 Today, we're going to talk about the spinal cord. And I brought along my friend here to help tell you some of the salient features and to show you the spine in the real skeleton. So let's turn about and have an about face and look at the spinal spinous processes along the vertebra. And we're not going to go into the details about how many cervical, thoracic, lumbar and sacral segments there are because you've already learned that. What I want to emphasize is the relationship between the vertebra and the canal where the spinal cord runs. So let's put the brain back in our model here. And this is a brain with the spinal cord still attached. And what I want you to realize, I'm going to put this in front like this, is that the spinal cord comes down and ends right about here where my thumb is. At about L1. And below that you have these rootlets of the cauda equina that continue on down through the canal and inside the dural sac down into the sacral region. And then these roots, sensory and motor, emerge from the foramina that are along the vertebra or in the sacrum. And therefore if you're going to remove cerebral spinal fluid, you want to do it down fairly low at about L4, L3 and that region. So let's go now and look at a plastic model showing the roots. So here is a plastic model of the vertebral column with the pelvis attached. And while we're looking at the cervical region with the bones of the vertebra here, I want you to take note of the red vertebral artery on either side. The vertebral artery comes up through the foramina in the sides here of the cervical vertebra and then enters through the foramen magnum, that's this hole right here, to form the two vertebral arteries on the medulla and then join to form the basilar artery. When we talk about the vascular supply to the brain, it will be important to remember that this part of the circulatory system, the vertebral basilar contribution supplies the brainstem in the posterior fossa of the calvarium and is very important. Now to focus on the spinal cord, which is represented in green here, the cord is large in the cervical region where you have the cervical enlargement for the arms and hands, the appendages, and then it becomes a little more difficult to see as we drop down here. But as I move the cord laterally and lift it up, you can see that down in this region here, they've added the spinal nerve to each segment. And notice that the nerves come out between a notch in each of the vertebra. There's a dip in the bottom one and a disc-shaped semicircle in the upper one and together they make the intervertebral foramina through which the spinal nerve emerges. And sitting in this foramina, not demonstrated here, is the ganglion where the sensory cell bodies of the nerves carrying sensation are located. In between the vertebral bodies here on the anterior surface are the discs. These are the discs that so many people have moving in one direction or the other, herniating. They can herniate posteriorly, anteriorly, and as they move toward the central canal, they put pressure, I'll just pretend to herniate here, they put pressure on the spinal cord. And if they put pressure on the nerve here, then people have either pain or partial paralysis, or both. So the relationship between the vertebral column, the disc, the emergence of the nerves and the foramina through which they emerge is very important. Now let's look at a gross specimen of the spinal cord. So I have here six spinal cords laid out from rostral or near the cervical end to caudal near the sacral end on this tray. And the first thing I want to tell you is that the spinal cord does not come, cut into little pieces like this. This was in the process of the pathology review of the spinal cord, that the attending person cut it to look at it, to examine it for any pathology. So the spinal cord is not in segments, but is all connected like this. And these cut marks do not represent different spinal cord levels. They're totally arbitrary as if you were cutting salami for dinner. Now, let's look at the outside. Remember, the dura covers the outside of the spinal cord. And here on the outer surface, we see these bumps. And there's a little bit of fat with them. But these bumps, there should be a pair on the right and left sides at each level. Those are the dorsal root ganglia. These are where the spinal nerves are emerging on each side of the spinal column. So there is a pair of dorsal root ganglia. Now dorsal root ganglia, a ganglion is a collection of nerve cell bodies outside the central nervous system. These cell bodies have one process or axon that goes out to the periphery, to a sensory organ in the skin. And the axon then comes into the spinal cord to carry on and carry the information up the spinal cord toward the brain. Or there is a cell body in the spinal cord whose axon emerges in the spinal nerve and goes out to innervate the muscle. So the dorsal root ganglion is sensory. But passing through this region here are also the motor fibers. So we want to keep in mind that each of these represents a level. Now I can't tell you exactly where, but this is approximately one segment of the spinal cord. Let's say it's thoracic. And this would be thoracic one, thoracic two, thoracic three, approximately like that. This is the cervical region of a spinal cord where in the cervical region, so that the brainstem would be in this direction. And here on the dorsal surface you see a groove. And emerging in that groove are all these little rootlets. These are dorsal or sensory roots. You see all of these. All of these would be coming in over one spinal nerve. So on right and left sides you have the filaments or the rootlets of the dorsal side. Now if we turn it over, so that's sensory. Dorsal is sensory. Dorsal is also called posterior. And we use those terms interchangeably. So these are dorsal roots or posterior roots. And if we turn it over, it's harder to see on this side. But most of them have been torn off in the removal of the cord. But on this side you can see the ventral roots. These are coming out on the ventral surface. And they are motor fibers going out to muscles. Unfortunately, most of them are missing. Here we are in the cervical region. And let's magnify this as much as we can. So that I can show you that there is gray and white matter in the spinal cord, just as there was in the cerebral cortex or the brainstem. The white in the middle is the gray matter. It's backwards, I know. And the beige on the outside is the white matter. So the white matter is in the dorsal region or posterior region, in the lateral region, and in the anterior or ventral region. And the dorsal part of the cord was where the sensory fibers were coming in. And the ventral part of the cord is where the motor fibers were going out. And there is sort of a butterfly pattern. And at each level, this is the cervical level, so there's a lot of gray, or white in this case as it appears, cells because we have so many muscles to innovate in the hands and in the arm. If we look now, let's say, at the thoracic level, where all you have are a few intercostal muscles, then you can barely see, you can barely make out any neuronal regions, any of the white H or butterfly pattern. It's very, very difficult to see because there aren't very many neurons because we don't do very much with our intercostal muscles, unless maybe you're a belly dancer. And as we move down farther, down toward the sacral region and the lumbar region, then, because we have lots of muscles in our legs to be innervated, again, we have more of the white area, which is the gray or anterior horn and posterior horns as we call them because of the shape of this H and the dorsal or posterior is going to be sensory and the ventral or anterior is going to be motor. And then, finally, we get down to the very, very sacral region where it's almost all gray matter, it's almost all white. And you can see that we have a lot of cells that are involved in the innervation of the sacral region. This model of the spinal cord can show you things that we were not able to see in the growth specimen. However, it gives you an exaggeratedly large impression of the spinal cord and I wanted you to see how narrow and tenuous and delicate our spinal cord really is. So let's just recap very quickly. Let's focus on the upper model here because it shows us, first of all, a spinal nerve and this is the bulge of the dorsal root ganglion and here is the ventral roots coming and passing through and joining in the spinal nerve and here are the dorsal roots that are passing through with the cell bodies in the dorsal root ganglion. So this spinal nerve has sensory and motor fibers. The motor fibers are in the ventral or anterior roots and they are actually leaving. Their cell bodies are in the spinal cord. Remember, this part of the spinal cord has the cell bodies and the axons go out and they carry on out to the muscle or if they're autonomics, they're going out to the smooth muscle and these axons are dorsal roots. They are coming in and they are connecting in the gray matter here and forming tracts out here in the white matter. So we have tracts, anterior, lateral and posterior tracts. They're carrying information up and down the cord so they're going up and down and we have roots that are coming in at each level that are coming in laterally. So the superior or posterior part of the spinal cord, the gray matter here, is particularly sensory and the ventral part or anterior part is motor neurons and in between, of course, are other neurons that we will not focus on. So damage to this part of the cord would give you a paralysis of the muscles innervated by these axons and a compression of the roots here would give you a sensory loss for the segment or the dermatome of the body supplied by these axons. And so this is the segmental nature of the spinal cord. Each segment has a dorsal and ventral root. So this model really represents two segments of the spinal cord. Now, important in understanding the function of the spinal cord and in clinical testing is the fact that we have reflexes. A reflex is an automatic response to a sensory stimulus. So if a sensory stimulus comes in over this root and it makes connections with the nerve cell body in the anterior region here and those roots come out and go to a muscle, you have a reflex sensory in and motor out. And there is only one connection between the axon coming in, connecting with this neuron and this neuron coming out. That is what we call a monosynaptic reflex. One synapse, one transmission junction. And so they're very fast, they're very automatic, and they need no thinking. So if you touch or pound on with your reflex hammer, the nerve that supplies the region of the knee or excuse me, you pound on the tendon that is supporting the muscle, let's say the patella, then the information comes in from the stretch of that muscle and tendon and causes a reflex jerk so that you don't overstretch your muscle and the knee jerk reflex or patella reflex occurs. So each segment of the spinal cord has a reflex associated with it and there are some reflexes that are more commonly tested than others. And that patella reflex is the most classic but they are all monosynaptic reflexes and they provide information about the level of the spinal cord and whether it's a sensory loss or a motor loss or both. And the absence and presence of reflex are very important clinical science, not only in the spinal cord but also we're going to see in the brain stem. Now to relate this model to a stained cross section of the spinal cord, this happens to be cervical spinal cord, just want you to see that many times in teaching neuroanatomy, the cord has been cut and it has been stained and we have special stains for the axonal myelin, the insulation of the axons that are carrying information over long distances. So the axons or the white matter, remember are on the outer part of the spinal cord and the cell bodies or the gray matter are on the inside of the spinal cord. And this is where we get this sort of classic H pattern or some people call it in some regions it looks more like a butterfly. So these are the cell bodies. So when we talk about a reflex, sensory information comes in, synapses on a neuron here, that neuron sends its axon out to the muscle and that's all there is to a simple monosynaptic reflex.