1a.6 Nervous System PDF

[00:00:06] >> This session takes an overview of the entire nervous system. So we're going to start with really overviewing everything, thinking about central nervous system and peripheral nervous system. The rest of today's talk will really focus more on the peripheral nervous system with attention...

[00:00:06] >> This session takes an overview of the entire nervous system. So we're going to start with really overviewing everything, thinking about central nervous system and peripheral nervous system. The rest of today's talk will really focus more on the peripheral nervous system with attention to the spinal nerves, the peripheral nerves, and the nerve plexuses. [00:00:29] Much more detail on all of these as we continue in anatomy, and much, much more on the central nervous system when you do neuroscience To overview the entire nervous system, we have to talk about the central nervous system, or CNS, which includes the brain and the spinal cord. [00:00:55] As well as the peripheral nervous system, which includes everything from the spinal nerve roots as well as the cranial nerves. And then we also have the autonomic nervous system as part of the peripheral nervous system. The autonomic nervous system is what runs our essentially unconscious functions, helping to maintain blood pressure, helping it to manage thermoregulation, managing your digestion. [00:01:23] More on the autonomic nervous system will come up as we talk about the viscera as well as in your physiology course. We will spend most of this anatomy course looking at portions of the peripheral nervous system when we talk about nerves. Everything starting from the spinal nerve root, including the anterior or motor roots and posterior or sensory roots, as they come together to make a single spinal nerve and then heading out into plexuses and other nerves. [00:01:59] We will see sensory nerves that carry sensation of different types from the periphery towards the spinal cord. We will also see our motor nerves, which help to carry those motor impulses out to our skeletal muscle. As well as, as we mentioned, these autonomic nerves that will help to regulate body functions. [00:02:23] And again, more on those within the physiology and in the viscera. As we progress through each body region, we will be talking about the spinal nerves and peripheral nerves that address that body area. One key component to know about our spinal nerves and our peripheral nerves is that any nerve is actually a bundle of many, many nerve fibers. [00:02:54] A single nerve fiber is a single axon, and that axon is covered with a myelin sheath that serves an insulating property. And then that single axon and myelin sheath are covered by what is called endoneurium. Then if you take a whole bunch of those units, so axon myelin sheath covered in endoderium, and we bundle them together, we then call it a fascicle. [00:03:25] So a fascicle is a bundle of neurons, and that is then covered with another connective tissue called the perineurium. Finally, to actually form a named spinal nerve or peripheral nerve root, we then take a whole bunch of these fascicles, bundle them together, and cover them with another connective tissue called the epineurium. [00:03:49] This is really important to understand because while a spinal nerve may look like a single structure, it is actually a bundle of a vast number of nerve axons. And this allows a single spinal nerve to then branch out and engage in nerve plexuses and also to provide innervation to many different areas. [00:04:16] When we talk about the organization of a spinal nerve, most of the time we are going to talk about the mixed spinal nerve. But we also have to be aware that very close to where that mixed spinal nerve emerges from the vertebrae, there are two little branches called an anterior primary ramus, or an APR, and a posterior primary ramus. [00:04:43] We will talk more about those in just a moment. As a spinal nerve branches off of the spinal cord, there are multiple components we need to be aware of. First of all, the posterior root of a spinal nerve actually comes from a collection of posterior rootlets, very small nerve fascicles that are coming directly attached to the spinal cord. [00:05:13] The anterior or motor root has the same structure. There are many rootlets coming off of the anterior horn of the spinal cord, so many fascicles that come together to make the one anterior motor root. So this is the first step we need to be really aware of. When we talk about a root of the spinal nerve, we talk about a mixed spinal nerve, which is the area once the two roots come together. [00:05:45] So when we talk about a spinal nerve or a nerve root, we're really talking about the component that has both motor and sensory. But before that spinal nerve connects to the spinal cord, there are two components, that anterior root, which are the motor fibers taking signals away from the spinal cord. [00:06:06] And then there is the posterior root, which are the sensory fibers that are returning to the spinal cord. And you may recall from previous courses, the posterior root has a spinal root ganglion. A ganglion is just a name for a collection of nerve cell bodies outside of the central nervous system. [00:06:29] So essentially, what we see is that the sensory nerve cell bodies exist within the posterior root, whereas the motor nerve cell bodies actually live in the anterior horn of the spinal cord. Obviously much more on that with your neuroscience. So once this anterior and posterior root come together to make the mixed spinal nerve, we actually have one more set of branching. [00:07:02] I'm going to clear those highlights so that we can better see the next step here, which is terms that you may not be familiar with, an anterior ramus and a posterior ramus. The majority of the spinal nerve fibers travel out this anterior ramus. So this is what's going to go into your plexuses to your limbs to the anterior part of the trunk to most of the body. [00:07:33] A very small portion of the axons or nerves are going to peel off in this posterior ramus. We will learn a lot more about the posterior ramus when we talk about the back muscles, because the posterior ramus really only serves two major purposes. It takes motor innervation to the intrinsic back muscles, things like your erector Spinae, and it brings the sensory information from the direct back of the body, that's it. [00:08:09] Everything else, anything in your arms and legs, anything on your abdomen, that is all going to be innervated by anterior rami. Now I know this can be a very challenging concept so we will definitely spend time taking this apart and digging in a little more deeply. As a physical therapist, you are going to find that knowledge of this spinal nerves is incredibly important to your practice no matter what area you are in. [00:08:41] And something that you will be learning about fairly soon in your basic courses is things about testing sensory information. One area of which we will talk about the dermatomes. So we talked about how a spinal nerve has sensory fibers that return to the spinal cord through the posterior route. [00:09:03] Well, all of these sensory fibers are carrying sensation from some specific region of the body. And the area of the body that corresponds to sensory signals at one spinal level is what's called a dermatome. We will look at this more on the next slide. We have a similar concept for motor output, so all of the areas that are innervated by a single spinal level are referred to as that myotome. [00:09:35] These terms will become more comfortable as we move through the body. And look at specific dermatomes and myotomes, such as looking at the L4 dermatome and the L4 myotome when we get to talking about the thigh. This image illustrates how the dermatomes apply to the body. So I'm going to give some specific examples from this image to help make sense out of it. [00:10:04] And know that you do not need to memorize this entire map right now, as you we're going to be learning a lot more as we visit each region of the body. So for example, you see this light green down here that says L3 and it shows up a patch that kind of comes down the medial thigh, the distal anterior thigh and around the knee. [00:10:26] What that tells us is if you touch yourself somewhere in that area and I welcome you to put your hand on your knee right now. That sensation is traveling up a nerve back to your back to your spine, and then it enters the spinal cord at the L3 level. [00:10:46] So that's what a dermatome tells us. It tells us if you touch your body or somebody touches your body or you poke it, pain as well, that sensation will be carried up some nerves, we will talk about that. As we get to each region, and then it will enter the spine at a certain level. [00:11:06] So for example, up here at C4, what that's telling us is that if you touch yourself just on the top of your shoulder, or if somebody pokes you on the top of your shoulder. That message is going to travel up the nerves and it will enter the spinal cord at the level of C4. [00:11:26] Now one piece to remember here is that in the cervical spine, those nerve roots enter above the named vertebrae. So C4 for example will enter between the C3 and C4 vertebrae. Once we get to the thoracic and lumbar regions, that nerve root will enter below its named vertebrae. [00:11:51] So when we talked about touching your knee, that L3 nerve root is going to enter below the L3 vertebrae above the L4. Once again, we will be diving into the vertebrae, the nerve roots, and all of that very soon, and we'll get much more details. This image represents another way that we often represent the dermatomes, and this addresses some sensory key points. [00:12:20] These are the points that are defined when we especially test during spinal cord injury. But what I will tell you is I find it much easier to remember a single point than to try to remember this random patch. So as you work to learn the dermatomes, and we will be doing some for the trunk but they will get more complicated as we get to lower and upper extremity. [00:12:45] I do recommend coming back to this map, because what this will show you then is that there is for example, a single point that is a really critical point for that dermatome. And if you can remember that and they usually line up with certain anatomical landmarks. It will help you in learning the map of the dermatomes. [00:13:09] Similar to dermatomes, each spinal nerve root also carries motor fibers from that anterior root that will go out to many different muscles. We will be learning all of the nerve roots associated with the innervation of each muscle that we learn. But what we can learn about our myotomes is that there usually are about two or three specific nerve roots that will help with each action. [00:13:40] So, this example shows the upper extremity motions, for example, shoulder and elbow. And in this case, what you see noted about elbow flexion is that it is motor nerves from the C5 and C6 levels that primarily go to your elbow flexors, right, muscles like your biceps and your brachialis. [00:14:03] It is nerves with anterior horn cells in the levels C6 and C7. So motor anterior roots coming from the C6 and C7 that go to your triceps, which are going to help with elbow extension. This may feel a little overwhelming right now, but we will be doing many different exercises as we move through each region of the body to learn what spinal nerve root levels contribute to innervation of each muscle, As PTS life in the nervous system can get a little complicated, and we have to think about the difference between our named peripheral nerves and those spinal nerve roots that we just talked about. [00:14:55] So rather than just having a single nerve coming from the spinal cord going directly out to the muscles, usually a spinal nerve root will split and contribute to multiple different peripheral nerves. As well, each of those peripheral nerves likely gets input from multiple spinal nerves. So we see an interesting sort of branching and rejoining pattern. [00:15:23] We will see this very specifically in our lumbosacral plexus for the lower extremity and the brachial plexus of the upper extremity. What this means is as we get especially out into the limbs, most muscles will be interveted by a single named peripheral nerve. However, that single peripheral nerve is receiving motor input from multiple nerve roots. [00:15:53] An example I like to use is your quadriceps. Most of us are familiar with our quadriceps and you may not know. Know yet, but you will learn soon that the primary peripheral nerve to the quadriceps is the femoral nerve. So the femoral nerve is like, I think about nerves like wiring, right? [00:16:13] So if your body was your house, the nerves are all the wiring in your house. So the femoral nerve is like the big bunch of wires that carry those impulses to your quads. Now the femoral nerve gets input from both the L3 and the L4 nerve root levels. [00:16:32] We will come to explore how this differentiation is really critical in our physical therapy thinking. But for now, I want you to really remember that any spinal nerve root is really a bundle of a whole bunch of nerve axons, and so is any named peripheral nerve. The named peripheral nerves are also a big bundle of lots of axons, which explains how fibers can come and go in multiple directions. [00:17:04] Now, just as we talked about a dermatome going with a spinal nerve root, each named peripheral nerve will also provide sensation to a certain area of the skin. These will overlap with the dermatome because each peripheral nerve is carrying these sensations back to the spinal nerve roots. So, for example, if we talk about that femoral nerve that we talked about for the quads, it also provides sensation to the anterior thigh. [00:17:41] Well, we mentioned that that femoral nerve carries L3 and L4 nerve roots, and you will notice that that anterior thigh overlaps with the L3 and L4 dermatomes. This can be a very confusing concepts, so we're going to spend a lot of time visiting and revisiting this idea of a named peripheral nerve versus a spinal nerve root. [00:18:10] They present over here another example from the upper extremity, which is looking at the C6 nerve root versus the musculocutaneous nerve. So how we look at this is the fact that the C6 nerve root goes to a bunch of different muscle areas. It will help to innovate elbow flexion, it will also help with wrist extension with shoulder flexion and shoulder extension. [00:18:41] The musculocutaneous nerve is going to go very specifically to the elbow flexors. So what we see is that the C6 is coming into the musculocutaneous nerve, but C6 also contributes to multiple other nerves. As we visit the brachial plexus much later in this course, you will see this map out, and by that time, these terms are going to be a lot more comfortable. [00:19:10] Once again, when it comes to sensory, we also see this relationship between C6, which gives sensation to the lateral forearm and the lateral hand. And we see overlap to musculocutaneous which gives sensation to the lateral forearm. So what it is, is that dermatomes and the sensory distributions of peripheral nerves are two different ways of mapping the skin sensation from the body. [00:19:40] As PTs, it's going to be important to know both of those maps, and we will be chewing this apart many, many times because there are some very complex concepts that are captured within this single slide. So if it feels overwhelming, don't worry, you're in good company. As I mentioned on the previous slide, part of the relationship between your spinal nerve roots which are coming right off the spinal cord at the spine, and your named peripheral nerves, like the median nerve or the musculocutaneous nerve, this relationship happens through nerve plexuses. [00:20:26] So what we see is that your nerve roots, and this is very specifically those anterior rami of the nerve roots, participate in a plexus. And many of you will be familiar from previous courses with the brachial plexus, which makes this complicated tangle of nerves that come together, and split apart, and come together. [00:20:49] But one of the things that you can do with the brachial plexus is you can trace from a specific peripheral nerve back to the nerve roots that it comes from. And so as a result as we mentioned on the previous slide, this is why we see a very clear relationship between the motor innervation of the C5 and C6 nerve roots and the musculocutaneous nerve. [00:21:17] Very similarly, we see the overlap between the C6 dermatome, that sensory distribution that goes back to the C6 nerve root, and the sensory distribution of the musculocutaneous nerve. This concept about spinal nerve roots, peripheral nerves, nerve plexuses, this is some something that we will continue to visit and revisit because it is a very challenging concept. [00:21:47] So we're wrapping up with our review of the nervous system. So we covered a lot in a very short time, starting with the fact that there's the central nervous system, the brain and spinal cord. And then there's the peripheral nervous system, the everything else, which includes your spinal nerve roots as well as your cranial nerves, and then everything that comes from those spinal nerve roots. [00:22:12] Remembering that the autonomic nervous system is also a component that's going to help to drive those essentially automatic functions, things like your heart rate, your digestion, your blood vessel dilation, or contraction. Much more on that as we move along. And then we spent a lot of time looking at the structure of spinal nerves and recognizing that there is a ventral or motor root and a dorsal or sensory root that come together to make this spinal root. [00:22:47] And then if that wasn't complicated enough, after they've come together to be a spinal root, then you have two branches that come off, an anterior primary ramus that's going to go to most of the body. And a little tiny, as you see over here, this posterior primary ramus is tiny that heads to the back muscles and some sensation on the back. [00:23:12] Then we also discussed how a dermatome is the sensory distribution of a single spinal nerve. And the myotome represents the muscles that are innervated by a single spinal nerve. And finally, we got into this very confusing world of looking at how do our named peripheral nerves, things like the median nerve, or the femoral nerve relate to our spinal nerves and how do the plexuses play in. [00:23:46] Obviously, these are very complex concepts that we are going to cover in much more detail as we get to the specifics in each body region.