The brainstem includes the medulla oblongata, the pons, and the midbrain.
Your book describes the midbrain first. But I prefer to start from the spinal cord and move up. So we'll start at the medulla and end at the midbrain.
Most of us tend to think about the brain as allowing us to do all sorts of voluntary things, or as an organ that provides us with cognitive abilities. But the brain is the main control center of our bodies. As such, it also has to be responsible for more basic life controls, like breathing, heart rate, blood vessel constriction, and sleep. Also, the brain has to allow us to integrate many different things that are going on in our world; we have to process sensory information and send it to appropriate, not inappropriate, centers; that way, if you are touched on your hand, you can respond with your hand-- not with your nose! So you will also see that our brain offers relays of information, from one area to another, allowing for divergence and convergence of information, which is the way we understand things.
Since the brainstem is the closest brain region to the spinal cord, it will have to deal with a lot of the basic information about life functions, like breathing, which occur in our bodies. It will also have to provide for some relaying of information up and down from the spinal cord to higher brain centers.
Finally, the brainstem, like all other CNS regions, contains both white matter and gray matter. The gray matter is found in clumps and clusters throughout the brainstem, with white matter running up and down and between all the gray matter. It isn't as orderly as in the spinal cord where the white matter encircles the gray. It also isn't as orderly as in the cerebrum or cerebellum. But, there's a lot of white matter, because a lot of axons have to go up to and down from the higher brain centers. Just as you saw in the spinal cord, though, the gray matter contains the cell bodies and dendrites, while the white matter contains the axons and myelin.
Three important gray matter centers located in the medulla:
There are other functions carried out by neurons (gray matter) in the medulla as well. But none quite as important as breathing or heart rate. So we'll skip the others in this class.
Many axons cross from one side of the brain to the other within the medulla:
You have learned that each side of the brain controls the opposite side of the body. Somewhere in the brain all this information has to cross over. The major site of that crossing over is in the medulla. Simply stated, all the information about touch in the body and all the information to control movements in the body crosses over in the medulla. Below, I have explained each of these in a bit more detail. The points described below are also shown, somewhat, in the tract figures 11.6 and 11.7 of your book (the right hand path on each figure only).
The gray matter of the pons mainly serves to relay information from one brain center to another. Neurons in the pons send information into the cerebellum or up to higher brain centers. These neurons also have to receive information from the cerebellum or from the higher brain centers.
There are also many bundles of white matter in the pons. This just shows how the pons is in heavy traffic location (a major axonal thoroughfare).
There are quite a few portions of the midbrain that we could discuss, but there's only one that I really want you to learn. The midbrain has a bulging region. Actually, this bulge is really made up of two smaller bulges on each side of the brain. The overall bulge due to these four gray matter regions is called the corpora quadrigemina ("corpora" means bodies, and "quadri" means four, since there are 4 bumps total-- 2 on each side, and I think that gemina means growths).
Each of the 4 bumps is considered to be a "colliculus" . I think that this term is used instead of nucleus because there's so much gray matter that it bulges. One set of these colliculi is more rostral than the other. Because of this, the one on top is called the superior colliculus, while the one on the bottom is called the inferior colliculus. The left and right superior colliculi plus the left and right inferior colliculi make up the entire corpora quadrigemina.
Now that I've explained this long terminology, let me tell you what these regions do. They are involved in some of the more basic things that we do with our senses... in particular, sight and sound. When we see something within our visual field, we can turn our eyes to it. And when we hear something, we can turn our heads toward the sound. These types of more reflexive movements to sensory information are carried out in the corpora quadrigemina. They can be more accurately described as orientation toward sensory information. The superior colliculi are involved in the visual orientation, while the inferior colliculi are more important for auditory orientation.
Bats and owls use sound to locate their prey. They are experts at sound processing and orientation toward sound. I hope that you can understand that these organisms have much larger inferior colliculi than we have.
Brainstem in general
There are some clumps of gray matter carrying out certain functions that are not restricted to one particular region of the brainstem. Instead, these gray matter regions span multiple brainstem regions so that their functions are served at many levels. What things?
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