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  University of Idaho
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  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

When one looks at the mid brain or the Mesencephalon, one sees a variety of different structures.  What we see in slide two are some particular fibers and structures that we see within a cross section of the midbrain.  This is, just one section.  As you notice here, this is a diagrammatic representation of a slide you might see in a pet scan.  So, again, what you have is a cross section of the brain, basically a horizontal slice and having some different structures within it.

Well, what are some structures that are involved in the mid brain?  As we can see in slide three, the midbrain contains two major groups of structures—the Tectum and Tegmentum.  So, let’s talk about the tectum first.  The tectum as we see in slide four is the dorsal part of the mesencephalon.  That is, it is the part that’s above the other parts.  And it contains two major structures, the superior colliculus and the inferior colliculus.  So, what’s the superior colliculus? Well, the superior colliculus (as we can see in slide five) basically receives fibers from the retina of the eye.  Basically what it does is integrate that information, and then send that information to the cortex.  The superior colliculus is extremely important in controlling eye movement, especially tracking movements.  So, what I’d like you to do is take a coin, any kind of coin that you have, hold your head still and flip the coin up in the air and just follow it with your eyes.  That is using the superior colliculus to help you do that.  So it’s especially important for tracking an object in time and space.  So a person with a really good superior colliculus might be a really good baseball hitter.  Specifically, you have a baseball that’s coming at you 90, 95 miles an hour and you have to make a bunch of movements to hit the ball, and hit the ball well.  Also, you have to determine if it’s spinning, if it’s turning left and right and on and on.  So, all of these things are going to be extremely important for you.  The other major important thing about the superior colliculus is the word superior.  Superior means above anatomically, so the inferior colliculus which we’ll talk about in a minute is below the superior colliculus.  Superior does not mean better than.  So again, the superior colliculus is basically involved with tracking movements.  And it is a very, very important structure in certain organisms such as frogs that are catching flies as they’re flying time and space.

Well, what about the inferior colliculus.  Well, the inferior colliculus (as we see in slide six) receives information from the cochlea of the ear and it sends it on to the cortex.  Like the superior colliculus, it has a role in organizing the auditory stimuli that come into the brain.  We don’t know as much about the inferior colliculus as the superior colliculus, but basically we do know that it helps to organize material so you can hear and hear well.

Well, now we’ve talked a little bit about structures in the tectum.  What about the tegmentum.  What is that?  Well, as we see in slide seven, the tegmentum is part of the mid brain that’s below the tectum.  So, anatomically it would be inferior to the tectum.  What it does is contain the rostral end of the reticular formation.  Remember the difference between rostral and dorsal?  So anyway, basically what the tegmentum does is basically contain nuclei that help control eye movements, and it has a variety of different structures and these are listed here, periaqueductal gray matter, red nucleus, substantia nigra, and ventral tegmental area. 

In addition to that, the tegmentum contains the reticular formation.  The reticular formation (as we talked about in the hindbrain section) are fibers that go from the medulla and the pons and extend to the thalamus.  Thus there are fibers in the reticular formation in both the hindbrain and the midbrain.  Generally, it has about 90 different nuclei. 

The reticular formation receives information from the cortex, the thalamus and the spinal cord.  Its basic function (as we see in slide nine) is for controlling your state of arousal.  In addition to that, the reticular formation also may play a role in sleep.  But what we really know about the reticular formation is that it acts as a filter, and is extremely important in focusing your attention and keeping you on track within certain objects in your environment.  As we can see here at the bottom of this slide, it allows you to concentrate on important things (such as myself) and ignore unimportant things such as the hum of your computer or some other things that may be going on in the room around you. 

Well, what about periaqueductal gray matter?  What does that do?  Well, periaqueductal gray matter basically consists mostly of neuronal somas, or what are called cell bodies.  These structures help control things that are related to species specific behaviors, such as mating and fighting. 

n addition to that, another structure in the tegmentum is the red nucleus.  It’s involved with motor movement and helps to control fine movements.  We’ll talk more about this structure as we talk about structures in the basal ganglia.  Basically the red nucleus is going to receive information from the cortex and cerebellum, and sends that information to the spinal cord.  This information will go down the spinal cord and end up in the final common pathway that goes to muscle groups.

Another part of the tegmentum is called the substantia nigra or black substance.  It’s part of the basal ganglia as well.  Like the red nucleus, it is involved with fine motor movement and provides input to the red nucleus and other structures.  And finally it sends other axons to the putamen and caudate nucleus which we’ll talk about a little later. 

So, we’ve talked about are some structures that are in the mesencephalon and each of these has extremely important roles.  One final point that relates to structures in the hindbrain and midbrain is the structure that we called the brain stem.  The brain stem basically consists of all the structures in the hindbrain and the midbrain.  An individual can get cortical damage.  That is damage to the upper parts of the nervous system (cortex), can be unconscious or in a coma.  However, because they have not damaged their brain stem, they can live for long periods of time, provided that they are fed and clothed and assorted other things by some external care workers.  Often these individuals can live for many, many years.  In fact, there have been cases where the damage to the cortical structure resulted in a coma has lasted seven to ten years. 

Well this kind of concludes our material and structures in the midbrain.  In the next section, we’re going to talk about forebrain structures.  First, we will talk about older forebrain structures and then newer forebrain structures.  Until we do, you have yourself a wonderful day and we will talk with you soon.