Now, with some overview of the white matter and gray matter of the spinal cord, I want us to turn to a very important consideration. And that is understanding what we find at each level of the spinal cord. And here's, I think, a very helpful figure. What it does is it shows representative cross-sections that are taken from different levels of the spinal cord. And I want you to notice a few features that we find here. Essentially, what we find is that in the region of the cervical spinal cord, and also, in the lumbosacral enlargement region, we see an expansion of the amount of gray matter that's present in the ventral horn. I think that should make good sense to you. This gray matter is providing motor output to the arms, in the case of the cervical cord, and to the legs in the case of the lumbosacral enlargement. So, more neurons, more local circuits means more grey matter volume that's governing what we do with our appendages on our arms and our legs. In the thoracic spinal cord, notice that the amount of grey matter in the ventral horn is reduced, compared to what we find in the cervical and in the lumbar and sacral levels. Okay, so, look at how much gray matter we have in the ventral horn, that will help you identify the section. I also want you to notice the amount of white matter that we have here in the spinal cord. Now, for the amount of white matter in the cord. I think it should make a certain amount of sense that it would decrease as we move from superior to inferior. And it's decreasing for 2 reasons. One reason is that the axons that are coming down from the brainstem and cerebral cortex and the lateral and anterior column of white matter are simply terminating along the way. So, if we looked at the upper levels of the spinal cord we would expect to find more axons than if we were to look at the lower levels of the spinal cord. Now, for a similar reason, our ascending pathways that are sending signals, let's say about pain and temperature or mechanosensation, up through the spinal cord are collecting at various levels. And as a consequence, in the superior segments of the spinal cord we have more axons conveying ascending sensory signals than we have at more inferior levels. Now, if we then look at the cross sections of the spinal cord, I think it should be obvious to you that we have a whole lot more white matter here in the cervical cord compared to the very bottom of the spinal cord in the sacral region. So, look for the amount of white matter that we find. And we see that there is more white matter at the upper levels than in the lower levels. Now, one key difference that I want you to look for is the lateral margins of the grey matter of the spinal cord. You'll note that in the thoracic cord there is a lateral protrusion from the gray matter right near the junction of the ventral horn and the dorsal horn. This is a feature that we call the lateral horn. And this is a distinctive feature of the thoracic spinal cord. And as we'll see, what's in that lateral horn are preganglionic sympathetic neurons, that are providing outflow from the nervous system to the viscera. We see a similar group of cells in the sacral cord, but they tend not to form a pointy protrusion into the white matter. And at that level we don't call that a lateral horn, but we do recognize another collection of preganglionic visceral motor neurons, only in the sacral cord they're parasympathetic, whereas in the thoracic cord they're sympathetic. All right. Another key distinction that we need to make when we look at the spinal cord is in the region of the dorsal column. So, remember the dorsal column is what we find out here with dorsal being shown to the top of the image and ventral or anterior to the bottom. What I want you to notice is that in the dorsal column, there is more white matter in the cervical region, compared to the lumbosacral enlargement for reasons that I just illustrated for you. But I also want you to note that in the cervical cord, there is a distinction between a lateral component of this white matter, and a medial component. And as we talk about our pathways for mechanosensation, what you'll discover is that the medial component is what we find at lower levels of the spinal chord. But as we get into the region of the cervical enlargement, we begin to add white matter from the sensory inputs that are attached to that cervical enlargement, serving the upper extremities. While that wet matter begins to add up in the lateral aspect of this dorsal column and the upper thoracic cord. And then, we really see it adding up as we move through the cervical enlargement. So, look for those divisions of the dorsal column when we look at cross sections through the spinal cord. And we'll, we'll give these divisions names. I don't need to tell them to you now but, but just note that in the cervical cord the dorsal column is subdivided, whereas in the lumbar and sacral levels, there's no such subdivision of the dorsal column. Okay, now, I think we're ready to name some of these details. And give you a chart that will allow you to explore on your own, cross sections through the human spinal cord. So, this is a chart that lays out for you some important internal features, in terms of grey matter and white matter. And for white matter, I'm giving you the names of some of these important tracks or pathways, that we will talk about in detail when we talk about mechanosensation and our systems for pain and temperature. Well, let me orient you to this chart. We have a list of internal features of the spinal cord in columns. And then, our spinal cord segments organized according to cervical, thoracic, lumbar, sacral and the one coccygeal segment. And what I've provided for you is a check mark to indicate that, that grey matter structure is present. So, as you've just seen, in an array of cross sections, the Dorsal horn is present in each level of the spinal cord, as is the Ventral horn. Okay? So, we have a check mark for each of these longitudinal divisions of the spinal cord. The lateral horn is that special feature indicative of the preganglionic sympathetic neurons. And that's only found in the thoracic cord. So, please note that. The lateral horn is a feature of the thoracic cord. Now, the rest of this chart pertains to white matter. So, just with respect to the overall abundance of white matter, as I've suggested, there is less and less white matter as we perceive from superior to inferior. And with respect to the individual pathways or the individual tracks of white matter that will be important for mechanosensation and pain and temperature sensitivity, as well as for motor control. We also see a general superior to inferior progression from more white matter superior, to less white matter on the inferior side. So these terms here Gracile tract and Cuneate tract, these are the pathways that we find in the dorsal column. The Gracile tract pertains to the lower extremity, the Cuneate to the upper extremity. So, the Gracile tract begins to build up and its complement as we pass through the portion of the spinal cord that receives input from the lower extremities. We don't have a Cuneate tract until we get into the upper part of the thoracic cord. And then, once we do, we rapidly add axons that are adding somatic sensory signals to that dorsal column. Now, I'm going to jump to the far right column, because this also is a somatic sensory pathway. This pathway called the Antero-lateral system is found, as it's name suggest in the anterior and lateral white matter of the spinal cord. And this is the system that is conveying pain and temperature signals from the body up into the brainstem into the forebrain. So, here we have a progressive addition of white matter, as we go from inferior to superior levels. Now, I'm talking about an ascending sensory pathway which is why my arrow is going up. Now, finally, we have 2 pathways that pertain to motor control identified here. A Lateral corticospinal tract which is really the major motor control pathway that I will emphasize for you in this course. But there's also a smaller, minor ventral corticospinal tract. The major pathway, the Lateral corticospinal tract descends with a progressive termination of axons as we go from the upper part of the cord to the lower part of the cord. And really, much of this pathway is concerned with what we do with our arms and our hands and to perhaps a lesser extent what we do with our legs and our feet. This pathway is all about controlling voluntary movements, which mainly involves our arms and our legs. So, there's not as denser projection to the Thoracic cord, but we still see the pathways that are working their way down to the lumbar segment. And then, once we get below the lumbosacral enlargement, this pathway really drops out of the spinal cord. Now, this minor pathway, the Ventral corticospinal tract, it's, a relatively small contribution. And the axons tend to project along the length of the spinal cord. Perhaps with some bias towards the lumbar levels. Alright, well, now that we've had a broad conceptual overview of the spinal cord, it's time to actually look at cross-sections and see these features directly. So, I would ask that if you have Sylvius Software, go ahead and open it and follow along with me. If you don't have the software That's fine. You can just continue to track with me as shift now to Sylvius and look through some representative cross-sections of the spinal cord.
