Central Nervous System—Internal Organization
In our last lecture, we looked at the external or gross features of the brain and spinal cord—what can be seen from the
outside—and discussed how the adult brain is subdivided into five areas. In this lecture, we want to look briefly at how the CNS is organized internally. Rather than surveying the literally hundreds of individual nuclei and areas in the human brain and spinal cord, here we are concerned with giving a brief description of some of the major internal structures that will be the focus of future lectures.
I. In the last lecture, we looked at how the CNS looks from the outside. Now we want to look at the internal organization. When early neuroanatomists cut open and viewed the internal part of the CNS, they noted that in fresh tissue some areas appeared “grayish” and other areas appeared “whitish” by contrast.
A. We still use these terms in modern neuroscience, saying that the CNS is made of two types of matter: gray matter and
B. To understand how these terms are commonly used, and to what they refer, we need to be familiar with the parts of a neuron. The major parts of a neuron are the cell body, dendrites,
and the axon. A generic neuron is drawn in Figure 3.1.
thC. In the 19-century, when stains were developed and
applied to sectioned brain tissue, it was discovered that gray matter consisted of aggregations of neuron cell bodies; groups of neurons that from functional and structural areas in the nervous system are called nuclei (singular = nucleus). A nucleus is
a collection of neuron cell bodies into a structure with unique cytoarchitecture, connections, and function.
1. Franz Nissl (1860-1919) developed a method for cutting the brain into thin sections and staining the sections with a dye that stained structures (organelles called “Nissl
bodies”) within neuron cell bodies.
2. Nissl stains can be used to stain neuron cell bodies, but not their processes (axons and dendrites), and thus can be used to identify nuclei in the brain.
D. Using other techniques, it was discovered that the white matter of the CNS was composed of neuronal axons; what actually looks “white” in the fresh brain is the fatty myelin
sheath that surrounds most axons (Figure 3.1).
1. Karl Weigert (1845-1904) first developed a method for staining the myelin sheath.
2. We can view white matter areas of the brain by using stains which selectively stain myelin (e.g., a Weigert stain), or stain structures inside of axons, or by injecting substances into the brain and filling the axons with a substance that can be visualized.
E. Finally, in a special method developed by Camillo Golgi (1843-1926), silver is used to fill or “impregnate” neuron cell
bodies and dendrites (but not axons in the adult CNS); the Golgi method can be used to distinguish between neuron types on the basis of their dendritic trees or arbors.
1. We have found that there are about 150 kinds of neurons based primarily on difference in their dendritic arbors, making neurons the most diverse cell type in the body.
2. In modern neuroscience, we can inject various dyes directly into individual neurons and visualize their dendritic trees and processes.
II. By using various methods or combination of methods, the internal organization of the CNS, which is not visible from the external surface, can be revealed.
A. When looking internally, we view the ventricles and individual nuclei throughout the brain and spinal cord.
B. Thus, we see that the brain and spinal cord are made up of a number of individual nuclei (gray matter) with bundles of axons coursing between them (white matter).
Any recent neuroscience text will cover most of the basic information covered in this lecture, although in much greater detail/ the authoritative book in neuroscience in Principles of
, edited by E. R. Kandel, J. H. Schwartz, and T. M. Neural Science
Questions to Consider:
1. Would an area stained with a Nissl stain be considered white matter or gray matter? Why?
2. Approximately 150 different neuronal types can be distinguished on the basis of their dendritic trees. What specific method could be used to visualize dendritic arbors?
Central Nervous System—Internal Organization
Welcome back to our course on the brain. In our last lecture we looked at the external or gross features of the brain, what we could see when we liked from the outside. We talked about how the brain and spinal cord make up the central nervous system. If we were to subdivide, liking at our brain model, we would subdivide the brain going from rostral to caudal. We divide it into telencephalon, diencephalon, mesencephalon or midbrain, metencephalon, myelencephalon, and the spinal cord.
In the last lecture, we also talked a little bit about different structures that work within these different brain subdivisions. What we want to do now is to look at the complexity of the internal organization of these areas. So if we look from the outside, and we talked about one or two areas in them, if we actually take the brain and cut it and turn and look at it, we see that it’s very complex on the inside. We have to have a microscope to de that. We have to have some other methods for doing that. That’s what we want to talk about in this lecture, is the internal organization of the brain itself.
Now, when early neuroanatomists look the brains out of cadavers they noticed, first of all, that the brain substance was sort of like cottage cheese. This was not very good for doing any king of dissection. One of the first things that had to happen so that we could understand something about he structure of the brain, was methods had to be developed so that we could look at it. One of the most important methods was discovering what kinds of chemical would fix the brain or make it hard so that it could be sliced up and viewed. So that was one of the first things they did.
When they did this and they cut into the brain and they turned and looked at it, it looked like the brain had very little natural contrast, but there were areas that looked a little bit “grayish”
or a little bit “whitish.” And we still use these terms in modern
neuroscience, but before we get to a definition of them I wanted to show you a real human brain and the fact that there is very little natural contrast.
I want you to look at this slice here, and this slice is taken in this plane so it’s like through the top like this, looking down. What you can see is there would be two hemispheres. This would
be the front of the brain. This is the back of the brain. And one of the first things you can see is that there is very little natural contrast. Now, these are human brain sections that were embedded in plastic, so they have a little bit of a yellowish tint. But you can see right around the external surface there appears to be a very thin ridge or margin that’s a little bit darker than
on the inside. And this will become clear what this means in a minute.
Also you can see these huge holes. These are the ventricles of the brain, the place Leonardo da Vinci thought cognition and perception were located. So there actually are those holes in the brain. If we look at another brain section, we see it again. Now, in this brain section, this is the front of the brain, this is the back of the brain. Here is our cerebellum right here; that’s
how we know that that’s the back of the brain. Here you can also see there is still a little bit of darker color around the outer edge, then it’s light. And you see some darker areas again.
That’s because there are different structures located within the brain substance itself, and when the early neuroanatomists cut up the brain, they saw this as well. When they cut up the brain they saw that there was little natural contrast but there
was enough contrast to make some areas look a little bit darker or grayer in a natural brain. Some of the areas looked a little bit whiter.
Now, looking down here, just because we talked about this in our last lecture and I always like to review information, I’m going to
put my finger right here on this structure right here, which is the superior colliculus. That’s a part of the midbrain, but it’s
now cut in this plane. That superior colliculus is one of the structures that are involved in visual reflexes.
So this is what the brain looks like in reality. This has been fixed and, again, embedded in a plastic so it’s changed the color
a little bit, but the early neuroanatomists noticed this difference between what they called gray matter and white
. For us to be able to understand what “gray” and “white matter
matter” refer to, we need to learn something about the parts of a neuron. A neuron is a nerve cell, so we need to look at the parts of a nerve cell.
This is a drawing of what I call a generic nerve cell, and what you see here is a cell body. Just like any other king of cell in the
body, there is a cell and a nucleus in the middle. Then extending from that cell body are there funny-looking structures which are called dendrites. Dendrites are noting more than true
extensions of the cell body surface. The name “dendrites”
stems from the fact that early neuroanatomists thought they looked like the branches of a tree and, in fact, that’s exactly
what they do look like. We will come back to this, because it’s
very important; our understanding of different neurons and different areas of the brain has come in large part from looking at the different kinds of dendritic trees that exist on different neurons. So here is our cell body, and here are our dendrites extending away.
Now, from the other part or side of the cell you see right at this junction here, which is called the axon hillock, there is a
single structure, which leads away from the cell. This is called an axon, and we will come back to this in a minute. Right now, we have the cell body and the dendrites, which are extensions of that cell body.
Early neuroanatomists developed stains, primarily in the th19-century, that were applied to brain tissue. What they
discovered is that the areas that were a little bit darker, this so-called gray matter, were actually groups of neurons or nerve cells. So the gray areas of the brain, the darker areas of the brain, are where the neuron cell bodies are located, and these neuron cell bodies form functional and structural groups. In the nervous system, we call these nuclei. Now, sometimes a word is
used in neuroscience in a way that is different from how it’s
used in general biology or used by the general population. This is one of them.
There are two different kinds of nuclei. One we will talk about in this course, and is a subject of this course, and one which is not. There is the nucleus of a cell, and that’s an organelle, which
is located within the cell, which is where the genetic material of that cell is located. We aren’t going to be talking about that.
When neuroscientists use the term “nuclei,” they are referring
to collections of neuron cell bodies that from a structure with unique characteristics and function. So if I were to say the superior colliculus is a nucleus, it means it’s a specific area of
the brain that has a specific function associated with it, and all it is, is a collection of cell bodies into that one structure or nucleus. So think of it that way, and if you understand the