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

As we go to slide two, we can see that the neuron is classified in a variety of different ways.  The first way to classify it is by the number of processes, second by function, third by speed, and fourth by nucleus to cytoplasmic ratio. 

There are also other ways that the neurons are classified.  Some are classified by the type of neuro-transmitter that’s excreted, and others are classified by the effects the neuro-transmitter has on the postsynaptic element.   So, let’s talk about each of these and kind of give you an idea about what they are.  We will talk about the number of processes first.

Neurons are classified for the number of processes three different ways by the number of processes that come off of the Soma.  The first type of neuron is what we call a unipolar neuron.  A unipolar neuron is shown in slide four.  It has one stock that comes off the Soma and splits into two branches.  As you can see here, there are two different shapes that you normally see.  The top unipolar neuron one has one process come off (which is an axon) and then it’s going to branch and do assorted other things.  The second one is shaped like a T except it is a little, short T, rather than a long T.  This is the shape of a T cell that you find in the spinal cord the first unipolar neuron is oftentimes found in newborns in the brain).

T cells are extremely important in sending information and we talk a little bit about that when we talk about the spinal column.  So, again, the first type of way that you classify neurons by processes is with a unipolar neuron having one structure coming off the Soma.  The second way that we look at neuron with the number of processes are what are called bipolar neurons While unipolar (uni meaning one structure came off the Soma); in a bipolar neuron you have one axon and one dendrite that comes off the Soma.  You can see this in slide five.  Basically, where we find bipolar neurons are in the retina of the eye. 

The next type of neuron that we see is what we call the multipolar neuron.  In a multipolar neuron what you have are multiple dendrites and one axon coming off a particular Soma.  So, what you’re having are multiple structures rather than one or two.  Again each of these neuron types is going to have their own specific role.  But, as you notice in each of these three examples, there’s only one axon (that may split) but there may be multiple dendrites. 

So, the first way that we’ve classified neurons was by the number of processes. The second way that we classify neurons is by function, and this is shown in slide seven.  By function what we have are three different types.  We first have afferent or sensory neurons.  These basically are going to get information from receptors and send them to the central nervous system (usually the spinal cord).  Next, we have internuncial neurons (or what we also call interneurons) that we talked a little bit about previously in the spinal cord.  Basically they are going to connect afferent neurons and the next type of neuron, which are called efferent neurons.  Efferent neurons are motor neurons.  Basically, they get information from the central nervous system and send it to muscle cells in the peripheral nervous system.  So again, (as we see on the next slide), what we have is an afferent neuron coming through, branching with some information going to the brain and some going to an internuncial neuron.  Internuncial neurons then connect to an efferent neuron which will go back out to the peripheral nervous system.

The third way that we classify neurons is by speed.  This is called the Glasser system. The Glasser system basically has three different types of neurons; type A, type B and type C.  Type A are the fastest neurons.  They go about 120 meters a second, or about 270 miles an hour.  Now, you may think that’s relatively slow in relation to systems, but since your body is only about six feet, that’s really pretty fast. The big thing about type A neurons is that they’re heavily milanated and they’re relatively large.  Usually you see these neurons when concerned with pain types of systems.  So, you’re going to see a lot of these are coming out of the spinal cord and going up and down to the brain.  So they are heavily myelinated neurons, relatively large, and that send information very, very rapidly. 

The second type are what we call type B neurons.  Type Bs are much smaller than type A neurons.  They are also milanated but they’re slower.  Their speed is only about 3 to 15 meters per second.  You might see these in pathways of the brain, whereas with Type A’s, you might see them in the spinal cord. 

Type C neurons are very slow.  They’re only going to go to .6 to 2 meters per second.  So, they’re not going very fast.  Again, they’re relatively small.  As we can see, but the big thing about type C neurons is that they’re nonmelinated.  Basically, they’re going to be concerned with dull or throbbing pain that’s going to last for a long period of time.  The classic example of this might be a toothache.

So, how do these neurons work together?  Well, let’s say that you cut your hand.  What you’re going to feel?   The initial cutting begins and the initial pain system is going to be from a type A neuron and that’s going to say, “Get my hand away from this sharp object.”  Then you’re going to have type Cs being activated, which are going to be slower.  Then you’re going to have a dull or a throbbing pain. 

The next way that we classify neurons is what we call nucleus to cytoplasmic ratio.  Basically, this is the ratio that compares nucleus size to the surrounding cytoplasm.  There are a couple of different cells that you see here.  In one type of cell which we call Stelate cells, you see large nuclei with relatively little cytoplasm, whereas in pyramidal cells you see a small nucleus with large amounts of cytoplasm plasma.  These are going to be related to the kind of cells that they work with.  Usually Pyramidal cells are concerned with motor movement and Stelate cells tend to be more sensory.

Ok, so we’ve talked now about the four major schemes in relation to classifying neurons.  There’s also a couple of other ways.  So, let’s talk about them for a second.  Basically, as we saw earlier, the type of neuro-transmitters secreted by the neuron is one way that we classify neurons.  So, we call them, for example, dopaminergic neurons or a gabanergic neurons or something to that extent.  Or, we also talk about their effect.  So, if they’re excitatory neurons, we will call them excitatory, and inhibitory if they inhibit.  One type causes excitatory signals in the next neuron fires while the other shuts that neuron down.  These classification systems aren’t used as much, but they are still used out there and you need to be aware of them. 

Ok, we’ve talked now about some types of neuro classification schemes.  In the next section we’re going to talk about the different neuron structures, how they work, and what they do.  So until then, we hope you’re enjoying the lecture and have yourself a great day.