Hello, again. Welcome to this tutorial on the internal anatomy of the spinal cord. Once again, we're confronted with the complexity of the nervous system. So, our core concept is that the brain is the body's most complex organ. And our learning objective for today is that I want you to be able to identify the major subdivisions of the spinal cord as seen in representative transverse cross-sections. So, this requires enough familiarity to the internal anatomy of the spinal cord that we can view in cross-sections so that you can understand what level of the spinal cord is in view and what are the important gray matter and white matter structures that we can find there. Alright. Well, let's begin by looking at a histological section of the spinal cord. This happens to be from the lumbar enlargement of the human spinal cord. And this is a section that's been stained for the presence of Nissl substance. Nissl is a term we use in neurohistology. It refers to rough endoplasmic reticulum. So, wherever we have rough ER within a cell, which is typically in the cytoplasm, and there's some rough ER within the nucleus as well. But mainly in the cytoplasm and perhaps into the proximal dendrites, that's where we will find the stains. So, this is going to be a stain for cell bodies, and for neurons, for also the proximal portions of the dendrites. So what's not stained here are myelinated axons. So, this is one way of looking at the nervous system that highlights the presence of cell bodies. And what we can see in this image is that within this spinal cord, we have gray matter that is central to the cord and that gray matter is surrounded by white matter. So white matter is found all in these peripheral regions on both sides of the spinal cord. Okay. Now let me give you some terms that will help you as we go and look at the structure within the spinal cord. So, the white matter that we find on the dorsal aspect, and this is dorsal, is called the dorsal column. The white matter that we find on the lateral sides of the cord, is called the lateral column. And the white matter that we find in the ventral or the interior parts of the cord, is called the ventral column. So, this word, column, appears repeatedly in our lexicon that we use to describe white matter in the spinal cord. So, when you see the word, column, think white matter. Now, occasionally, we apply the word, column, to talk about the organization of cell bodies but I'll try to be very clear when that's the case. Otherwise, when you see the word column in your readings or in our discussions, think about white matter. Okay. So now, let's turn our attention to the gray matter of the spinal cord and we can recognize basically two broad regions of gray matter in the spinal cord. There's something that we call the Dorsal horn, which can be found in this aspect of the grey matter of the spinal cord. And then, there is a Ventral horn. And the Ventral horn, then, would be pretty much the rest of this, excluding, perhaps, what we might consider to be an intermediate zone between the Dorsal horn and the Ventral horn. We'll come back and talk about that as we get into different levels of the spinal cord. Bu the Dorsal horn and the Ventral horn provide us with our basic dichotomy between a sensory zone of gray matter, which is what we find in the Dorsal horn, and a motor zone, which is what we find in the Ventral horn. So, in the Dorsal horn, we have longitudinal layers of cells that are organized along the long axis of the spinal cord one layer on top of the other almost like a long series of strips of cells. And these strips have been recognized by anatomists over the years, and the scheme that seems to have, have stuck the best was proposed by an anatomist by the name of Rexed. And we call this scheme Rexed's layers or Rexed's laminae of the Dorsal horn. So, that's what the roman numerals are over here to the left side. So, the Dorsal horn is essentially laminae 1 through 6. We'll spend some time talking about these laminae when we talk about mechanisms of pain and temperature processing, especially lamina number 2, also known as the substantia gelatinosa or the gelatinous layer of the Dorsal horn. So, we'll come back to that in a later session. For now, I just want you to get the idea that the Dorsal horn is gray matter that is receiving incoming sensory signals from nerves that are running in through the Dorsal roots and synapsing upon cells in the Dorsal horn. Now, let's turn our attention to the Ventral horn, which is where we find our lower motor neurons, that is, the motor neurons that innervate muscle, and the local circuit neurons that coordinate the output of those lower motor neurons. And here, we have less of a layered organization to these cells, more of a column organization of the cells. So, for example, we may find a column of cells that all innervate the same muscle. Not necessarily the same muscle fiber but perhaps a distributed set of muscle fibers that are within the same anatomical unit that we would recognize as a muscle. So, if the Dorsal horn is organized into long strips of cells, then the Ventral horn is organized more into columns. And so here in lamina 9, we actually have something more like a rod-shape of cells. And there are other more rod-shapes collections of cells that we recognize as laminae 7 through 9. Now, we still use the term laminae, but as I'm suggesting, the organization in the ventral horn is more columnar or rod-shaped, than it is laminar. Alright. So there are connections between the Dorsal horn and the Ventral horn made by local circuit interneurons, and in one special case, there is an incoming sensory axon that synapses directly on alpha motor neurons that mediates our very fast myotactic reflexes. So, we'll talk about that in a later tutorial. For now, I just want you to have an overview of the organization of the gray matter in the spinal cord. And since it's gray matter, we expect to see cell bodies and that's why I'm showing you this Nissl stain. But now, let's turn our attention to the white matter that surrounds that gray matter. And for that purpose, I want to show you a myelin stain section. So, so this is a typical way that a myelin stain section might look at a cross-section through the nervous system. One can often see a great detail that is otherwise not made visible in Nissl stain if you stain with silver salts or perhaps gold chloride or otherwise make the white matter look dark. So, please don't be thrown off by this convention. Essentially, wherever we see dark staining, we are looking at white matter. So, that means that where we see the lighter stain, that is going to be gray matter. So, this is essentially the inverse visual appearance of what we just looked at with the Nissl stain. Okay, I think that should be clear enough. So, what we have here is white matter that surrounds gray matter. And we recognize, basically, three zones of white matter around this gray matter. On the dorsal side of the spinal cord, or the posterior side, we have the dorsal column of white matter. On the lateral side, roughly to about this region, we have what we call the lateral column. And then, on the ventral or the anterior side, we have white matter that we recognize as the ventral or the anterior column of white matter. Now, in each region of white matter, we're going to find different kinds of sensory or motor pathways. And we'll get into that in quite a bit of detail as we progress over the next couple of weeks. So, I won't give you all that detail now except to just make a few broad statements about how these tracks are organised within this white matter. Essentially, the dorsal column is a sensory zone. There are sensory axons that are ascending the spinal cord in the dorsal columns. In the lateral column, we have a mixture of sensory axons, and one very large and very important motor pathway that occupies actually most of this dorsal, excuse me, most of this lateral column, and that's the lateral cortical spinal tract. We'll spend a fair amount of time on the organization of that one pathway. Now, likewise on the ventral or the anterior column, we have a mixture of both sensory and motor pathways. And here, with respect to the sensory pathway, there is one very important pathway that we'll spend a fair amount of time talking about. And that pathway we find roughly in this region of the ventral or anterior column. And this is where we find our pain pathways that are ascending the spinal cord, conveying sensations about pain and temperature to processing stations in the brain stem and in the forebrain. Much of the rest of this system is a collection of small descending motor pathways that are conveying signals mainly from the brain stem to the ventral horn of the spinal cord. Alright. So, stay tuned, we'll talk about those sensory and motor pathways in detail as needed, as we get into a discussion of sensory motor systems.
