Lecture 18 - Spinal Cord and Spinal Nerves PDF

**[NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling.]** Lecture 18 - Spinal Cord and Spinal Nerves [Video 1] Introduction Welcome to the next online lecture. In this lecture we\'re going to be looking at the spinal cord and the spinal ne...

**[NOTE: Transcripts are made from the auto-generated Lecture Captions, so are not edited for grammar/spelling.]** Lecture 18 - Spinal Cord and Spinal Nerves [Video 1] Introduction Welcome to the next online lecture. In this lecture we\'re going to be looking at the spinal cord and the spinal nerves. We\'re are going to begin by looking at the structure of the spinal cord and the protective coverings for the spinal cord. We\'ll then follow by looking at the arrangement of the spinal nerves and the various plexuses that they form just outside the spinal chord, along with the protective coverings for the spinal nerves. So let\'s take a look. Slide 1 The material for this online lecture module can be found in chapter 12 of your textbook on the spinal cord and spinal nerves. Slide 2 Now, while most of you are somewhat familiar with the spinal cord, we\'re going to start from the beginning. The spinal cord has a long structure comprised of nervous tissue, so both neurons and their associated glial cells or those supporting cells in nervous tissue. It\'s represented in this posterior view of the torso by this pink line. It\'s part of the central nervous system and it\'s the portion that connects the brain to the peripheral nervous system. The spinal nerves are shown here extending from the spinal cord out towards the periphery. In the left-hand image you can see the bony structures which surround the spinal cord. These bony structures are called the vertebrae, and they\'re stacked on one another to create what\'s called the vertebral column. This image is showing a lateral view of the vertebral column. The smooth side of the vertebrae is the anterior side, and the side with the spiny processes is the posterior side of the vertebral column. In the region where you can see the spiny processes, each individual vertebra has an opening called the vertebral foramen, and it\'s basically a hole within this region that faces superior, inferior where the spiny processes are. When you stack the vertebrae on top of each other, the successive holes create a canal, and this canal is called the vertebral canal, and this is where the spinal cord itself is located. Notice that the vertebral column has several curves in it. Each curve is divided into a region. The regions than dictate the names of the vertebrae in that part of the vertebral column, as well as the spinal nerves that exit that part of the vertebral column. The superior curve is called the cervical region. Interior to that we have the thoracic region, followed by the lumbar region, and then the sacral and coccygeal regions. So the vertebrae in those areas will be called the cervical vertebrae, the thoracic vertebrae, the lumbar vertebrae, et cetera. And the spinal nerves will be called the cervical nerves, the thoracic nerves and so on. So while the vertebrae are important for providing protection for this very delicate tissue of the spinal cord, they also create a bit of a challenge because the vertebral canal curves like this, the spinal cord must also curve to maintain its position in the middle of the vertebral column. Part of how we keep the spinal cord in the middle of the vertebral column is by using connective tissue layers that surround the spinal cord, the meninges, which we\'ll be talking about more in a moment. I also want to highlight another feature of their vertebral column that\'s on this image because you can see them really well and that is the spaces or openings between the vertebrae. These openings are called the intervertebral foramina. This is where the spinal nerves will exit the vertebral canal and extend out to the periphery. So you can see in the other image here that the first spinal nerve of each region has been labeled for you. So cervical nerve, one, thoracic nerve one, lumbar one, and sacral one. The numbering is based on which level of the vertebral canal they exit through the foramina, not the level at which they exit the spinal cord. This makes sense because the spinal cord actually has ended by the time it reaches the top of vertebrae L2. The vertebral canal is actually much longer than the spinal cord itself. Part of the reason the spinal cord is so much shorter than the vertebral column is that there\'s a difference in the speed of growth of these structures during development. The spinal cord stops growing in length around age four or five, whereas the vertebral column continues to elongate until early adulthood, basically until you reach your maximum height anywhere between 18 and 22 years old, depending on the individual and their genetics and their diet and so on. This difference in length will be important when we start to talk about the anatomy of these structures. So we\'ll talk more about the cauda equina in the filum terminale in a moment. Slide 3 The spinal cord is your communication link between the brain and the peripheral nervous system. If you think of the nervous system as a roadmap, the spinal cord is this sort of superhighway where all the small roads converge in an organized way to get to the brain or other parts of the body. It\'s providing a pathway for nerve impulses. Sensory information will gather from the periphery and enter the spinal cord and motor information will go out to the muscles glands from the spinal cord. Another job is to integrate incoming and outgoing information and produce responses. So it helps to determine where sensory information, either from our internal or external environment needs to go after it\'s been collected from the periphery and can also create motor responses. The spinal chord extends from the foramen magnum down to the top of the second lumbar vertebrae. Now I know we haven\'t talked about any bony structures yet, but the foramen magnum is a very large, circular shaped hole in the base of the skull, specifically in the bone called the occipital bone. So the spinal cord is continuous with the brainstem. So where that medulla oblongata ends at that level of the foramen magnum, the spinal cord will begin. If you\'re trying to landmark Where L2 or the second lumbar vertebrae is. It\'s about the level of the most inferior rib on your rib cage. So even though the spinal cord ends at L2, the spinal nerves still continue down the rest of the vertebral canal and exit at their respective vertebral level. The spinal cord and spinal nerves have cervical, thoracic, lumbar, and sacral sections, just like I\'ve already mentioned for the vertebral column and 31 pairs of spinal nerves will exit the spinal cord in total. Slide 4 On this image, you can see the brain and the spinal cord and just as I mentioned before, the spinal cord is going to go from the foramen magnum to the level of the second lumbar vertebrae. While the spinal cord is somewhat circular in cross-section, it\'s not quite round. Instead it slightly flattened on one side and the diameter is not uniform along its length. There\'s actually two enlargements that you can see here on the diagram. The first one is the cervical enlargement. The cervical enlargement occurs between vertebrae C4 and T1. We also have the lumbar sacral enlargement. This occurs between the level of T9 and T12. These enlargements in the spinal cord are associated with the extra nervous tissue that\'s needed to supply the additional structures of the upper and lower limbs. They\'ve got extra muscles, extra bones and other tissues that need to be supplied with neurons. So in those areas of the spinal cord, it\'s become enlarged in order to accommodate all of those extra structures. So here you can see the cervical enlargement as well as the lumbosacral enlargement. Just inferior to the lumbosacral and enlargement, the spinal cord tapers into a cone like structure known as the conus medullaris. That\'s the tip or end of the spinal cord. Even though the spinal cord has ended the nerves of the lumbosacral enlargement and the conus medullaris continue down the vertebral canal and they exit at their respective foramina. So they\'re either exiting via the intervertebral foramina we just talked about, or through holes in the sacral bone called the sacral foramina. The hair-like nature of these nerves coming out of the bottom of the spinal cord is referred to as the cauda equina because it looks sort of like a horses tail. You\'ll also notice here on the diagram there\'s another structure called the filum terminale. It\'s an extension of the pia mater connective tissue covering around the spinal cord and it anchors the spinal cord to the inferior part of the vertebral column, called the coccyx. It acts as a tether for the spinal cord to stop any movement in the superior direction. Also on here you can see the peripheral nerves exiting the spinal cord. They begin as roots of the spinal nerves, and then they merge together to form the spinal nerves. Slide 5 Like the brain, the spinal cord is surrounded by layers of connective tissue meninges to provide some level of protection. The meninges are continuous with the meninges of the brain. So you can sort of think of them as a sack or covering that covers the brain and then is continuous down around the spinal cord. In the image here we have a cross section of a portion of the spinal cord where you can see the spinal nerves exiting the spinal cord here in yellow. Now, the dura mater is the most superficial of the meningeal layers, and it\'s made up of dense, irregular connective tissue. It\'s also the thickest and strongest of the meningeal layers. One thing that\'s different about the dura mater and this spinal cord compared to what we see in the brain, in the cranial cavity, is there\'s an actual space between the dura mater and the bone. Specifically that connective tissue on the layer of the bone is called the periosteum. So in the spinal cord there\'s actually a space between the periosteum and the dura mater. So remember in the brain we have the periosteal layer and it closely adhered with the meningeal layer, so it essentially became one functional layer and they only separated at the dural venous sinuses. Well in the spinal cord, they\'re separated by space. So the periosteal layer does not exist essentially as part of the meningies in the spinal cord, instead, it\'s a connective tissue layer that\'s just attached onto the bone. So the dura mater of the spinal cord is simply the same as the meningeal layer that we see in the brain. So that\'s the only part that\'s really continuous down the spinal cord for the dura mater. So in the spinal cord there\'s actually a space between the periosteum and the dura mater and this space is referred to as the epidural space. In the vertebral canal, the space is filled with fat, blood vessels and areolar connective tissue. It helps to protect the spinal cord as well as holding the spinal cord in place. Remember, we have to hold it within the centre of the canal as it moves through the various curves in the vertebral column. You may have heard of the term epidural before. So this is when you have anesthesia injected into this space to block pain receptors exiting certain levels of the spinal cord. Women sometimes get an epidural during childbirth. Since it targets a specific area, it should only affect the spinal nerves that are exiting that region. So that\'s how it provides pain relief during delivery, but the mother remains awake. Deep to our dura mater is the next meningeal layer called the arachnoid mater. It\'s a thin, avascular layer, that means it doesn\'t have a blood supply or blood vessels through the layer itself. It contains simple squamous epithelial cells and a delicate network of collagen and elastin fibers. It\'s named arachnoid mater because the fibers are arranged into a web-like pattern. So it\'s very similar to what we see in the brain. Between the dura mater and the arachnoid mater is a space called the subdural space. It\'s a very small space and contains only a small amount of serous fluid. Deep to the arachnoid mater, we had the final layer of the meninges, the pia mater. This contains our blood vessels which supply the spinal cord. It tightly adheres to the spinal cord itself, so it covers every little bump and groove in the spinal cord. The pia mater has small extensions of itself that move towards the dura mater and connect it to the dura mater. They occur between each of the routes that exit the spinal cord. As you can see in this image, these extensions are referred to as the denticulate ligaments. These ligaments also help anchor the spinal cord laterally to prevent side to side movement. And again, as we mentioned before, the filum terminale is another extension of the pia mater, which extends to the coccyx, and along with the attachment of the meninges to the foramen magnum or the base of the skull, it helps to anchor the spinal cord longitudinally and prevents superior and inferior movement. So the pia mater is very important for maintaining the spinal cord within the vertebral canal. Between the arachnoid and pia mater is the subarachnoid space. So this is continuous with the subarachnoid space of the cranial cavity and contains that cerebrospinal fluid. There, cerebrospinal fluid as a couple of jobs, it cushions the spinal cord and allows for protection, and it also delivers nutrients and removes waste products. Slide 6 So this is looking at another view of meningies as a cross section. You can see this is the posterior side and this is an anterior side. Now, even if you didn\'t have the directional information on a given diagram, you should be able to landmark specific features of the bones or the spinal cord itself to determine what direction you may be looking at it. For example, the body of the vertebrae is on the anterior side and the spinous process of the vertebrae is on the posterior side of the body. After we finish the bones, you\'ll also be able to tell by the sharpness of the spinous process what level of the vertebral column you\'re actually in. So here you can see the spinal cord and the spinal nerve that\'s exited through those intervertebral foramina, that space that exits out towards the periphery. You can also see the ventral root and the dorsal root of the spinal nerve, which we\'ll talk more about in a moment, along with that dorsal root ganglion that we\'ve looked at in some previous images. So surrounding the bone itself, as I mentioned, is a connective tissue layer called the periosteum. And then deep to that is where we had that epidural space. So the epidural space, as I mentioned, it\'s filled with blood vessels fat and loose connective tissue, and it\'s going to provide some cushioning for the spinal cord in the meninges. Then it shows the dura mater, as well as the arachnoid mater and the subdural space between those, that space that\'s in between there is so small that it\'s only filled with the tiniest bit of serous fluid within it and if you actually had an anatomical specimen, it\'d be very hard to see the difference between those two layers. The space just deep to that is the subarachnoid space and that space is going to be filled with cerebrospinal fluid. And then very tight to our spinal cord is where you have the pia mater, and then you can see those denticulate ligaments that are coming off the pia mater and attaching to a space that\'s actually just inferior to this spinal nerve that\'s exiting the spinal cord. So let\'s pause here and try some practice questions. [Video 2] Slide 7 So now we\'re going to look at the internal anatomy of the spinal cord in a little bit more detail. So a lot of what I\'m going to say here is actually written on the following slide just so that you know, when you\'re taking notes, the spinal cord has two major portions, an outer white matter portion and an inner gray matter portion. Even without the bony structures, you should be able to orient yourself to the direction of the spinal cord that you\'re looking at in a cross section based on a few features. So on the posterior side we have a narrow groove called the posterior median sulcus. On the anterior side of the spinal cord, we have a larger groove called the anterior median fissure. You can also notice that the sulcus in the anterior median fissure partially divide the spinal cord into two halves. Each half of the spinal cord is pretty much a mirror reflection, creating symmetry between the two sides. The white matter of our spinal cord contains myelinated axons, which run the length of the spinal cord and can either be a ascending or descending creating nerve tracks and then we have our nerve tracks grouped together in an organized way and they become what\'s known as columns. This allows for information to be transmitted in an organized fashion throughout the spinal cord. So you can see that we have three columns within the white matter. This is again mirrored on both sides of the spinal cord. There are the posterior, lateral, and anterior white columns. Within each of these columns, there will be groups of either a ascending or descending tracts that are bundled together because they have similar functions or serve a similar area of the body. Connecting the right and the left sides of the spinal cord are the white commissures. In the image here they\'ve labeled the anterior one, but there\'s a posterior one as well. This allows signals to move from one side of the body to the other. The gray matter of the spinal cord is in the central region of the spinal cord and is butterfly shaped or H shaped. It\'s composed of three horns, the posterior horn, the lateral horn, and the anterior horn. So just to recall, the grey matter is made up of neuron cell bodies, dendrites, supporting cells, axon terminals, and there\'s also some shorter in neurons that connect various parts of the spinal cord called interneurons. The horns of the gray matter are also highly organized, and this is based on nerve function. The lateral horns, as you can see here, are very small and are found only in the thoracic and lumbar regions of the spinal cord. This horn is associated with neurons of the autonomic nervous system, which we\'ll be talking about more in a couple of weeks. The posterior horn, which you can see here, is where the axons of the sensory neurons enter the spinal cord, and they can either synapse here with interneurons or they can combine with a nerve tract in the white matter and then a ascend or descend in the spinal cord depending on the function of that particular sensory neuron. Since sensory nerve cell bodies are found only in the dorsal root ganglion, so outside the central nervous system in this region right here, the cell bodies in the posterior horn are actually all the cell bodies of the interneurons that are going to transmit the information around in this gray matter area. The anterior horn, a much larger horn, is sometimes referred to as the motor horn, and its largest because it contains the cell bodies for the somatic motor neurons that go to skeletal muscles. Connecting the right and left sides of the spinal cord in the grey matter region is the gray commissures. In the middle of the gray commissures, we have what\'s called the central canal, and this central canal is continuous with the ventricles of the brain and it contains cerebrospinal fluid. In addition, at each spinal level, we have something called the rootlets that either leave or enter the spinal cord. They\'re part of the peripheral nervous system. On the anterior side we have what\'s referred to as the anterior or ventral root, and that\'s where all the motor neurons are going to exit the spinal cord. On the posterior side we have the posterior or dorsal root, which contains all of the sensory neurons that are entering into the spinal cord. Within the dorsal root, we had the dorsal root ganglion. Since almost all sensory neurons are pseudo-unipolar, their cell bodies are in a sort of extension off the axon and they get grouped together outside the central nervous system in this dorsal root ganglion. If you can see this bulge in one of the roots of the spinal cord, you know that\'s the dorsal side of the spinal cord. When the dorsal and ventral roots merged together. That\'s what\'s called a spinal nerve. So therefore, all spinal nerves have a mixed function or they have both sensory function as well as motor. Slide 8 So this slide is just summarizing some of the points I just made on the image. The white matter is divided into two halves, and within those halves we have three columns. Each column is subdivided into nerve tracks, also known as the fasciculi. The ascending and descending nerve tracks are myelinated axons of white matter. The gray matter is made up of posterior, anterior, and lateral horns. The lateral horns contain the cell bodies of the autonomic nervous system. The posterior horns are where the sensory neuron information is being brought into the spinal cord and it will synapse here with some interneurons or some white matter tracks that will carry the information through the spinal cord. And the anterior horn, also known as the motor horn, is where we have the cell bodies for all of our motor neurons that are part of the somatic motor nervous system. The white and gray commissures are axons that cross from one side of the spinal cord to the other, in both the gray and white matter areas and the central canal is in the center of the gray commissure, and that is continuous with the ventricles. Slide 9 So this image is really depicting how the peripheral neurons are interacting with the spinal cord. So here we can see this is our spinal nerve. And we also have a spinal nerve that sort of opened up so you can see it in the inside on this side. And then these are going to represent some sensory receptors that are found within the skin. So if we have some stimuli here activating these sensory receptors then the information is going to be brought back through these sensory neurons towards the spinal cord. Specifically the cell bodies are going to be in this bulge in the roots of the spinal cord, known as the dorsal root ganglion. And then the rest of the axon continues onto the spinal cord and enters via the dorsal roots of the spinal nerves. So that will enter into the posterior gray horn of the spinal cord. So as I said, the cell bodies are not located in this region from the sensory neurons, they\'re out here, but what we do have in this posterior gray horn are going to be some cell bodies of the interneurons. So here\'s one example of an interneuron here. This is going to be an interneuron that connects to a somatic motor nerve. So say we had a reflex where sensory information goes in and then a motor response goes out. It could go through that pathway. It can also exit the posterior gray horn and go into the white matter and go into a sensory ascending track, so it might bring the information towards the brain. It can also cross over, so this is an interneuron crossing over through that gray commissure and entering into the sensory ascending tract on the opposite side of the body as well. So you can see it\'s an interneuron here that\'s crossing over, but it\'s bringing the sensory information from one side of the body over to the other side of the body to move up the spinal cord in that direction. So again, it\'s just highlighting those cell bodies of those interneurons. Now on the motor side of things, of course, motor information is coming from the brain first and typically we\'re getting that from the regions of the cerebrum, the motor cortex. So that would come down in a descending tract in the white matter areas. And then the information enters into the anterior gray horn and it will synapse with the somatic motor neuron and remember, this is a single neuron system after we leave the spinal cord, so this somatic motor neuron will go all the way out to our target skeletal muscle. Again, all of our somatic motor neurons are going to be found in the anterior gray horn. So the information comes down the spinal cord. You have a synapse here and then all the cell bodies of all the somatic neurons will be found in these anterior gray horns. So as I said, the somatic motor neurons are going to exit the anterior gray horn and enter into the anterior or ventral root of the spinal nerve. And their target is going to be skeletal muscle because it\'s the somatic motor nervous system. The lateral gray horn is where we\'re going to have the motor neurons for the autonomic nervous system. So you can see that\'s highlighted here in green. So those are the cell bodies located there. And then they\'re also going to exit via the anterior or ventral root of the spinal nerve. And eventually these are going to synapse out in the body at the ganglion, that\'s the autonomic ganglion. So remember this is a two neuron system. So after the synapse, it will then go out to the target tissues, which are going to be their cardiac muscle, smooth muscle or glands. This image over here is really just highlighting how we can clump together many of the axons that are ascending or descending in the spinal cord and how these create these regions known as the fasciculi. So you can see that a ascending nerve tracks here are in green and descending nerve tracks are in purple, and we actually have mirror images on the right and the left halves of the spinal cord. So you can see these green areas where all the axons are bundled together and you can also see there\'s certain subdivisions of these regions and that\'s really lumping together axons that serve similar areas of the body. So for example, if you had a series of sensory information coming from an upper limb, it might lump them all together in one area of the spinal cord. And then of course we have the descending tracts as well. So those are shown in purple and this is going to be motor information coming to that anterior gray horn region. So you can see how it just packages all of those together into bundles so that we can more highly organized the information as it\'s going up and down the spinal cord. As you can imagine, because the regions of the spinal cord are organized in this way, damage to any particular area of the spinal cord will result in a characteristic loss of some sort of sensation or motor control. We could have damage from acute spinal cord injury caused by trauma or even a tumor can cause a spinal cord injury. If it happens in a certain region, it will only impact potentially motor function or sensory function. Or it could be specific to an area of the body or a specific function of the body because the tracks in the white matter are grouped together and organized in this fashion. There are couple of interesting diseases associated with the anterior horn of the spinal cord and they are in part caused by the degeneration of the gray matter within that horn. The first one is polio. You may have heard of polio as everyone typically gets vaccinated as a child. Polio is caused by a virus called the polio virus and the virus attacks the cell bodies of the motor neurons that are in the anterior horn and some of the cranial nerve cell bodies. So it results in motor loss and eventually paralysis. Another disease that you may have heard about also has a degeneration of the anterior horn of the spinal cord, as well as the motor axons descending the spinal cord, this is ALS or Amyotrophic Lateral Sclerosis, also referred to as Lou Gehrig\'s disease. So it also attacks the motor neuron cell bodies in the brain and the spinal cord and eventually the people lose their ability to do things like speaking, swallowing, and breathing and typically within five years, the disease has progressed to the point where the person can no longer survive. Slide 10 Again, this slide is summarizing some of the points that were made on the previous image. So the internal anatomy of the spinal cord allows sensory and motor information to be processed in an organized way. Sensory neurons are going to pass into the posterior horn through the dorsal root, synapse with interneurons or enter into the white matter and ascend or descend the spinal cord. Somatic motor neurons are found in the anterior horn and exit via the ventral root. And then of course, our autonomic neurons are going to be found in the lateral horn and also exit via the ventral root. So let\'s pause here and try some more practice questions. [Animation] No caption file [Video 3] Slide 11 So that was just a short little introductory animation on the spinal nerves. So now we\'re moving on to the peripheral nervous system by looking at the spinal nerves. There are 31 pairs of spinal nerves. And as I mentioned earlier, they\'re named and numbered for the region of the vertebral column where they exit the vertebral canal. The first pair exit the vertebral canal between the skull and the first cervical vertebra. So it\'s actually superior to the initial vertebrae in the vertebral column. That one is referred to as C1. Then as we begin to move down the vertebral column, the spinal nerves, our name for the vertebrae that are beneath them. So C1 would be above vertebrae, C1, below it, C2, would be above vertebra C2, and so on and so on. Until we get to C7. There are only seven cervical vertebrae, but there are eight cervical nerves. So the eight cervical nerve actually exits below C7, and then from then on, the names of the spinal nerves are represented as the vertebrae that are superior to them. So at vertebrae C7, There are actually spinal nerve C7 above it and spinal nerve C8 below it. The next vertebrae is the thoracic vertebrae number one, T1. So it\'s nerve T1 will exit below that. The numbering then continues down the spinal cord for the rest of the spinal nerves. We do have four pairs of spinal nerves that exit in the sacral foramina. Those are the holes within this sacrum that I mentioned before. Originally the sacrum was individual bones, but as we develop, the bones fuse together. The rest of the spinal nerves exit through the intervertebral foramina or those spaces that I pointed out earlier between each of the vertebrae on the posterior side of the vertebral column. This leaves us with 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Slide 12 These diagrams show the numbers of the spinal nerves as they move from most superior to most inferior on both images. The image on the right also colour codes various regions of the spinal nerves. So we had 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. The image on the left also shows the various plexuses that we\'re going to be covering. We will cover them in more detail in a moment, but you can see here is the cervical plexus, the brachial plexus, the lumbar and sacral plexuses, which also get lumped together to form the lumbosacral plexus, and we also have the coccygeal plexus. It\'s also highlighting the spinal nerves that are part of each of the plexus underneath their names. On the right hand image. In addition to numbering the pairs of spinal nerves, it also gives you a breakdown of the general function in each of those various regions. You can see how A person that has a spinal cord injury, the level of injury is really important for determining the impact on the function that the person will have. This in addition to what we talked about earlier with the columns in the spinal cord being damaged. The function will also be determined by whether or not the person has a complete spinal cord injury where they\'ve actually transacted or cut the cord entirely or a partial spinal cord injury, where only part of the court itself has been damaged. This is part of the reason that spinal cord injury loss of functions vary so much between people. If you had a complete transection in the cervical region, it\'s going to result in quadriplegia, or loss-of-function in both the upper limbs as well as the lower limbs, and it may also require the individual to have a ventilator for breathing if the diaphragm connection has been affected, an injury in the upper thoracic region would result in becoming paraplegic. So you would still have the use of your upper limbs, but you would lose lower limb function. Slide 13 Much like this structures in the central nervous system, which have protection of bony structures, meninges and cerebral spinal fluid. That peripheral nervous system also needs protection from the delicate axons that are traveling to various parts of the body. The axons in a nerve are bundled in parallel, and they are typically grouped together by either motor or sensory function. Surrounding each axon, we had the Schwann cells that make up the myelin sheaths. Each axon is then surrounded by three connective tissue layers. Immediately around the individual myelinated axons, we have a layer called the endoneurium, it separates the axons from each other. Then these individual axons are grouped together into bundles called fascicles, which are held together by the next connective tissue layer called the perineurium. Surrounding the fascicles are arteries and veins, which provide blood supply to our nervous tissue and there\'s also adipose or fat deposits and loose connective tissue in this area as well. Then the groups of fascicles are bundled together to form the spinal nerve, which is surrounded by the most superficial of the connective tissue layers called the epineurium, which is a dense connective tissue layer and is actually continuous with the dura mater of the spinal cord. So imagine we have the dura mater surrounding the brain coming down the spinal cord and then the layer of the dura mater actually becomes the epineurium as it extends out over the peripheral nerves. You can think of it as like the sleeves of a coat, if the body of the coat is the dura mater protecting the brain and the spinal cord than the epineurium would be the sleeves of the coat protecting the peripheral nerves. Slide 14 As the axons leave the spinal cord or just before they enter, they form smaller bundles called rootlets. These rootlets are on both the dorsal and ventral surfaces of the spinal cord. And about six to eight rootlets will merge together to form either the dorsal or the ventral roots. These roots then pass through the subarachnoid space. They pierce the arachnoid mater and merged together to form what we refer to as the spinal nerve. Each dorsal root, as I\'ve mentioned previously many times now, contains a ganglion. Slide 15 Next to warn you look at the branches of the spinal nerves. So on this image we can see many of the structures that we\'ve just learned. We can see the rootlets exiting on both the anterior and posterior side of the spinal cord. Those rootlets will merge to form the ventral root of the spinal nerve as well as the dorsal root. And in the dorsal root we have the dorsal root ganglion. Now when those two roots come together, that\'s what forms the spinal nerve. Shortly after the ventral and dorsal roots come together to form the spinal nerve, the nerve then splits into two main branches called rami or singular ramus. The posterior or dorsal rami of the spinal nerve is innervating the dorsal trunk muscles or the deep muscles of the back responsible for movements of the vertebral column, and also some of the skin and connective tissue on the back. The other branch is the anterior or ventral rami of the spinal nerve. The destination of these branches depend on where you are in the vertebral column, as we\'ll see on the next slide. So here we can see in this image we\'re going to have a region of the spinal cord that\'s in the thoracic region. So the ventral rami of these nerves are going to the intercostal muscles, so these are the muscles that are in between your ribs. Another thing that I want to just highlight quickly on this image is this structure right here. These are ganglions of the sympathetic nervous system. So we are going to be talking about these more a little bit later, but you can see how they go superior, inferior along the length of the spinal cord. So we\'ll talk about these sympathetic chains a little bit later, but I just wanted to highlight here what they look like. Slide 16 So again, the dorsal ramus innervates the muscles of the vertebral column. The ventral rami in the thoracic region are actually called the intercostal nerves, the intercostal nerves are innervating the intercostal muscles which are involved in breathing, and they\'ll also innervate the skin of the thoracic region. The ventral rami in the rest of the spinal nerves become the roots of plexuses, which are essentially an intermingling of nerves which acts at the spinal cord. There are five plexuses in total, the ventral rami of C1 to C4 form the cervical plexus. The ventral rami of C5 to T1 form the brachial plexus. The ventral rami of L1 to L4 form the lumbar plexus and the ventral rami of L4 to S4 form the sacral plexus, and then the final plexus is the ventral rami of S5 and the coccygeal nerve. So picture the plexuses as nerves coming out of the spinal cord from several different levels, but redistributing their axons after leaving the spinal cord so that the nerves that actually reach the target tissue are carrying impulses from several different levels of the spinal cord. So let\'s pause here again and do some more practice questions and then we\'ll come back and look at the various plexuses. [Video 4] Slide 17 The most superior plexus is the cervical plexus. It\'s one of the smaller plexuses with contributions or roots from the ventral rami of spinal nerves C1 to C4 shown here in red. The rami then branch off and intermingle and merge with other branches within the plexus itself. Some of them also run parallel to some of the cranial nerves, as we can see here with the hypoglossal nerve. The main functions of the nerves from the cervical plexus are to innervate the muscles and skin of the head and neck regions, as well as the superior parts of the shoulders and chest. One important nerve that originates as part of the cervical plexus is called the phrenic nerve. The phrenic nerve arises from the cervical plexus with some contribution from C5, as you can see here. C5 is actually part of the brachial plexus. This nerve is important because it innervates the diaphragm, which is central to the breathing process. As you can see, this diagram has a lot of complex connections between the nerves, but you\'re only responsible for labeling the specific peripheral nerves that I highlight and each plexus. However, in general, you should understand that each peripheral nerve is a complex combination of sensory and motor components of the various spinal nerve levels and that it\'s possible to trace back from a peripheral nerve to see which spinal levels that have contributions to that nerve. This is important for understanding certain diseases and injury because we can see where the contributions to those nerves are coming from. So in our phrenic nerve you can see all the different branches going back to the different levels of the spinal cord. Slide 18 The next plexus we\'re going to talk about is the brachial plexus. It has roots at levels C5 to C8 as well as T1. So our first original rami exit the spinal cord and merge together to form three separate trunks. These trunks then branch out it again to form a six different divisions which are represented by the purple and green areas. Then they\'ll merge again in another combination to form three cords. Finally, they split into the five main branches, which become the peripheral nerves highlighted on this slide, which supply the shoulders and the upper limbs. These branches are the auxiliary nerve, the radial nerve, the musculocutaneous nerve, the ulnar nerve, and the median nerve. Each supplies a different area and as with most nerves, the name of the branch or the peripheral nerve, indicates the structures that it innervates. So the auxiliary nerve innervates the deltoid and teres minor muscles, which are muscles of the shoulder that are involved in arm movement. The radial nerve supplies the muscles on the posterior aspect of the arm and forearm. The musculocutaneous nerve supplies, the flexors of the forearm like the biceps brachii. The median nerve supplies most of the anterior forearm and some of the hand muscles associated with the movement of the thumb. And finally, the ulnar nerve supplies the forearm and most of the hand muscles. Slide 19 This slide is showing both the lumbar and sacral plexuses or the lumbosacral plexus, because they\'re often associated with each other. The lumbar plexus doesn\'t have as much mixing as the brachial plexus. It has ventral rami originating in levels L1 to L4. The major nerves of the lumbar plexus are the femoral and obturator nerves. These supply the lateral abdominal walls, the external genitals, parts of the lower limbs. The sacral plexus has ventral rami in the regions of L4 to S4. The two main nerves that arise from the sacral plexus or the common fibular nerve, or the peroneal nerve, and the tibial nerve. These are grouped together as you can see here, and anatomically they\'re held together by a sheath and referred to as the sciatic nerve. You can see that there\'s some contribution from L4 into this particular nerve. L4 is also part of the lumbar plexus, and that\'s why it overlaps in both these plexuses. The sciatic nerve is the largest peripheral nerve in the body, and it supplies many of the muscles and skin of the lower limbs. Slide 20 Finally, we have the coccygeal plexus. The coccygeal plexus has rami originating at S5 and the coccygeal nerve. It provides innervation for the muscles of the pelvic floor, as well as it provides sensory information from the skin over the coccyx, which is the most inferior bone in the vertebral column. Conclusion So that\'s what we\'re going to end this online lecture today. Today we looked at the spinal cord, external and internal anatomy. We followed by looking at the protective coverings, the meningies, for the spinal cord and we looked at how the spinal nerves interact with the spinal cord. We then followed by looking more closely at the spinal nerves and their arrangement along the spinal chord, along with their protective coverings in the peripheral nervous system. We finished by looking at how those spinal nerves interact with each other just outside the spinal cord to form the various plexuses. In the next online lecture module, we\'re going to be looking more closely at the autonomic nervous system. So until then, take care.