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

We need to remember that axons are structures that send information to other neurons or other muscle cells.  However, unlike what you probably remember from Introductory Psychology, axons can also receive information.  Like the Soma, the axons are also composed with the bilayer and the inside of the axon or the actual membrane of the axon is called the Axolemma.

There are a lot of different structures that we have in axons.  As we see in slide three, a lot of them are similar to the Soma.  They have mitochondria, they have voltage gated channels, they have passive channels, and on and on.  However, one structure that the Soma does not have is called the axon hillock.  This is located at the base of the axon and is the place where action potentials begin.

The axon hillock then extends into the body of the axon.  And as we see in slide five, this structure can branch (called the collateral).  Now the axon continues until the branching begins to form smaller and smaller and smaller branches.  These smaller branches at the very end of the axon are what are called teleodendria or what are also called end feet.  Again, the axon contains the microtubules or neurotubules which transport material from the Soma to the pre-synaptic element which is located at the end of each axon. 

Slide six shows you a diagram of the different structures in the axon.  As you can see here, we have the Soma, then the axon hillock, which extends into the body of the axon itself.   Then you have the branching, and at the very end of the axon we have a structure called pre-synaptic element. 

So, let’s talk about the pre-synaptic element for a minute.  As we see in slide seven, they are given a lot of different names, but the most correct name in the literature that we’re seeing now is called the pre-synaptic element.  But, (like you probably learned in your book from Introductory psychology and is also discussed in Carlson) they are also called terminal buttons, terminal boutons, synaptic knobs and many other names. 

Again, these structures contain a lipid bilayer, but they also contain a wide variety of other structures including synaptic vesicles, a wide variety of channels, autoreceptors and reuptake channels, amongst other things. 

Slide eight kind gives you an overview of the different structures.  As you can see on the left hand side, we have the microtubule bringing down vesicles (which are the little round knobs).  We also have a variety of different receptors and channels.  In addition, on the pre-synaptic element (at the very end on the top) we also have reuptake channels.  Reuptake channels basically reabsorb different types of neuro-transmitter.  We also have autoreceptors which monitors how much neurotransmitter is located in the synaptic cleft.

So let’s talk about each of these in a little detail starting with figure nine (which are the synaptic vesicles).  Again, synaptic vesicles are different types of neurotransmitters or neuropeptides that basically are surrounded by a sack or a vesicle.  Again, these materials are made by Golgi bodies and then sent down via axonal transport.  However, synaptic vesicles must have calcium for them to release their neurotransmitter.  We will talk about that in more detail in the next sections.

There’s also a wide variety of different ion channels.  As you can see in slide 10, there’s sodium and potassium channels which you saw in the earlier parts (and we’ve talked about in earlier systems), but we also have other channels (such as calcium channels) as well.  These are also different from channels that are associated with receptors, such as GABA receptors.   Ultimately these ion channels are extremely, extremely important for neurotransmitter release.

On slide 11, we also see another structure called an autoreceptor.  These are receptors in the pre-synaptic membrane.  They’re designed to monitor the amount of neurotransmitter in the synaptic cleft.  What they’re going to do is help in the process of what is called up or down regulation which we will talk about in the next major module. 

There are also a wide variety of other receptors.  As we can see in slide 12, these are located in the presynaptic element as well.  The GABA receptor is a classic example.  The synapse that the GABA neuron makes with the presynaptic element is called an axoaxonic synapse.  In essence these receptors shut down action potentials.  We will talk about this in much greater detail in the next major module.

Slide 13 gives you a picture of the synapse.  This slide is from Carlson.  Again as you can see, all the different structures are there and all of what they do. 

Now, in addition to all the structures that we’ve talked about so far, axons can be one of two types.  They first can be myelinated axons.   Myelin is basically a fatty covering or a sheet that surrounds the axon.  What it does is helps to increase the speed the action potential.  And as we might have anticipated, the more myelin there is, the faster the speed.  There are also spaces between these myelin sheets.  These are called Nodes of Ranvier and is where the axon is actually exposed to the system.

Now, myelinated axons are different from the next major structure that we see in slide 15.  These are called nonmyelinated axons.  Many axons don’t have any kind of myelin and they are slower than myelinated axons.  However, the fatter the axon is, the faster the action potential will go.  This has to do with resistance inside the axon.

In conclusion, this section talks a little bit about axons and the structures that are involved.  Realize that all of this information is tied together with the neurophysiology that we’ll talk about in the next major module.  But until we get to that point, what I’d like you to do is kind of review these major structures and make sure that you understand what each of them does.  In the next section, we’ll talk about the next major type of structure that comes off the Soma called dendrites.  So until then, enjoy your afternoon.