Hi everyone! In this video, we're going to be covering chapter 14, which is primarily focusing on the spinal cord and the spinal nerves. So as always, here are your learning objectives. Please make sure that you guys use these, fill them out, whatever, in order to further guide your studying.
So let's get started. We mentioned in the previous chapter, in chapter 13, that the spinal cord is going to be a part of the central nervous system. The other part, of course, being the brain. So the brain we're going to talk a little bit more about in the next chapter, but we know that the spinal cord is crucial for integration and processing of information in the central nervous system because lots of the information that's coming from the rest of our body first has to go to the spinal cord before it can make its way up to the brain.
The cool thing about the spinal cord is that it can function with the brain. but also it can function independently from the brain. And so this ability is actually super important for reflex arcs and for our reflexes.
And that's because if we think about it, if we touch something really, really hot, right, whenever that happens, you pull your hand away really, really quickly. And that's because that sensory information is coming up all the way through the arm, going to the spinal cord. The spinal cord is being like, oh crap, right, integrating really, really quickly. And then immediately it goes back out the efferent neurons to the muscles in order to pull those muscles and pull that hand away by contraction, right?
By muscle contraction. So that happens really quickly because in that case, the spinal cord does not have to function or work with the brain. But instead, it just happens all in one point at the spinal cord, which is great because if we had to wait for that sensory information to go all the way up to our brain and then all the way back down. it would take forever and by that point we could have suffered from some serious damage, right? Especially if we did have like our hand on a hot burner.
The spinal cord is about 45 centimeters in length. It does pass through the foramen magnum, which is at that kind of base portion of our occipital lobe, sorry, occipital bone on our skull. And then We also know that it's going to extend, the spinal cord will extend all the way down to about L1, L2. So lumbar region, L1, L2, and then it ends right in there. We know that the brain tends to have the higher level thinking that can modulate also without the spinal cord.
But sending signals just in the brain and not needing to go to the spinal cord is something that happens kind of regularly. but then we have a lot of that constant communication between the brain and the spinal cord that's going to help kind of make sure that we stay alive and stay focused. The anatomy of the spinal cord is the next thing that I want to point out. As we're looking at this, you'll notice that we can refer to the different portions of the spinal cord based on these different portions of the vertebral column.
We have the cervical region here that is going to basically come from the foramen magnum all the way down to about C7. and then we have the thoracic spine and then we have the lumbar spine. But as you'll notice the lumbar spine, the spinal cord itself ends between L1 and L2 and so that's where it comes to this little tip which is called the conus medullaris and then at that point we just have the lumbar spinal nerves which are coming off of the spinal cord and then kind of filling in this space between basically L2 and L5.
as it makes its way down to the sacrum. All of this part right here, these spinal nerves, these lumbar spinal nerves, are referred to as the cauda equina because it kind of looks like a horse's tail. So equine is referring to horse, and then cauda is referring to tail. And when we kind of look at it closer, we can see that cauda equina in here, and this is like in an actual cadaveric specimen.
Another thing that you guys will notice is that we have these different areas of enlargement. So we have a cervical enlargement and a lumbar enlargement. So think about why that might be.
So the reason why we have a cervical enlargement is because we have lots of neurons and axons and cell bodies kind of coming from the spinal cord here that are going to make their way to the upper limb, right? All the way through the arm. And then all the way down to the hands. And so since there's kind of those limbs up here, we have to have a little bit more oomph, I guess, in the spinal cord. And the same goes for the lumbar enlargement.
This is kind of preparing. housing all the cell bodies and axons for the lower limbs. That's why we'll see these two enlargements here.
The thoracic part of the spinal cord is going to be really thin, but we can also see the thoracic spinal nerves coming out. The last thing I want to mention is the phylum terminale, which is basically this extension of pia mater, which is the last part of the most deepest layer of the meninges that basically takes the tip of the conus medullaris, the tip of the spinal cord, and goes all the way down to the sacrum. That's the phylum terminaliae.
That's going to be the end of the spinal cord. By connecting it all the way down, we're going to be able to ensure that the spinal cord doesn't displace and move up and down or jump around in the vertebral column. In this slide, I want to draw a cross-section of the spinal cord with all of you just so that we can kind of practice and then we can see a better picture of it on the next slide.
So the way that I kind of draw my spinal cords is a little bit different and so let's kind of put it all together. So I kind of just draw it very basic all right I know it kind of looks funny I'm sorry it's not symmetrical at all but then we have our gray matter okay kind of like that and the white matter on the outside. So just kind of first off this is going to be anterior or ventral and then this side is going to be dorsal or posterior.
Okay so this is the back side this is the front of it. So then from here We're going to start to draw in the various components like the spinal nerves or what becomes the spinal nerves. On this dorsal side, we're going to see something come off like this. It's going to bulge out and then come down.
Let's draw it here, bulge out, come down. On the ventral side, it just comes up like this. Then we have that.
Again, dorsal, ventral. So this big portion is going to be where all of these cell bodies are located for various afferent or sensory neurons. So since this is a bunch of cell bodies in the peripheral nervous system, we call this a ganglia. And so since it's on the dorsal root, we often refer to this as our dorsal root ganglia or the DRG. Okay, so this whole portion is the dorsal root with the dorsal root ganglia.
And then on this front side, we have the ventral root. Okay, so this whole thing is the ventral root. Where the dorsal root and the ventral root come together, we get the spinal nerve. All right, and so kind of relating back to our previous lecture.
where we had same right we can add now same Dave and this is going to help kind of orient you on not only the sensory afferent motor efferent part of same but now we can see where the sensory and motor portions are going to be entering the spinal cord or exiting the spinal cord so this Dave portion now means dorsal afferent and then ventral efferent. If we were to do this, we know that our motor neurons are going to be coming from the CNS and exiting out the ventral side. Then from this as well, we can see that our sensory neurons are going to be coming in to the CNS, right?
Into the CNS through the dorsal side, okay? So we can kind of split our spinal cord in half if you will. And so our dorsal is going to be afferent and then our ventral is going to be efferent. So same Dave. That's just kind of a quick way to kind of remember all these different kind of floating components and kind of keeping everything straight as you continue to learn.
So looking at a better picture right than the one that I just drew. we can easily see the white matter and the gray matter. Gray matter is going to be kind of in the middle here. White matter is on the outside.
Specifically within the gray matter, we have various kind of portions or columns that we'll call them, or nuclei. But this H-shaped mass in the center is usually what we refer to as the gray matter. We can also see our dorsal root coming off into the dorsal root ganglion.
We see our ventral root coming off and then both of those will come together to form the spinal nerve. Okay, so again remember same Dave, sensory afferent, motor efferent, dorsal afferent. So our sensory information is going to be coming through here and then our motor efferent is going to be traveling out the ventral side here. The next thing I want to talk about are the spinal meninges. The meninges are these specialized membranes that are going to provide protection, physical stability, and shock absorption for the central nervous system.
We have meninges covering the spinal cord as well as surrounding and covering the brain. Just to keep in mind, the spinal meninges and the meninges that are covering the brain are the same. They are continuous with each other. We will see all three of these layers surrounding the the brain as well. If we pay close attention, you guys will notice that within the spinal cord, so this is a picture of like a human cadaver, we have basically our meninges kind of peeled away and then we can see these little kind of substances within kind of near the spinal cord called the denticulate ligaments.
And the denticulate ligaments are actually going to help anchor the spinal cord in position as well. So it's just kind of another thing to kind of mention as we're looking at the spinal cord and looking at the different meninges. So let's break down those meninges. Okay, so if we're looking at it here, you'll notice that we have our gray matter and our white matter.
We could see our spinal cord. We have the ventral root. We have our dorsal root with the dorsal root ganglion. And then those come together to form the spinal nerves. So the innermost layer or the innermost meninges is going to be the pia mater.
which is basically continuous with the spinal cord. It's really hard to dissect this off of the spinal cord. And then the next layer is going to be the arachnoid mater.
If we were looking at this in dissection, it actually would look a lot like a spider's web. It's kind of attaching here, which is why it's been given the name arachnoid mater. And then finally, the last layer, I guess the most superficial layer, is going to be the dura mater. And the way that I remember the dura mater is it's durable. It's the most durable.
It's really hard to basically rip or tear. Really the only way that I've ever been able to kind of cut through or get through the dura is by cutting it with a scalpel or scissors because it is so tough. This translates to tough mother.
And so it's super durable, super tough, which is going to be really helpful for protection purposes. So please make sure that you guys know these three layers of the meninges. We will repeat it again as we go through the brain anatomy.
So based on what we know about the spinal cord already, right, we know kind of the general anatomy, we know how long it is, we know where it ends. At what location of the vertebral column, where would you do a spinal tap or an epidural based on your knowledge of the spinal cord? So we know that the spinal cord ends between L1 and L2.
So to be safe, if we're going to be sticking the needle in somebody, we're going to want to put that needle between L3 and L5. So either between L3 and L4 or L4 and L5, either one's fine. But generally, we want to make sure that we go beneath the spinal cord so that we don't hit it with the needle and accidentally cause our patient to have paralysis. So if we look here.
Most of the way that they kind of do the spinal tapping or epidurals is by making sure that the patient is kind of like in the fetal position, like rounded over, so that they can kind of better visualize their superior iliac crests from their hip bones. So the superior iliac crest is kind of telling you where L3 is. And so if you kind of use that as an anatomical landmark, you are able to kind of figure out exactly what. what level of the spinal cord you're at and that's where you would want to do the epidural. Okay so usually to collect cerebrospinal fluid you have to go a little bit deeper than the epidural space um so to do an epidural like to kind of paralyze the lower parts of the body for like pregnancy purposes you would just go here into the epidural space um but if you needed to collect cerebrospinal fluid for like meningitis testing you need to go a little bit further into the subarachnoid space to collect that csf um but again either way you want to kind of enter between l3 and l4 or l4 and l5 which is far enough away from our spinal cord here so that it doesn't potentially get injured.
So now let's look at the spinal nerves. There are 31 pairs of spinal nerves. In the cervical region, we have eight cervical spinal nerves. Eight on the left, eight on the right. In the thoracic, we have 12. So again, left and right.
Lumbar, we have five. Sacral, we have five. and then we have one coccygeal nerve.
Okay so again 31 total spinal nerves that are going to be carrying motor information as well as sensory information away from the CNS for motor and then back to the CNS for sensory. So here we can start to look at the anatomy of a peripheral nerve. And so these nerves remember are just basically bundles of axons. So if we start with our axon we can see our Schwann cells because it's in the PNS remember Schwann cells are only found in the PNS.
And then we can see that we kind of bundled all these axons together. So it kind of follows that same organization that we were looking at when we were looking at the muscle anatomy, right? So the most microscopic level we have the axons of our neuron. We have our myelinated sheath which is going to make the action potential move a lot faster and then kind of covering the axon in the myelin sheath like a blanket it's going to be connective tissue called the endoneurium okay so we have a connective tissue sheath surrounding each of our axons called the endoneurium if i bundle a bunch of these axons together i'm going to get a fascicle right and then surrounding the fascicle is going to be another connective tissue sheath called the perineurium.
If I then take a bunch of fascicles and bundle them together, I get the entire peripheral nerve. And surrounding that entire peripheral nerve is a layer of connective tissue called the epineurium. So again, kind of that same exact organization that we talked about last week with the muscle. Epi is above, right? So in the outermost is going to be surrounding the epineurium or surrounding the nerve itself.
We have fascicles that are covered by perineurium and then axons covered by endoneurium. Also within the nerves you might see some blood vessels which makes sense because all of our neurons need blood supply in order to ensure that they get oxygen and nutrients to complete and do their jobs. The next thing I want to talk about are dermatomes.
And so basically what this is showing us is that each pair of spinal nerves is going to be monitoring specific surface areas of our body. And so each of the spinal nerves again, it's going to be carrying sensory and motor information. A lot of the sensory information is that cutaneous or tactile information that we can just feel on our skin.
If we touch the palm of our hand or our wrist or our forearm, we're going to have specific spinal nerves that are going to be feeding back to the spinal cord into the brain that touch information. That's what these dermatomes are talking about. As you can see, it's very sporadic. You're not expected to memorize this at all. but you can see that a lot of the cervical nerves are going to be kind of helping with like the neck region, maybe some back of the head, top of the head.
The face is mostly cranial nerves, so we'll talk more about that in the next chapter. And then we get into the body here, which is the thoracic nerves. The lower limbs is going to be primarily lumbar with some sacral, and then the genital region is going to be sacral.
These are typically more clinically relevant when we're talking about like surgery or deficits in terms of like the spinal cord if the spinal cord gets damaged at all you can kind of figure out where the damage is occurring based on where the patient is no longer feeling touch right so if let's say they had damage to like their lumbar spinal cord let's say like at L1 so that means everything below L1 isn't going to feel that touch sensation or it's not going to get back to the brain because everything's damaged. The lower limb isn't going to have any touch sensation. Again, that's something that helps with diagnosis purposes as well as for surgery.
We want to be able to do nerve blocks at different areas of the spinal cord. We want to make sure that... if we're doing the nerve block at the right location to get that area basically numb or so that we don't feel it if we are undergoing surgery in that area next we have some nerve plexuses that i want to talk about we have four major nerve plexuses in the body which is basically a meshwork of nerves that leave the spinal cord and kind of form like these intertwined complexities where axons are crossing and things like that, but they are super kind of important for understanding some muscle innervation as well as some sensation innervation throughout our upper limbs and neck and different regions of our body.
So the four nerve plexuses that we have are the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus. Oftentimes we could take the lumbar plexus and the sacral plexus and combine them in order to get the lumbosacral plexus. For today, I really want to focus on the brachial plexus because it is more important than some of the other ones.
Brachial, again, we're going to be looking primarily at the upper limb. If we look at the brachial plexus here, you'll notice that it goes from C5, cervical spinal nerve 5, all the way down to T1. We have these spinal nerves coming out between the vertebrae, so C5, C6, C7, C8, T1. And then you'll kind of see how these spinal nerves come together in order to form trunks. So we have basically the way that they like to say it for the brachial plexus anatomy is we have roots here.
Then the roots come together to form trunks. The trunks will then split to form divisions. The divisions will come together to form cords. And then the cords will split to become the actual nerves themselves.
So the main ones I want you guys to kind of look at is this lateral cord here, which is kind of coming from if we look back C8 and, or sorry, C6 and C7, right? As we look at them coming together. I want you to see the musculocutaneous nerve coming off of that lateral cord, right? And then we also have the median nerve okay The median nerve is kind of an important one to just kind of keep in mind or remember because it is the one that travels all the way down the forearm, all the way down through basically the connective tissue of the wrist, the aponeuroses and stuff there.
And when that area gets inflamed, it can lead to carpal tunnel syndrome. So median nerve is associated with carpal tunnel syndrome. And then...
We also have the ulnar nerve, which rides along that medial surface, that pinky side. Also with the ulnar nerve, this is the nerve that you may hit on your elbow. When you hit your funny bone, everything shoots down that pinky side of your forearm all the way down to the pinky. For me, if I'm compressing that ulnar nerve for too long, I might feel numbness in my pinky.
as well as a tingling sensation if you hit that funny bone, but again, it's just hitting the ulnar nerve. The medial cord is going to be that primary one going into the ulnar nerve here. Last, we have the posterior cord on the backside, which is going to be contributing to the axillary nerve, which goes to the armpit region, and then our radial nerve, which again is now on the thumb side of the forearm.
This last plexus is the lumbar and sacral plexuses. We have some key nerves that I just want to point out to you here. With the lumbosacral plexus, we're focusing on T12 all the way down to L4. We have our genitofemoral nerve coming off around L2, as we can see. Then the genitofemoral nerve is going to branch into the femoral.
nerve and the genital branch as well, or sorry, the femoral branch and the genital branch. The femoral nerve is going to come off a little bit later around L4 in order to go down through the femur near the femoral artery and things like that. We also have the lateral femoral cutaneous nerve coming off like L2. And then again, these are just kind of to give you an insight onto how these plexuses are very complicated.
and how if we damage specific parts of the spinal cord or different parts of these roots, we could see kind of long-standing damage within the body as well. And that is chapter 14.