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.
Medulla oblongata
Three important gray matter centers located in the medulla:
 | Respiratory center
Inspiration, not expiration, is the main portion of respiration that
has to be commanded regularly. We have to work to breathe in, but exhaling is a
rather passive event (as you'll see next semester) during routine breathing. A major
function of the respiratory center is to command the rhythmic inspiratory events.
Another portion of the inspiratory center is involved in changing our breathing rate
according to our bodys' needs.
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| Cardiac center
Heart rate is adjusted in the cardiac center. This way, our brain
can integrate all sorts of information from within the body and determine when to adjust
heart rate.
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| Vasomotor center
This medullary center commands blood vessel constriction. You'll
understand the importance of this function more next semester.
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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).
| Touch/pressure/proprioception crosses
over in the medulla (dorsal column information)
When the sensory information that is carried up to the brain in the
dorsal columns arrives in the medulla, it has a relay point here. The sensory
information synapses on neurons in one of two gray matter regions in the medulla (either
the nucleus gracilis or the nucleus cuneatus, depending on whether the information was
coming from the fasciculus gracilis or the fasciculus cuneatus). These gray matter
regions are called nuclei; a spot of gray matter within the white matter of the medulla
may look like a central clump (a nucleus), right? As the information gets carried up
further to the brain from these two nuclei, it is sent to the contralateral side of the
brain. So, the dorsal column tracts run up the spinal cord ipsilateral to their
sensation, their axons synapse in a particular nucleus in the medulla, and the neurons in
the nuclei send their axons over to the contralateral side to run up the brain the rest of
the way on the opposite side of the body from the original sensation. All this
sensory information crosses here in the medulla.
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| Motor information to control our bodies
crosses over in the medulla (lateral corticospinal information)
The axons running down from the brain to descend in the lateral
corticospinal tract of the spinal cord make it all the way down the brain to the medulla
before they cross over. So, if the brain wants to control movement on the right side
of the body, the left side of the brain has the neurons to do it; they just send their
axons down the left side of the brain to the medulla, and then the axons cross over to the
right side within the medulla, before continuing down the right side within the
corticospinal tracts.
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Pons
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).
Midbrain
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?
| Alertness
This function is rather important. Right? Hello? Are
you there? Are you still with me? If you are starting to fade, then this
function is not working well right now for you!
Our alterness is controlled by gray matter that runs throughout the
brainstem that has been given the name of the reticular formation.
So, they gray matter is called the reticular formation. The neurons within it help
us to control our levels of alterness. How about when you are overly alert?
Like the feeling you get when you have had way too much caffeine and are flying off the
walls? At that time, your reticular formation is overly active. And when
you're tired and about to fall asleep, that's when your reticular formation is not active.
It can be good for our reticular formation to be quiet (or low in
activity), because we need that to fall asleep. In fact, the activity of our
reticular formation regulates our sleep.
Does it seem appropriate that our sleep/wakefulness is a function that
is housed in our brainstems? I hope so. That is a function that doesn't need a
higher center to run it. It should be rather automatic and associated with our
energy exertion and with the time of day.
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| Voluntary Breathing
Although our medulla has a portion of the breathing control within
it, the control over breathing extends into the pons as well. So both the medulla
and the pons help us to breath voluntarily. What does that mean?
I assume you understand automatic breathing. Like while you're
sleeping. And I also assume that you understand that if you exercise, the amount
your breathing speeds up is not something that you can control-- that is also done
involuntarily to ensure you get enough oxygen.
But voluntary breathing is used for us to be able to speak or blow up a
balloon. When you speak, you have to breath more irregularly-- a quick breath in, a
long exhale during which you make sounds, and then another quick breath in. This is
not done involuntarily. The pons and the medulla control it.
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| Supplying their cranial nerves
Other nuclei (gray matter areas) within the brainstem contain
motor neurons or sensory neurons that have axons running out the cranial nerves in the
brainstem. For example, all the motor neurons to our facial muscles have to lie
somewhere-- they lie in nuclei within the brainstem. And, the motor neurons to our
eye muscles also have their cell bodies in brainstem nuclei.
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