Now, I want you to understand the organization of the spinal cord. Here we have a cross section of the spinal cord.
There are three features in this cross section or axial section that I want you to understand. First is that the spinal cord
is made up of centrally located gray matter that we see in through here. Then it communicates here to the opposite side
that we see in through here. This has a butterfly appearance based on how it’s organized within the spinal cord.
In the central aspect here, there is a central canal. Cerebrospinal fluid is found within the central canal.
Lastly, note that the peripheral aspect of the spinal cord in through here, here more laterally, here anteriorly,
and over on the opposite side anteriorly and laterally as well, this represents peripherally located white matter.
Ascending and descending tracts are travelling in these peripheral areas, whereas nerve cell bodies are housed
in the gray matter. So white matter is peripheral. Gray is internal in the spinal cord. This is just the opposite of how
it’s organized in the brain where the white matter is found peripherally. Then your gray matter is found more deeply
within the brain. Now that you have a basic understanding of the arrangement of gray matter to white matter
in a cross section through the spinal cord, it’s important for you to realize that the ratio of gray to white matter
will vary depending on which segment or cross section of the spinal cord that you are observing. So here in this series
of axial or cross sections, you have a section through the upper portion of the spinal cord of the cervical area.
Here’s the thoracic area. Here is the lumbar area. Then this lower region represents the sacral axial or cross section
through the cord. First concept here is that when you look at the ratio of gray to white matter in the cervical area,
as you see here, you have a lot of white matter here and little gray matter in comparison. If you divide the area
represented by gray matter by the much larger area represented by the white matter, this ratio would be low.
The concept for you to remember is that the ratio of gray matter to white matter increases inferiorly.
So, as you take a look here in the thoracic area, you start to see a greater ratio of gray to white. As you go over
here to the lumbar area, a greater ratio of gray matter to white matter. Then when you get to the sacral area,
you see a large area of gray matter. You see in comparison very little white matter. So here in the sacral area,
the ratio of gray matter to white matter is very, very high. The rationale for this is that by the time you get to the
cervical area, descending and ascending tracts that are coming from the much lower levels of the spinal cord are
being joined in by ascending and descending pathways that are responsible for conveying information from the more
upper aspects of the body. So you have a lot more ascending and descending traffic in the more superior axial sections
of the cord. The white matter then will start to overwhelm the amount of gray matter that you have up in the upper
levels of the spinal cord itself. Now, I want you to understand some of the details that relate specifically to gray matter.
The first concept here with respect to the gray matter is that it’s organized into horns. Here, we’re looking at the
dorsal horn. The dorsal horn receives sensory input and also receives input that is involved in the coordination
of reflexes. The larger area located ventrally is the ventral horn. This is a more expanded area of the gray matter.
The ventral horn is housing alpha and gamma motor neurons. What this means is we have a functional division
of labor between the horns. The dorsal horns are responsible for processing and relaying sensory information,
whereas the ventral horns are going to be responsible for motor output. In thoracic segments T1 through T12
and L1, L2, and perhaps L3, there is an intermediate horn or gray horn. We see that in this thoracic spinal cord
section, axial section. The intermediate horns contain motor neurons that are a part of visceral motor output.
Sympathetics are being relayed out of the cord from motor neurons then that are sympathetic in nature,
efferent in nature that reside here.