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

The Soma, as we can see in slide two, contains a membrane.  This membrane is going to separate the external environment from the internal environment.  This membrane is made up of what we call a lipid bilayer.  As we can see in slide three, this layer contains two layers.  The first layer is what we have is what we call a hydrophillic which is water loving, and the other is called a hydrophobic end and which is water fearing.  The membrane is about five nanometers thick.  And as you can see from the figure, the inside and the outside are hydrophillic and the inside between the two layers is hydrophobic. 

Well, in addition to this bilayer, we have lots of stuff that’s inside this membrane (shown on slide four).  There are proteins which are chains of amino acids.  These include lots of things, RNA, DNA, receptors, sodium potassium pump that you learned about, channels and assorted other things.  In addition to that, there are lots and lots of peptides.  Peptides are very small proteins.  They have about 30 to 40 amino acids.  These are the classic neurotransmitters that we usually talk about.

Ok, we’ve talked about some structures that are inside the membrane.  Now let’s talk about the neuron in and of itself.  As we can see in slide five, there are a variety of major structures within a neuron. You have the Soma, you have the dendrites, and you have the axon.  Each of these has other structures within it.  All of these structures, as you can see in slide six, are located there.  So let’s talk about each of these in detail, what they are, and how they work.

First of all, let’s talk about the Soma.  The Soma or the cell body basically has places where messages from other neurons can be received.  This structure is called a postsynaptic element.  However, and more importantly, the Soma is the place where cell metabolism takes place.  As you can see here, there are a large number of structures within it.  Each of these structures does a lot of different things. 

Now there are two classes of cell components inside the Soma.  There are inclusions and organelles.  So let’s talk about each of these in a little detail.  Inclusions, as we can see on slide nine, include a variety of different things. There’s lipofusion, which are fats in the cell.  They ultimately accumulate over time and can destroy the cell.  There are also pigmented areas of the cell.  The classic example of a brain structure that has a lot of pigment is the substantia nigra (or what is called black substance).  Organelles, on the other hand, are structures in the neuron that keep it healthy and living.  Organelles are in the soma, the nucleus, and throughout the cell and they do a wide variety of different things.  So let’s talk about some of these organelles listed in slide 11.  There are things such as neurofilaments, nissel bodies, smooth endoplasmic reticulum, golgi bodies, and many, many other structures.  So let’s talk about these in some detail.

Neurofilaments, as we see in slide 12, basically form the skeleton of the neuron, and give the neuron its shape.  It’s like a skeleton that we have in our own body.  As we age, these neurofilaments tangle up.  This tangling may be related to Alzheimer’s disease and other things because the tangles cause the cell to die. 

The second major set of structures (as we see in slide 13) are called neurotubules.  These are going to receive material (vesicles from golgi bodies) and send them from the Soma to the presynaptic elements which are located at the very end of the axon.  In essence, these structures are long narrow tubes.

The next major structure, as we see in slide 14, are what we call nissel bodies or what we call rough endoplasmic reticulum.  These are used to make proteins for the cell, and they make many different things. They replace RNA, they help repair the structure if you have damage to the neuron.  They also help make neuro-transmitter.  In addition, as you can see here, when you get a lot of stress in your environment, you need to have more things made.  What happens is that stress begins to damage the nissel bodies, and they begin to decrease.  So, they’re used up faster, and cells under stress also swell a lot.  Consequently, the nucleus to cytoplasmic ratio is also decreased.

Golgi bodies, as we can see in slide 15, make packaging for protein.  These are what we call vesicles.  In general, golgi bodies package the proteins that are made by the nissel bodies.   Then those vesicles from golgi bodies are put into tubules and then we get what we call axoplasmic flow.   That is, the vesicle is going to go from one structure to another structure.

So, what are some types of axoplasmic flow?  Well, as you can see in slide 16, there are three types.  There is fast, slow, and retrograde.  What’s going to happen is neurotubules are going to send these vesicles from the Soma to the presynatpic element, (the structures at the very end of the axon).  The neurotubules have actin in the walls and the lipids have myocin.  Ultimately, the combination of the two causes a contraction peristalsis (which is like the contractions that you have within large intestine to move food through one into the other).  In essence, it sends material from one end all the way down to the other end.

As we said, there are three types.  The first type is fast peristalsis, so fast axoplasmic flow.  Here materials move about 400 mm. a day (which is about 16 to 20 inches).  However, you can get up to 2800 mm. a day. So, this flow can be very, very rapid.

Slow axoplasmic flow is a little slower.  It is only going to go about 40 mm. per day.  This flow works by diffusion.  So, of the two of these, fast is more important you are going to be carrying more material; and most of this material you’re carrying is neurotransmitter. 

Now there’s a third type of axoplasmic flow and as you can see in slide 18, this is called retrograde flow.  Once you get information sent to the presynaptic element or the terminal button (which Carlson describes in your book), the material is oftentimes reabsorbed.  it’s used to help recycle products.  These products are going to be reabsorbed back in the presynaptic element and the material then goes back to the Soma.  Usually this material travels about 200 mm. a day, and again it works by diffusion.

Now, this material returned to the Soma provides a very good feedback loop to the Soma.  For example, if you’re getting a faster return (that is, you’re getting lots and lots of stuff coming back), what it tells the Soma is that it needs to send more material and speed up.  On the other hand, if it’s not getting very much stuff, it tells the Soma that it needs to slow down so it conserves energy.

Related to this are two hypotheses.  The first of these, as we can see in slide 19, is the vesicular hypothesis.  The vesicular hypothesis contended that neurotransmitter was synthesized in the Soma, wrapped up, put in the tubules and then sent down to the presynaptic element, and then ultimately released. 

The nonvesicular hypothesis said that everything was done in the presynaptic element.  There was no packaging, and that neurotransmitter wasn’t actually released into the cleft in packets.  Now, why is this important to our discussion about the Soma?  Because what happened is that we discovered that both of these hypotheses were correct.  The Soma did make proteins and send them down by axoplasmic flow to the presynaptic element.  However, there are also other proteins that do not use this vesicular mechanism.  As a result, both of these hypotheses are now known to be correct.  Both of these are extremely, extremely important in the production and secretion of neurotransmitters which we’ll talk about a little bit later. 

So now we’ve talked about a little bit of structures in the Soma, let’s talk about a couple of others.  The first of these are what we call lysosomes.  Lysosomes are oval shaped organelles.  They’re basically enzyme packages and are kind of the garbage cleaners that we have within neurons.  They help keep the insides clean.  When you get damage to an organelle, the lysosomes come along and basically chew it up.  Material is then removed from the cell by waste removal mechanisms and then taken out by a particular glial cells.  Ultimately, lysosomes are also important in cell death.  What happens is that when the cell is dying, the lysosomes release all the enzymes, and, in essence self-destruct the cell.  So, this is something that when you’re dying is going on inside your brain.

Mitochondria, on the other hand, as we see in slide 22, provide energy for the cell.  As we know, mitochondria are located throughout the body, but so are lysosomes and other structures as well.

Ok, now we’ve talked about are a variety of different structures that we have within the Soma.  Each of these will be important later.  We will continue to talk about them as we start to talk about neurophysiology.  In the next section, we’re going to begin talking about other structures (axons).  Until then, we hope you’re having a great day and we’ll look forward to talking with you soon.