© 2010
  University of Idaho
  All rights reserved.
  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

So let’s begin by talking about glial cells, what they are, how they work, and what they do.  Let’s begin by going to slide two.  First, as we can see here, glial cells are not neurons, but they are very, very important.  There are also a lot more glial cells than there are neurons, and depending upon which anatomical books that you use, there’s as many as two to fifty times as many.  Glial cells are unlike neurons though.  They have no action potentials but they do have threshold changes, and they’re extremely important to helping with the action potential.  In general, glial cells are there to help neurons, they act in support functions, and they do a lot of other things. 

Now as we can see in slide three, there are two major groups of glial cells.  There are microglia and macroglia.  So let’s talk about microglia first.  Microglia, are phagocytes.  They eat up and destroy things that have already died.  Basically, they migrate to areas that are damaged, either from a stroke or some other brain trauma.  In essence, what we have is some kind of lesion and get what is called gliosis.  That is, glial cells concentrate in some area and clean up the damaged material.  But as they do, they form scar tissue. 

In addition, microglia also serves as part of the immune system.   So, they’re going to help kill things that are bad in the brain and other structures.  In addition to microglia, there is also another type of glial category.  That is what we call macroglia.  There are a wide variety of different types of macroglia, and we can see these types on slide five.  They include cells such as astrocytes, oligodendrocytes sites, ependymal cells, and Schwann cells.  So, let’s begin by talking about each of these in more detail.  Let’s start with astrocytes on slide six.

As you can see on slide six, astrocytes are called hollow stars and they basically appear transparent and actually look like a star.  They can make contact with both gray and white matter and they don’t move around very much.  There’s a variety of different types of astrocytes.  The first major type is what are called fibrous astrocytes.  These are the types that you find in myelinated tissue.  However, they don’t make the actual myelin tissue in and of itself.  The other type is called protoplasmic astrocytes.  These are, again, star shaped and have lots and lots of cytoplasm.  They basically function and bond with capillaries. They are phagocytotic and kill things that are bad for the nervous system.  In general, astrocytes are thought to take material, digest it and push it through the capillary blood system for removal.  So, what they’re in essence doing is removing waste and other types of things that are out there.

The next type of macroglia are called oligodendrocytes.  Oligodendrocytes are located in the central nervous system and it’s a many branching cell.  In the central nervous system, oligodendrocytes will myelinate approximately 30 to 40 different axons.  The problem is that they don’t regenerate well.  So when you damage these particular cells, you do not have any replacements.  A classic example would be a problem with demyelinating disease.  Once the oligodendrocyte is gone, it doesn’t come back.  As a result you have a lot of problems.

The third type of macroglia is what is called the Schwann cell.  Schwann cells are sort of like oligodendrocytes except that they’re found in the peripheral nervous system.  They also make myelin, but unlike the oligodendrocyte, these cells are only make one myelin sheath. So, one Schwann cell would make one myelin sheath.  They also regenerate faster than an oligodendrocyte.  Basically, they provide conduits for an axon to follow in the peripheral nervous system.  When you damage structures in the peripheral nervous system you get better neural regeneration.  Thus, sensation and movement come back over time, rather than the problems you see in the central nervous system.

So, as we see on slide 10, in the central nervous system when the oligodendrocytes are destroyed, the axon distribution is very confused.  As a result, there are no conduits and growth cones from the nerve cell (primarily from the axon) are random.  So, they don’t go back to the places where they were.  In the peripheral nervous system they go back to the same locations more often.

The last type of macroglia are called ependymal cells.  These form the linings of the ventricles of the brain.  They also have cilia and appear to aid the movement of cerebral spinal fluid through the ventricles on the brain and into the spinal cord.  As we see here, the cerebral spinal fluid is made very rapidly and is very dynamic.  You can actually watch where it goes if you use particular types of tracers within it.  As we can see here, just using diffusion can’t explain the speed of CSF fluid.

So, now we’ve talked about two major groups of glial cells.  Let’s talk about some functions of glial cells.  These are shown on slide 12.  So let’s talk about each of these in general. Let’s first talk about myelination.  As we can see in slide 13, myelination occurs because of the oligodendrocytes and Schwann cells.  They myelinate the axons that we have and they cause a saltatory function.   That is, the action potential will jump from one note of Node of Ranvier to another Node of Ranvier.  So, what you get is a skipping or jumping effect.  This causes action potentials to go much faster, even in small diameter neurons.

In addition, glial cells are extremely involved with neurotransmitter uptake and degradation.  As we can see in slide 14, glial cells remove neurotransmitter after it has been released by the presynaptic elements.  In addition, it also provides compounds that degrade

neurotransmitter (e.g., norepinephrine).  It also has good evidence for uptake and storage of different types of neurotransmitters such as glutamate and aspartate.  But there is much weaker evidence for release of neurotransmitters.

In addition to the uptake of neurotransmitters, glial cells are also extremely, extremely important with ion uptake.  This is going to be important in the communication systems with neurons (primarily potassium).  As we can see in slide 15, glial cells are only permeable to potassium.  So, they’re not going to be permeable to sodium, calcium, or other types of ions.  In addition, they have a membrane potential.  That is, the inside to the outside difference (basically minus 80 or 90 millivolts).   They have a concentration of 3 milli molar of potassium.  So, it’s going to depend upon a particular ion concentration, and diffusion

So, let’s talk about this concept a little bit because it’s extremely, extremely important.  As we can see on slide 16, what we’re going to do is take a glial cell (usually an astrocyte) and put them in a water bath of three milli molar.  That is, the potassium concentration outside of the glial cell is potassium is 3 milli molar.  So, as we see here, as we se on slide 17, we have a glial cell with a measuring device in it and a water bath with some level of potassium on the outside. Because of that, the resting number in potential (that is, the inside of the glial cell compared with the outside of the water bath) is 80 millivolts.

Well, let’s say that I increase my levels of potassium from 3 milli molar to 30 milli molar.  Now what happens?  Well, what we do now is increase the amount of potassium in the fluid.  As a result, more potassium is going to enter the glial cell by diffusion.  As a result, the glial cell is going to become more positive.  So, as we can see in slide 19, when we use our little measuring device, we increased our potassium concentrations on the outside.  As a result, more has gone into the inside of the glial cell.  As a consequence, the inside of that glial cell is now more positive, going from a minus 85 to a minus 60. 

I can also do the same thing with concentrations.  That is, I can decrease my potassium concentration to .3 milli molar by just adding more water.  As I do, I reduce the amount of concentration on the outside of the glial cell.  More potassium leaves the glial cell and moves back out into the extra cellular fluid.  As a result, the glial cell becomes more negative.  So, as we can see here in slide 21, when I added more water to my water bath, we have our potassium concentration to .3 milli molar.  As we can see, the potassium has left.  Now the inside of the glial cell is even more negative than was before.

So, we have two concepts here that are extremely important.  The first concept is that as you make the membrane more positive, that is called depolarization.  So, I make something more positive, that is going from a minus 50 to a minus 30 milli molar.  That is going to be more positive and what we call depolarization.  I can also make this membrane more negative and when I do that, it is called hyperpolarization. 

A second concept that is important is that we can manipulate the concentrations of potassium in the interstitial fluid.  As a result, we can drive the glial cell.  Well, the question is how do we do that?  Why is that important?  Well, let’s walk through this.  As we see in slide 23, what we begin to do is stimulate the neurons around that glial cell.  When that glial cell does that, it releases potassium.  So, the more action potentials you get, the more potassium that begins to be released.  As a result, we have more potassium and this fluid that surrounds the glial cell. 

So as we do that, we increase the depolarization of the glial cell and the glial cell becomes more positive.  However, the cellular potassium pumps start and we decrease the potassium levels.  That potassium is going to diffuse from inside the glial cell and what we’re going to get is hyperpolarization.  So, what we’re going to have is combinations of different things that manipulate the membrane potential inside the glial cell.

Now, why is that all important?  Well, as we can see in slide 24, the glial membrane potential basically is going to depend upon that concentration of sodium in that interstitial fluid. So, what they’re going to do is help regulate the amount of potassium that’s in the interstitial fluid.  As a consequence, it’s going to help neurons by buffering them.  Ultimately, by regulating the neurons and helping steady out in a more prescribed manner, you’re going to prevent seizures and other different types of disorders from occurring.

Well now we’ve talked a little bit about glial cells, what they are, and how they work.  In the next sections we’re going to begin to talk about neurons and all of the particular structures that go with them.  So until we do, we hope you’re having a good day.