent neurons that are devoted to delivering impulses away from the brain and spinal cord and towards effector organs, such as muscles and glands. So they... conduct impulses towards target organs that can either be muscles or glands. Interneurons are sort of the liaison between the two. So they conduct messages, nerve impulses, back and forth between the sensory and the motor. So inter is derived from the term between, right? So this is the neuron that conducts messages in between, right? Passes along nerve impulses. between the sensory and the motor neurons. Okay, so the one of the best ways to illustrate the functional differences of neurons, the different functional neurons, is what's called a typical reflex arc. So we're looking at the in this image here, we're looking at the neural pathway involved in a typical reflex arc, where the stimulus again here is a pinprick right to the skin. And so that behaves as our, that acts as our stimulus. The stimulus is the pin, which is ultimately, that stimulus is ultimately detected by the pressure and pain receptors in your skin. And so the function of receptors is to convert that stimulus, regardless of its form, into a universal message that the brain can understand. And the brain and spinal cord can only understand the universal language of a nerve impulse. So that's really the function of receptors, right, is to transform stimuli into nerve impulses, right? Regardless of the form of that stimuli, whether it comes in the form of visual stimuli, light stimuli, pressure stimuli. Regardless of the shape and the form of the stimulus, the job of the receptor is to transform that stimulus, regardless of its form, into a universal message known as the nerve impulse, which the brain and spinal cord can understand and interpret. So that's the job of the receptor, to transform stimulus into a... nerve impulse. Then that nerve impulse, that traveling electrical signal, is ultimately propagated, spread, and conducted by the incoming sensory neuron here towards the spinal cord of the CNS. Then that same nerve impulse is relayed to the outgoing motor neuron by way of the interneuron. So the interneuron helps pass along, relay the nerve impulse from the sensory to the motor. It kind of passes along the nerve impulse from the sensory to the motor neuron. And then the motor neuron will conduct. the nerve impulse away from the spinal cord and then towards some target or effector organ, which in this context would be skeletal muscle, the skeletal muscle, okay, which results in pulling, let's say, pulling your hand back. And the target organ doesn't always have to be skeletal muscle. This is just one example, right? So So the target organ could also be glands as well, right? So there are also neural pathways involved in maintaining homeostasis, right? When let's say the stimulus is temperature fluctuations, right? Let's say there's a surge in body temperature. And so in that context, your effector would be sweat glands that are stimulated to release sweat product, right, to reduce body temperature back down to normal set points, okay? So that also involves incoming sensory neurons that convey nerve impulses to the CNS, and it also involves your outgoing motor neurons that conduct impulses towards a sweat glands, okay? So again, In this context, we're using skeletal muscle to illustrate the function of neurons. But again, this same kind of example will play out with other target effectors too. It can also be a gland. Now we can also classify neurons based on their shape or morphology, so their appearance. So this is known as structurally classifying neurons based on their shape or morphology. So these are the three general shapes or types of neuron structure. So the one we've consistently been viewing time and time again, we've been seeing this in the brain. is the structural class known as the multipolar shape, right? So multipolar translates to many, right? So this is in reference to the fact that a multipolar neuron will have many extensions emerging or radiating from the cell body, okay? So here we see many and lots of finger-like dendrites, lots of... cytoplasmic extensions radiating from the cell body. And then as per usual, the singular axon. But ultimately, there are many extensions coming off the cell body. You've got all the dendrites and even the singular axon. So there's multiple lots of extensions radiating off the cell body. So this is the multipolar shape. So. If we refer back to our image of the reflex arc here, the neural pathway of a reflex arc, you'll notice that it's the interneurons and the motor neurons. It's these two functional neurons that actually resemble or take on this shape. So we can describe our interneuron. the middleman, right, and our motor neuron as being multipolar in shape, right? They both have many extensions coming off the, radiating off the cell body, okay? So these two functional neurons, the interneuron and the motor neuron, are multipolar in structure, right? The next type of structural neuron is known as the bipolar shape. Okay, so by, of, or pertaining to, to, right, translates to two. So this is where you have two extensions radiating off the cell body. Okay, so this is where you actually just have one consolidated dendrite, and then you have a singular axon, okay? So one consolidated combined dendrite, right, conducting impulses to the cell body, and then the axon conducting impulses away, right? So one and then two, just in total, you only have one and two, two extensions coming off the cell body, okay? So this is the bipolar shape, okay? They're pretty rare in the body, and they're... limited to your special senses. So those are the main locations where you'll find the bipolar neuron. They're pretty localized to your special senses. They're limited to your special senses in terms of their location. So you'll find them in the ears, the eyes, and also the nose. Okay, so quite rare in the body. This is essentially your cranial floor here, right? So you can see, and this is where the roof of your nasal cavity meets your cranial floor. You can see that you have bipolar neurons situated in the roof of your nasal cavity. Okay, so here's my cell body with two extensions coming off the cell body. One dendrite leading towards the cell body and then one axon running away from the cell body. And the last structural classification is the unipolar. In this case, uni translates to one, singular. So this neuron only has one extension coming off the cell body, right, and that is exclusively just the axon. So here the dendrites are kind of relegated all the way to one end and they have no connection. direct connection to the cell body. It's just the axon that has the connection, direct connection to the cell body. Alright, so here in the unipolar shape, the dendrites are kind of relegated to one end, one extreme, and you can say they don't really have Again, a direct connection to the cell body. It's only the axon that does so. So hence the the term unipolar. OK, now the last remaining functional neuron known as the sensory neuron is unipolar in shape. All right. So the sensory neuron, if you take a peek at the sensory neuron here, you'll note that it has the cell body with. connected to just the one singular axon. It only has a connection to the one axon, direct connection. And the dendrites you can see are off to the other side, right, the other extreme. So we still maintain the core features of a neuron. they still possess the dendrite, the cell body, the axon. But in these different morphologies, you'll notice that their general position is kind of moved around or their general number has changed. So maybe the quantity of the structures has changed or general position, right? But by and large you still maintain the same three core features. Every neuron has the cell body, the dendron, the axon, but there are different kind of arrangements and positions, right? So that's how we arrived at all these different structural classifications. And again, all the way on the end of the axon is our axon terminals. So that's again, all right, checkpoint number one. The nervous system has three overlapping functions, which are the following sequence. do you think represents a logical sequence of these three functions? All right, so this brings us to a discussion of the various types of... of axons that you'll come across. You'll notice in the previous images here, there are some axons that are completely bare, right? They're completely kind of naked and bare, no covering, and there are some axons that possess this kind of membrane-like covering, right? There's a membrane surrounding the axon. So this membrane, this covering, is actually called myelin. And so the idea is that you can have two types of neurons. You can have what's called axons with the myelin membrane, so that's called myelinated axons, versus axons without, which are referred to as unmyelinated axons. So myelin is a mixture of... proteins and phospholipids. Okay, so recall that phospholipids are one of the three types of lipids. It's a kind of a fatty substance, right? So really, at the end of the day, myelin is a mixture of proteins and phospholipids, kind of a fatty material, right? So it will take on kind of a whitish appearance. So the presence of lipids and in the body contribute to a white coloration. So myelin is the protein and lipid membrane, right, or sheath that wraps around and covers axons. And when you have... when you have neuronal axons in the peripheral nervous system. Okay. So for instance, here, right, you can see that my axon is situated beyond the spinal cord, right? Outside of the spinal cord here, which is still part of the CNS. So My axon here is actually situated in the PNS, right? The peripheral nervous system. So whenever axons are located in the... PNS, right, the peripheral nervous system, when they exist outside of the brain and spinal cord, right, as depicted here, as illustrated here, the cells responsible for laying down the myelin, for producing the myelin sheath, right, around the axons are called Schwann cells, okay? This is a type of neuroglia. It's one of, again, one of the supportive background players. So the Schwann cell is a type of neuroglia or support cell, okay, only found in the peripheral nervous system. So they only lay down myelin around axons in the peripheral nervous system, the PNS. So you can see here, there's an image of the Schwann cell here. You can see even the nucleus of this supportive Schwann cell. And it's situated kind of on the outskirts of the myelin sheath, okay? And what it does is it effectively lays down myelin inward, right? And so over time that kind of manifests as a coiling effect where the myelin continues to kind of coil and wrap. around and around and around itself, right? Until a nice kind of thickened kind of sheath is formed. So kind of in cross-section, if we were to cut along the cross-section of a neuron and the sheath, we would see the axon in the middle, right? And then we'd see essentially layers upon layers of a coiled myelin, right? And then sitting on the outskirts, And the edges here in the periphery, those are the Schwann cells sitting on the outside, laying down myelin internally. So we call this outer layer of Schwann cell nucleus and cytoplasm, we call it the neurolemma. Let's write that a little bit bigger here. But neurolemma, so that's all it is. Neurolemma is a term. reserved for the cytoplasm and the nuclei of Schwann cells that lie, that kind of lie outside and then surround your myelin sheath. Yeah, so they all lie beyond or outside the myelin sheath. Okay, so all myelin is is a kind of a membrane comprised of proteins and lipids. Okay, that kind of that almost kind of insulate the axon. The presence of myelin is going to have implications on the rate of nerve impulse conduction, okay, how quickly nerve impulses are conducted, right? So you can you'll see that the presence or absence of myelin will have a direct impact or bearing on how quickly nerve impulses are conducted along axons, okay? So We want to consider that lipids or fat are non-conducting, right? Fat does not conduct electricity, right? And we established that nerve impulses are electrical signals, traveling electrical signals. So when a nerve impulse is... propagated along a myelinated axon, the nerve impulse does not get conducted through the myelin. Okay, so once a nerve impulse arrives at an area of myelin, that traveling nerve impulse is prompted to hop or leap over the myelin entirely. Then it will be propagated. It will be conducted for a little bit along a bare area of the axon. And then as soon as it arrives at another piece of myelin, it's forced to hop over that myelin once again. So the idea is that this electrical signal called a nerve impulse is not conducted by myelin. As soon as it encounters a piece of myelin, it's prompted to leap or hop over the myelin, and it's conducted briefly along the area where the axon is kind of exposed, right? So the axon itself conducts nerve impulses because the axon kind of behaves in a similar fashion to an electrical wire, right? The axon is able to conduct... electricity, but the wrapping around it is not. So ultimately, nerve impulses are only conducted along the areas of the axon where it's not covered by myelin, where it's completely bare. So these exposed areas here, these exposed areas is where the myelin is absent, and it's where nerve impulses are conducted. we call these exposed areas nodes of Ranvier. They're gaps. They're gaps where the axon is exposed. There is actually a name assigned to this kind of leaping motion, and it's actually called saltatory conduction. So this is a type of nerve impulse movement or conduction where... instead of the nerve impulse traveling along the entire length of the axon, it's allowed to leap or jump from node of Ranvier to node of Ranvier to node of Ranvier. And this kind of leaping motion, this saltatory type of saltatory conduction, allows nerve impulses to propagate or travel much more quickly at a much faster rate along an axon. So the impact or the effect of myelination on an axon is actually that it increases the rate of nerve impulse conduction. Because when nerve impulses are allowed to leap from node to node to node, that's much faster than... a nerve impulse having to travel along the entire length, right, of the axon. So anytime we come across the kind of the constricted areas where it appears kind of pinched in, that's a node of Ranvier, okay? So that's where the nerve impulse is forced to hop to. So non-myelinated axons actually conduct nerve impulses on a slower scale. compared to their myelinated counterparts. Checkpoint number two. In a living neuron, which component provides resistance to current flow? Next checkpoint. The portion of an axon that communicates with its target cell is the So that brings us to a discussion of the supportive cells known as the neuroglia, right? But we are now discussing the neuroglia of the CNS. So these are neuroglia support cells in your brain and spinal cord. We have what's called a type of neuroglia called the astrocytes, which are named after their kind of star shape. like astrology, study of stars. And they have these little extensions that resemble kind of foot-like processes. These foot-like processes allow the astrocytes to attach or anchor blood vessels to neurons to provide them with blood supply, right? To provide them with nutrients and oxygen, right? So you can hear in our image here, you can, you can see the astrocyte and you see how the foot-like processes help anchor this blood capillary to the neuron, right, to provide blood supply to the neuron. Okay, so that's the overall function of astrocytes. Then you have what are a type of neuroglia called microglia, and they're named after their small size, micro, and they can actually phagocytize or engulf foreign invaders, so foreign microbes. So any... Any microbes that breach the brain tissue, the brain barrier, they actually can physically engulf or ingest, eat these foreign microbes, these invading microbes. And then the third type of neuroglia is known as the oligodendrocytes. And they are the CNS equivalent of the Schwann cell. So recall that Schwann cells lay down myelin around axons in the PNS oligodendrocytes, lay down myelin around axons located in the CNS, the brain and spinal cord. That brings us to our next checkpoint. Complete the following analogy. Electrical wire is to electrical tape as peripheral neurons are to. All right, so we're going to discuss the the concept of a nerve impulse, kind of define what it is. We established earlier that it is a traveling wave of electricity, right, of electrical signals. And so the term here, the description self-propagating, refers to the ability to spread itself. So what we mean by self-propagating is that action potentials. They actually self-spread or self-propagate, self-spreading, so to speak. And nerve impulses are alternatively known as an action potential or an AP. And the nerve impulse is simply a traveling wave of positive behavior. charges, okay, as kind of depicted in this image on this animation. So that brings us to a discussion of what's called a cell's membrane potential, okay, and we'll talk about kind of kind of the how this plays into our discussion of nerve impulses. First want to consider what a membrane potential is, right? And the relationship between membrane potential and a nerve impulse or an action potential, right? So membrane potential refers to how the concentration of charges are in a cell compared to its external environment, okay? So all it is Membrane potential is really just the difference in the concentration of charges inside the cell versus outside of the cell. So in this chapter, we're particularly concerned with the cell, the body cell that is the neuron. OK, you'll notice that in this neuron, there is a difference in terms of how. charges are concentrated inside the cell versus outside of the cell in the extracellular environment. Okay, notice that in the interior of the cell, there's an excess of negative charges, right? Whereas in the external environment, there's an excess of positive charges, right? So the internal environment, the interior of a neuron at rest skews negative and its exterior, its external environment skews positive. Okay, so at rest, cells, body cells, and specifically nerve cells or neurons, always have an excess of negative charges on the inside compared to the outside. So there's a difference, there's a clear difference in how these charges are concentrated, right, between the inside and the outside. So really what we want to do is kind of break down the term membrane potential. Okay, notice that these charges, the positive and negative charges, are separated by a barrier, right? That barrier is the membrane. That's where we derive the term membrane and membrane potential because there's a membrane or a barrier that separates the charges, the positive and the negative charges. And the term potential also bears similarity to our working definition of potential. potential, right? The ability to achieve or the ability to, um, yeah, achieve, um, some task or the ability to reach a goal, right? So, um, so we know the term potential to, to represent the ability to achieve, uh, in the context of, uh, of, you know, uh, the nervous system. Uh, it also, it also aligns with that definition, um, because charges, positive and negative charges in your cells are separated, right, by the membrane. When you bring these charges together, when you mix these charges and bring them together into the fold, there is a potential to, again, produce something great, right? So for instance, we know that... the two ends of a battery are completely polarized, right? Positive charges are concentrated on one end. Negative charges are concentrated on the other end of the battery, okay? When you, again, conduct a current through them and when you ultimately bring... the positive and the negative charges of the battery together, there's a potential to produce something great, right? There's a potential to power a machine, power some object, right? So the same logic applies to the concept of membrane potential for the traveling nerve impulse or action potential to influence the activity of neurons, right? That is what can be achieved, right, ultimately by bringing the positive and negative charges together. Since we established that the nerve impulse or an action potential is a traveling wave of positive charges, right, it actually has the potential to briefly reverse the resting state of the neuron, right, so that the inside skews less negative. and eventually more and more positive, right? So, yeah, so as this traveling wave of positive charges called the nerve impulse is spreading and propagating through, let's say, the axon of your neuron, it has the potential to reverse the resting state, right? It has the potential to reverse how the charges were originally concentrated so that... The inside, the interior, skews less and less and less negative and more and more positive. It has the potential to move or to change the membrane potential from negative towards positive. The resting membrane potential refers to how the charges were concentrated as at rest, what we establish here, where you have more negative on the inside, more negative charges on the inside. So resting membrane potential refers to this particular state where you have more negative charges on the inside. And as an action potential, as that wave of positive charge is spreading through, it has the ability or the potential to reverse. the resting state from negative towards positive. Now in our kind of image in the right hand side here, this is an image of the sodium potassium pump that is found in all cell membranes, even the neuron, right? And this is by and large what maintains the the resting membrane potential, right? So, so if we consider, or if we pose the question, why are cells more negative on the inside at rest, right? Why are resting cells more negative on the inside, right? And it's really, that is really all coordinated and determined and by the sodium potassium pump here, okay? So the potassium, sodium potassium pump is located in all body cells and it's what maintains this negative resting state, okay? And ultimately, for every three positive sodium ions that are pumped out, only two positive potassium ions are pumped in. So that yields a net negative one charge. And so this is really what is keeping the resting. membrane potential negative, the sodium potassium pump.

structural divisions of the nervous system. The central nervous system is the division comprised of the centrally located brain and spinal cord, which are situated centrally or along the midline. The other half of the structural division is known as the peripheral nervous system, which is exclusively comprised of nerves.

Nerves are situated beyond or outside of the spinal cord, hence the term peripheral. If you consider the concept of peripheral vision, it's what you can see along the edges of your vision. So the peripheral nervous system is comprised of nerves that exist or are located beyond or outside of the brain and spinal cord. So here, We have two sets of nerves. One is known as the cranial nerves.

They emerge from the base of their brain, so they're physically situated beyond or outside the brain. You have a second set of nerves known as the spinal nerves, and those emerge from either side of the spinal cord, from left and right sides of the spinal cord, and they too exist beyond or outside of the spinal cord. Hence, they are both part of the peripheral nervous system because they're situated outside of the brain and spinal cord. We want to consider that the nerves are a two-way delivery system for electrical messages called nerve impulses, right?

Nerves are a two-way highway or delivery system, okay? They conduct impulses towards the brain and spinal cord. along the incoming direction, but they also conduct impulses away from the brain and spinal cord along the outgoing direction as well. So we can classify our peripheral nervous system into two broad systems based on function.

So nerves that function in conducting impulses towards the brain and spinal cord along the incoming direction we refer to that as the sensory division of your PNS. Nerves that are responsible for conducting messages away from the brain and spinal cord along the outgoing direction is known as the motor division of the PNS. So your sensory division is alternatively known as your afferent division. So afferent.

which translates to incoming, right? Think of arriving, A for arriving. So this is in a nutshell, our sensory division is our input system, right? That provides us with the means to acquire information. This is how we acquire information from our environment.

And this input system also allows us to interpret and process that incoming sensory information so we can make sense of it. Okay, so your sensory division is what's known as your senses, right? So think of your special senses, sense of sight, smell, taste. Now your motor division, alternatively known as your efferent division, so efferent in this context.

translates to outgoing or efferent e for exiting. Motor division provides us with a means to respond to that incoming sensory info. We can respond to incoming sensory info through movement. Right.

Through in the form of muscle contractions. Right. So, for instance, if the stimuli were heat or pain or pressure, we can respond by moving a body part essentially away from the source of the pain and the object inflicting the pain.

Right. So if we sustained, let's say, a prick or thumbtack to the skin. We can respond by ultimately flinching our hand away, jerking our hand back and contracting the muscles in our limbs.

So this is the part of your motor division that's controlled by your somatic nervous system. Skeletal muscle manipulation is coordinated by a branch of your motor division called the somatic nervous system. Okay, so soma actually translates to body.

So if you think about the division of your nervous system that moves the body, right, through skeletal muscle manipulation, that's all. coordinated by a branch of your nervous system called the somatic nervous system. So the target organ or the target effector of the somatic nervous system will always be skeletal muscle.

Skeletal muscle is the only muscle type under voluntary control. Okay, we do want to consider. In the context of reflexes, okay, where skeletal muscle is involved, for instance, the knee jerk reflex, or the reflex of again, jerking your hand back when you're when you're exposed to heat, for instance, or pain.

This is by and large involuntary, right? So there is an exception to the rule. And so the exception is that skeletal muscle can be involuntary when it comes to reflexes.

But outside of reflexes, yes, skeletal muscle is under your complete voluntary control where you have to will something to happen. You have to consciously will it to happen. That is true.

But skeletal muscle can be involuntary. with respect to reflexes. So when we're flinching back or jerking our hand back from pain stimuli or pressure stimuli, that is completely under involuntary control.

We did not have time to react and think about it and consciously will it to happen. So that's something to consider as well. That yes, skeletal muscle is largely under our voluntary control, but skeletal muscle can be involuntary when it comes to reflexes. So in this particular example, let's talk about how we can respond to incoming sensory info through voluntary and involuntary skeletal muscle contraction.

Okay, so So, um, The incoming sensory stimuli could be, for instance, an object that you see, let's say a glass of water, right? We can respond through voluntary means, right, through voluntary muscle manipulation by consciously picking up the glass of water, right? Contracting our skeletal muscles in the arm to pick up the glass of water, right?

So that would be an example of your somatic nervous system coordinating a response. to sensory stimuli that is completely voluntary. You willed it to happen, right? So you process and detected a glass of water, you're responding, right?

That's your sensory stimuli, that's your sensory info, and you're responding. voluntarily by moving your arm to pick up the glass of water. Now, if the incoming stimuli were, let's say, a hot surface, you would involuntarily respond, again, by flinching your hand back away from the hot surface. And again, that was entirely under your involuntary control.

So that's an example. that's an example scenario coordinated by your somatic nervous system. Okay, so your somatic nervous system also coordinates your reflexes. There are also an entirely different set of involuntary response to stimuli, okay, and that's coordinated by what's known as your autonomic nervous system, okay. So this is entirely and completely under involuntary control, okay?

So when you respond to sensory information through completely involuntary means, that is all coordinated by your autonomic nervous system. This is the division of your nervous system that operates on autopilot, okay? And it's self-regulating, self-coordinating, right? So auto, again, translates to self, right?

So This is the autonomic nervous system, abbreviated as the ANS. This is the branch of your motor division that is entirely under involuntary control. And the autonomic nervous system oversees the activity of your heart, cardiac muscle, smooth... muscle, which dominates hollow organs, such as blood vessels, the digestive tract, among other things, and also glands. For instance, sweat glands, salivary glands, adrenal glands, cardiac muscles, smooth muscle, and glands.

These are the target organs or effectors under the influence or the control of the autonomic. nervous system. So for instance, if we respond by adjusting the rate of heart muscle contractions, or the force of heart muscle contractions, right, that's coordinated by the autonomic nervous system, right?

If we respond to sensory info by Activating our sweat glands, right? That's, again, coordinated by the autonomic nervous system, right? So the autonomic nervous system is absolutely essential for our survival.

It's the branch of the nervous system that is constantly monitoring your dynamic state of equilibrium, okay? This is constantly, constantly maintaining and seeking to control what's known as your homeostasis. Okay, so it's continually monitoring and surveying your environment, right?

and helping us respond to any internal and external changes, right? Controlling and monitoring countless variables, right? In order to ensure our survival and that we maintain that balanced steady state of equilibrium known as homeostasis.

And these are the two subdivisions of the sympathetic, known as the sympathetic and the parasympathetic. They actually... antagonize or counteract each other's effects in order to help us maintain the steady state known as homeostasis. So there's a constant interplay where the sympathetic and the parasympathetic are constantly counteracting or offsetting one another to maintain this balance. Let's talk a little bit about nervous tissue.

So this is a brief kind of refresher on nervous tissue. So consider that the... The building block or the structural and functional unit of your nervous system is what's known as the neuron or the nerve cell, right? Without which the nervous system would cease to exist structurally, right? And it would cease to exist functionally.

It would cease to function, right? So we refer to the neuron or the nerve cell as really the... structural and functional unit of the nervous system. In a nutshell, the building block of the nervous system. The neurons are cells that are specialized for transmitting electrical signals, which we refer to as nerve impulses.

And in terms of their main structural features, they're comprised of a nucleated cell body. Right. And. cytoplasm that radiates extends into long processes called dendrites. Okay, so dendrites are really just extensions of the cytoplasm.

Okay, so they're cytoplasmic extensions. Okay, so they almost kind of resemble microscopic fingers, right? They're kind of finger-like microscopic projections. In this figure, you can see that there are many, many radiating dendrites, right, compared to the singular axon. The axon is essentially the neuron's singular cytoplasmic extension, okay.

that acts as an output channel. So in terms of their overall function, we rely on dendrites to conduct nerve impulses to the cell body. So think of the dendrites as sort of the input channels. They receive incoming nerve impulses and convey them to the cell body.

So think of dendrites as sort of the input channel to receive information from various receptors in the body, and they conduct those nerve impulses to the cell body. The axon, on the other hand, is the, as we see, the long branch here, right, that extends from the cell body as its output channel, right, to conduct impulses to other neurons or other cells away from the cell body. So the output channel is the axon that conducts impulses away from the cell body towards other cells or other neurons. Dendrites allow the cell body to receive.

information and the axon allows the cell body to send information away. We will discuss the supportive cells a little bit later. These are anatomically referred to as neuroglia, okay, glia, which translates to glue, because these are the support cells that hold the nervous tissue together by providing support to the neuron.

Okay, so these are sort of the background players, behind the scenes players, the supportive cells called neuroglia. Literally, these cells are the glue that hold the neurons together. Okay, they help the neuron function as well. And so they're always much smaller in size compared to the neurons, right?

So... these smaller surrounding cells kind of scattered around the neuron. These are supportive cells called neuroglia, which we'll discuss more a little bit later. And this is a tissue slide, a photo micrograph of nervous tissue.

And you can see that the you can see the nucleated cell body, the nucleus in the middle of the cell body. And this is kind of the general appearance of neurons. Sometimes they can appear kind of almost pyramidal. kind of almost triangular underneath the tissue slide.

So this is the appearance of neurons under microscopy. And they're kind of pyramidal at times. And it is difficult to distinguish between dendrites and axons within a tissue smear, unless it's very prominent like so here. So you can, we can... we can interpret that this is the singular axon.

Okay. There is an area of the cell body that tapers into the long axon. Kind of see this triangular tapered area, right?

There's a part of the cytoplasm that tapers into the axon, right? tapers into the axon, and this is the part of the cytoplasm called the axon hillock, okay? So the axon hillock is just a tapered portion of the neuron's cytoplasm, okay, that basically transitions into the axon, okay? That's actually where nerve impulses are actually generated by the by neurons in the axon helic. So this particular portion right here is the axon helic.

Here's another kind of figure of a neuron here. You can make out the finger-like dendrites, nucleated cell body. Here's the axon helic that tapers into the singular axon.

Now what's downstream here? All the way at the end, we often refer to them as nerve endings. Okay, but these are anatomically known as axon terminals. They're the endings of the axon. This is the part of the neuron that connects to target cells or fellow neurons.

This is where the neuron communicates with other neurons or other cells. Now we can classify neurons functionally. So that's what we're going to discuss here. The three different types of neurons based on function. So there are are neurons that function exclusively in conducting impulses from receptors such as pain, heat, temperature, all types of receptors in the environment towards your central nervous system, towards your brain and spinal cord.

Okay, so they conduct impulses in the along the incoming direction. Okay, so those are known as sensory neurons, okay, that carry nerve impulses from receptors, right, in the body, in your, in your, there's all kinds of receptors, right, there's mechanical pain receptors, there's temperature receptors. And they conduct impulses from those receptors towards the CNS. You have a set of neurons known as motor or efferent neurons that are devoted to delivering impulses away from the brain and spinal cord and towards effector organs, such as muscles and glands.

So they... conduct impulses towards target organs that can either be muscles or glands. Interneurons are sort of the liaison between the two. So they conduct messages, nerve impulses, back and forth between the sensory and the motor.

So inter is derived from the term between, right? So this is the neuron that conducts messages in between, right? Passes along nerve impulses. between the sensory and the motor neurons.

Okay, so the one of the best ways to illustrate the functional differences of neurons, the different functional neurons, is what's called a typical reflex arc. So we're looking at the in this image here, we're looking at the neural pathway involved in a typical reflex arc, where the stimulus again here is a pinprick right to the skin. And so that behaves as our, that acts as our stimulus.

The stimulus is the pin, which is ultimately, that stimulus is ultimately detected by the pressure and pain receptors in your skin. And so the function of receptors is to convert that stimulus, regardless of its form, into a universal message that the brain can understand. And the brain and spinal cord can only understand the universal language of a nerve impulse. So that's really the function of receptors, right, is to transform stimuli into nerve impulses, right?

Regardless of the form of that stimuli, whether it comes in the form of visual stimuli, light stimuli, pressure stimuli. Regardless of the shape and the form of the stimulus, the job of the receptor is to transform that stimulus, regardless of its form, into a universal message known as the nerve impulse, which the brain and spinal cord can understand and interpret. So that's the job of the receptor, to transform stimulus into a... nerve impulse.

Then that nerve impulse, that traveling electrical signal, is ultimately propagated, spread, and conducted by the incoming sensory neuron here towards the spinal cord of the CNS. Then that same nerve impulse is relayed to the outgoing motor neuron by way of the interneuron. So the interneuron helps pass along, relay the nerve impulse from the sensory to the motor.

It kind of passes along the nerve impulse from the sensory to the motor neuron. And then the motor neuron will conduct. the nerve impulse away from the spinal cord and then towards some target or effector organ, which in this context would be skeletal muscle, the skeletal muscle, okay, which results in pulling, let's say, pulling your hand back.

And the target organ doesn't always have to be skeletal muscle. This is just one example, right? So So the target organ could also be glands as well, right? So there are also neural pathways involved in maintaining homeostasis, right? When let's say the stimulus is temperature fluctuations, right?

Let's say there's a surge in body temperature. And so in that context, your effector would be sweat glands that are stimulated to release sweat product, right, to reduce body temperature back down to normal set points, okay? So that also involves incoming sensory neurons that convey nerve impulses to the CNS, and it also involves your outgoing motor neurons that conduct impulses towards a sweat glands, okay?

So again, In this context, we're using skeletal muscle to illustrate the function of neurons. But again, this same kind of example will play out with other target effectors too. It can also be a gland.

Now we can also classify neurons based on their shape or morphology, so their appearance. So this is known as structurally classifying neurons based on their shape or morphology. So these are the three general shapes or types of neuron structure. So the one we've consistently been viewing time and time again, we've been seeing this in the brain. is the structural class known as the multipolar shape, right?

So multipolar translates to many, right? So this is in reference to the fact that a multipolar neuron will have many extensions emerging or radiating from the cell body, okay? So here we see many and lots of finger-like dendrites, lots of...

cytoplasmic extensions radiating from the cell body. And then as per usual, the singular axon. But ultimately, there are many extensions coming off the cell body.

You've got all the dendrites and even the singular axon. So there's multiple lots of extensions radiating off the cell body. So this is the multipolar shape.

So. If we refer back to our image of the reflex arc here, the neural pathway of a reflex arc, you'll notice that it's the interneurons and the motor neurons. It's these two functional neurons that actually resemble or take on this shape. So we can describe our interneuron. the middleman, right, and our motor neuron as being multipolar in shape, right?

They both have many extensions coming off the, radiating off the cell body, okay? So these two functional neurons, the interneuron and the motor neuron, are multipolar in structure, right? The next type of structural neuron is known as the bipolar shape. Okay, so by, of, or pertaining to, to, right, translates to two.

So this is where you have two extensions radiating off the cell body. Okay, so this is where you actually just have one consolidated dendrite, and then you have a singular axon, okay? So one consolidated combined dendrite, right, conducting impulses to the cell body, and then the axon conducting impulses away, right? So one and then two, just in total, you only have one and two, two extensions coming off the cell body, okay?

So this is the bipolar shape, okay? They're pretty rare in the body, and they're... limited to your special senses.

So those are the main locations where you'll find the bipolar neuron. They're pretty localized to your special senses. They're limited to your special senses in terms of their location.

So you'll find them in the ears, the eyes, and also the nose. Okay, so quite rare in the body. This is essentially your cranial floor here, right? So you can see, and this is where the roof of your nasal cavity meets your cranial floor. You can see that you have bipolar neurons situated in the roof of your nasal cavity.

Okay, so here's my cell body with two extensions coming off the cell body. One dendrite leading towards the cell body and then one axon running away from the cell body. And the last structural classification is the unipolar.

In this case, uni translates to one, singular. So this neuron only has one extension coming off the cell body, right, and that is exclusively just the axon. So here the dendrites are kind of relegated all the way to one end and they have no connection.

direct connection to the cell body. It's just the axon that has the connection, direct connection to the cell body. Alright, so here in the unipolar shape, the dendrites are kind of relegated to one end, one extreme, and you can say they don't really have Again, a direct connection to the cell body. It's only the axon that does so. So hence the the term unipolar.

OK, now the last remaining functional neuron known as the sensory neuron is unipolar in shape. All right. So the sensory neuron, if you take a peek at the sensory neuron here, you'll note that it has the cell body with.

connected to just the one singular axon. It only has a connection to the one axon, direct connection. And the dendrites you can see are off to the other side, right, the other extreme.

So we still maintain the core features of a neuron. they still possess the dendrite, the cell body, the axon. But in these different morphologies, you'll notice that their general position is kind of moved around or their general number has changed.

So maybe the quantity of the structures has changed or general position, right? But by and large you still maintain the same three core features. Every neuron has the cell body, the dendron, the axon, but there are different kind of arrangements and positions, right? So that's how we arrived at all these different structural classifications.

And again, all the way on the end of the axon is our axon terminals. So that's again, all right, checkpoint number one. The nervous system has three overlapping functions, which are the following sequence. do you think represents a logical sequence of these three functions? All right, so this brings us to a discussion of the various types of...

of axons that you'll come across. You'll notice in the previous images here, there are some axons that are completely bare, right? They're completely kind of naked and bare, no covering, and there are some axons that possess this kind of membrane-like covering, right?

There's a membrane surrounding the axon. So this membrane, this covering, is actually called myelin. And so the idea is that you can have two types of neurons.

You can have what's called axons with the myelin membrane, so that's called myelinated axons, versus axons without, which are referred to as unmyelinated axons. So myelin is a mixture of... proteins and phospholipids.

Okay, so recall that phospholipids are one of the three types of lipids. It's a kind of a fatty substance, right? So really, at the end of the day, myelin is a mixture of proteins and phospholipids, kind of a fatty material, right?

So it will take on kind of a whitish appearance. So the presence of lipids and in the body contribute to a white coloration. So myelin is the protein and lipid membrane, right, or sheath that wraps around and covers axons.

And when you have... when you have neuronal axons in the peripheral nervous system. Okay.

So for instance, here, right, you can see that my axon is situated beyond the spinal cord, right? Outside of the spinal cord here, which is still part of the CNS. So My axon here is actually situated in the PNS, right? The peripheral nervous system.

So whenever axons are located in the... PNS, right, the peripheral nervous system, when they exist outside of the brain and spinal cord, right, as depicted here, as illustrated here, the cells responsible for laying down the myelin, for producing the myelin sheath, right, around the axons are called Schwann cells, okay? This is a type of neuroglia. It's one of, again, one of the supportive background players.

So the Schwann cell is a type of neuroglia or support cell, okay, only found in the peripheral nervous system. So they only lay down myelin around axons in the peripheral nervous system, the PNS. So you can see here, there's an image of the Schwann cell here. You can see even the nucleus of this supportive Schwann cell. And it's situated kind of on the outskirts of the myelin sheath, okay?

And what it does is it effectively lays down myelin inward, right? And so over time that kind of manifests as a coiling effect where the myelin continues to kind of coil and wrap. around and around and around itself, right?

Until a nice kind of thickened kind of sheath is formed. So kind of in cross-section, if we were to cut along the cross-section of a neuron and the sheath, we would see the axon in the middle, right? And then we'd see essentially layers upon layers of a coiled myelin, right? And then sitting on the outskirts, And the edges here in the periphery, those are the Schwann cells sitting on the outside, laying down myelin internally. So we call this outer layer of Schwann cell nucleus and cytoplasm, we call it the neurolemma.

Let's write that a little bit bigger here. But neurolemma, so that's all it is. Neurolemma is a term.

reserved for the cytoplasm and the nuclei of Schwann cells that lie, that kind of lie outside and then surround your myelin sheath. Yeah, so they all lie beyond or outside the myelin sheath. Okay, so all myelin is is a kind of a membrane comprised of proteins and lipids. Okay, that kind of that almost kind of insulate the axon.

The presence of myelin is going to have implications on the rate of nerve impulse conduction, okay, how quickly nerve impulses are conducted, right? So you can you'll see that the presence or absence of myelin will have a direct impact or bearing on how quickly nerve impulses are conducted along axons, okay? So We want to consider that lipids or fat are non-conducting, right? Fat does not conduct electricity, right? And we established that nerve impulses are electrical signals, traveling electrical signals.

So when a nerve impulse is... propagated along a myelinated axon, the nerve impulse does not get conducted through the myelin. Okay, so once a nerve impulse arrives at an area of myelin, that traveling nerve impulse is prompted to hop or leap over the myelin entirely.

Then it will be propagated. It will be conducted for a little bit along a bare area of the axon. And then as soon as it arrives at another piece of myelin, it's forced to hop over that myelin once again.

So the idea is that this electrical signal called a nerve impulse is not conducted by myelin. As soon as it encounters a piece of myelin, it's prompted to leap or hop over the myelin, and it's conducted briefly along the area where the axon is kind of exposed, right? So the axon itself conducts nerve impulses because the axon kind of behaves in a similar fashion to an electrical wire, right? The axon is able to conduct...

electricity, but the wrapping around it is not. So ultimately, nerve impulses are only conducted along the areas of the axon where it's not covered by myelin, where it's completely bare. So these exposed areas here, these exposed areas is where the myelin is absent, and it's where nerve impulses are conducted.

we call these exposed areas nodes of Ranvier. They're gaps. They're gaps where the axon is exposed. There is actually a name assigned to this kind of leaping motion, and it's actually called saltatory conduction. So this is a type of nerve impulse movement or conduction where...

instead of the nerve impulse traveling along the entire length of the axon, it's allowed to leap or jump from node of Ranvier to node of Ranvier to node of Ranvier. And this kind of leaping motion, this saltatory type of saltatory conduction, allows nerve impulses to propagate or travel much more quickly at a much faster rate along an axon. So the impact or the effect of myelination on an axon is actually that it increases the rate of nerve impulse conduction.

Because when nerve impulses are allowed to leap from node to node to node, that's much faster than... a nerve impulse having to travel along the entire length, right, of the axon. So anytime we come across the kind of the constricted areas where it appears kind of pinched in, that's a node of Ranvier, okay?

So that's where the nerve impulse is forced to hop to. So non-myelinated axons actually conduct nerve impulses on a slower scale. compared to their myelinated counterparts.

Checkpoint number two. In a living neuron, which component provides resistance to current flow? Next checkpoint. The portion of an axon that communicates with its target cell is the So that brings us to a discussion of the supportive cells known as the neuroglia, right?

But we are now discussing the neuroglia of the CNS. So these are neuroglia support cells in your brain and spinal cord. We have what's called a type of neuroglia called the astrocytes, which are named after their kind of star shape.

like astrology, study of stars. And they have these little extensions that resemble kind of foot-like processes. These foot-like processes allow the astrocytes to attach or anchor blood vessels to neurons to provide them with blood supply, right? To provide them with nutrients and oxygen, right? So you can hear in our image here, you can, you can see the astrocyte and you see how the foot-like processes help anchor this blood capillary to the neuron, right, to provide blood supply to the neuron.

Okay, so that's the overall function of astrocytes. Then you have what are a type of neuroglia called microglia, and they're named after their small size, micro, and they can actually phagocytize or engulf foreign invaders, so foreign microbes. So any... Any microbes that breach the brain tissue, the brain barrier, they actually can physically engulf or ingest, eat these foreign microbes, these invading microbes.

And then the third type of neuroglia is known as the oligodendrocytes. And they are the CNS equivalent of the Schwann cell. So recall that Schwann cells lay down myelin around axons in the PNS oligodendrocytes, lay down myelin around axons located in the CNS, the brain and spinal cord.

That brings us to our next checkpoint. Complete the following analogy. Electrical wire is to electrical tape as peripheral neurons are to. All right, so we're going to discuss the the concept of a nerve impulse, kind of define what it is. We established earlier that it is a traveling wave of electricity, right, of electrical signals.

And so the term here, the description self-propagating, refers to the ability to spread itself. So what we mean by self-propagating is that action potentials. They actually self-spread or self-propagate, self-spreading, so to speak.

And nerve impulses are alternatively known as an action potential or an AP. And the nerve impulse is simply a traveling wave of positive behavior. charges, okay, as kind of depicted in this image on this animation. So that brings us to a discussion of what's called a cell's membrane potential, okay, and we'll talk about kind of kind of the how this plays into our discussion of nerve impulses.

First want to consider what a membrane potential is, right? And the relationship between membrane potential and a nerve impulse or an action potential, right? So membrane potential refers to how the concentration of charges are in a cell compared to its external environment, okay? So all it is Membrane potential is really just the difference in the concentration of charges inside the cell versus outside of the cell. So in this chapter, we're particularly concerned with the cell, the body cell that is the neuron.

OK, you'll notice that in this neuron, there is a difference in terms of how. charges are concentrated inside the cell versus outside of the cell in the extracellular environment. Okay, notice that in the interior of the cell, there's an excess of negative charges, right?

Whereas in the external environment, there's an excess of positive charges, right? So the internal environment, the interior of a neuron at rest skews negative and its exterior, its external environment skews positive. Okay, so at rest, cells, body cells, and specifically nerve cells or neurons, always have an excess of negative charges on the inside compared to the outside. So there's a difference, there's a clear difference in how these charges are concentrated, right, between the inside and the outside. So really what we want to do is kind of break down the term membrane potential.

Okay, notice that these charges, the positive and negative charges, are separated by a barrier, right? That barrier is the membrane. That's where we derive the term membrane and membrane potential because there's a membrane or a barrier that separates the charges, the positive and the negative charges.

And the term potential also bears similarity to our working definition of potential. potential, right? The ability to achieve or the ability to, um, yeah, achieve, um, some task or the ability to reach a goal, right?

So, um, so we know the term potential to, to represent the ability to achieve, uh, in the context of, uh, of, you know, uh, the nervous system. Uh, it also, it also aligns with that definition, um, because charges, positive and negative charges in your cells are separated, right, by the membrane. When you bring these charges together, when you mix these charges and bring them together into the fold, there is a potential to, again, produce something great, right?

So for instance, we know that... the two ends of a battery are completely polarized, right? Positive charges are concentrated on one end. Negative charges are concentrated on the other end of the battery, okay? When you, again, conduct a current through them and when you ultimately bring...

the positive and the negative charges of the battery together, there's a potential to produce something great, right? There's a potential to power a machine, power some object, right? So the same logic applies to the concept of membrane potential for the traveling nerve impulse or action potential to influence the activity of neurons, right? That is what can be achieved, right, ultimately by bringing the positive and negative charges together.

Since we established that the nerve impulse or an action potential is a traveling wave of positive charges, right, it actually has the potential to briefly reverse the resting state of the neuron, right, so that the inside skews less negative. and eventually more and more positive, right? So, yeah, so as this traveling wave of positive charges called the nerve impulse is spreading and propagating through, let's say, the axon of your neuron, it has the potential to reverse the resting state, right? It has the potential to reverse how the charges were originally concentrated so that... The inside, the interior, skews less and less and less negative and more and more positive.

It has the potential to move or to change the membrane potential from negative towards positive. The resting membrane potential refers to how the charges were concentrated as at rest, what we establish here, where you have more negative on the inside, more negative charges on the inside. So resting membrane potential refers to this particular state where you have more negative charges on the inside. And as an action potential, as that wave of positive charge is spreading through, it has the ability or the potential to reverse.

the resting state from negative towards positive. Now in our kind of image in the right hand side here, this is an image of the sodium potassium pump that is found in all cell membranes, even the neuron, right? And this is by and large what maintains the the resting membrane potential, right?

So, so if we consider, or if we pose the question, why are cells more negative on the inside at rest, right? Why are resting cells more negative on the inside, right? And it's really, that is really all coordinated and determined and by the sodium potassium pump here, okay?

So the potassium, sodium potassium pump is located in all body cells and it's what maintains this negative resting state, okay? And ultimately, for every three positive sodium ions that are pumped out, only two positive potassium ions are pumped in. So that yields a net negative one charge.

And so this is really what is keeping the resting. membrane potential negative, the sodium potassium pump.

Transcript for:Overview of Nervous System Fundamentals

Transcript for:
Overview of Nervous System Fundamentals