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

The second and related thing is that the nervous system is made up of a wide variety of cells called nerve cells.  More formally for this class they are called neurons.  You’ll hear me talking about neurons for the remainder of the class and not nerve cells in general.  Now that we’ve talked about a couple basic things, let’s look at a schematic of the nervous system.  This is shown in slide three.  I’d like you to copy this slide and save it.  The slide gives you an idea about the system and you can kind of keep track of where we are.

As you can see here, the nervous system (as we see in slide four), consists of two major parts; the central nervous system which consists of all the neurons in the brain and spinal cord, and the peripheral nervous system which consists of all the neurons located outside the brain and spinal cord.

Since the peripheral nervous system is the smaller of the two, I’d like to talk about it first.  We’ll do that beginning in slide five.  The peripheral nervous system is made up of two major divisions: the somatic nervous system and the autonomic system.  Let’s talk about the somatic system first.  As we see in slide six, the somatic system contains three major sets of neurons.  These neurons are basically in two major groups.  We have afferent neurons, efferent neurons and intranuncial neurons.  These sets of neurons are broken into two groups.  The first group is located within what are called the cranial nerves.  As we see in slide seven, the cranial nerves intervate the head, the neck, and some internal organs, and there are twelve sets.  We can see where the cranial nerves go In slide eight.  As we see, some of them are going to be controlling motor movement within the head, face, and neck and others are just going to be purely sensory.  These provide information to the brain and the spinal cord. 

In addition, we have a wide variety and number of spinal nerves.  These are located and come out of the spinal cord.  Listed on slide nine, is the number of spinal nerves that come out of the major sections of the vertebrae.  As we can see, there’s cervical vertebrae, thoracic, lumbar, sacral and cocclogeal vertebra.  Each of these are important for us within the spinal cord.

So now that we’ve identified the two major groups of neurons (cranial nerves and the spinal nerves), let’s talk a little bit about the neurons that make these up.  As we see,  the first set of neurons are called afferent neurons.  Afferent neurons are sensory in nature.  Basically they receive information from sensory structures (ala receptors) and they send that information toward the central nervous system. 

The next major set of neurons are what are called efferent neurons.  Efferent neurons are motor in nature.  They take information from the central nervous system and send it to particular muscles of the body.  You also need to note that efferent neurons can be both sensory and inhibitory.  For example, if you’re going to bend your arm, one set of efferent neurons are going to say “contract” to one set of muscles, and another set of neurons say “relax” to the opposing set.  That way the arm will move.  If you don’t do that and they both contract at the same time, the arm remains in one place. 

The last major set of neurons are what are called intranuncial or what are also called inter-neurons.  These neurons are located within the spinal cord and what they do is connect afferent pathways to efferent pathways.  They also send information to the brain and are involved in what are called reflex arcs. 

Typically these types of neurons are T-shaped.  That is, they have a soma or cell body and two axons.  One set of axons is going to send information to an efferent neuron and another set of axons sends the information to the brain.  We will talk about that in more detail as we talk about reflex arcs and how they work when we talk about the spinal cord.

So, we’ve talked about the somatic system, let’s continue with a discussion of the other aspect of a peripheral nervous system called the autonomic system.  This begins with slide 13.  The autonomic system basically consists of neurons that go to and from various internal organs. What they do is regulate heart rate, blood pressure, digestion, and on and on. 

There are two major parts of the autonomic system; that is the sympathetic nervous system and the parasympathetic system.  The way to think of the autonomic system is to think about what happens when a lion chases you.  So I’d like you to do is imagine what you and your ancestors might have been like 3,000 or 4,000 years ago when we were all elementary organisms in the wilderness.  Out in the wilderness we were all eating berries and assorted other good things.  Things were fine and life was good.  As you were eating those berries, you looked out of the corner of your eye and saw a lion.  The lion also sees you and begins to chase you.  So, what needs to happen in your system for you to survive?  Well let’s look at the sympathetic system and kind of see what happens. 

The sympathetic system basically is an arousal system and it does some very major things.  It increases your heart rate, increases your respiration, etc.  As a result, you get blood with lots and lots of oxygen going to structures that’s going to help you run away from the lion.  In addition to that, it also decreases other structures.  The classic example is it decreases the digestive track.  The reason is when you are out running away from the lion, you do not need to be worrying about getting blood to digest all those berries that are now in your stomach.  If you do that, you put less blood to your muscles in your legs, as a result you become an eat eeee. 

So let’s say that you have been running away and you find a tree and you run up a tree. Now you have the lion down below you and you’re way up high and safe.  So, you start throwing sticks and sort of other things at the lion and the lion goes away.  Now we have a system where you’re really, really active.  What we need to do is slow the system down.  That is when the parasympathetic system begins to take over.  The parasympathetic system basically causes you to relax.  It’s going to decrease your heart rate and respiration.  Now we need to replenish all the energy that we’ve used trying to get away from the lion.  So we begin to increase the digestion of those berries and other things we have within our stomach. 

If we look at an overview on slide 15 you can use your zoom to look at specific things.  We see that the sympathetic system and the parasympathetic system activate specific types of structures.  You can see in the visual system one dilates the pupil while the other system contracts the pupil.  This also occurs for other systems as well

Well, now we’ve talked about the peripheral nervous system and the structures that make it up.  Let’s talk now about the central nervous system and its brain structures.   As we talked about the last time the central nervous system consists of the brain and the spinal cord. Let’s talk about the brain first. 

The first thing we need to know is that the brain has about a hundred billion neurons or what we call nerve cells, and the neurons are the functional elements of the brain.  However, the brain doesn’t only consist of neurons, it also has other things in it.  The first thing it has are approximately 120 billion glial cells.  The brain also has several membranes.  Those are called the meninges.  Finally, the entire brain is surrounded by a particular type of fluid called cerebral spinal fluid.  So let’s look at these structures in a little bit more detail and get an idea about what some of these structures are and what they do.  So let’s begin by talking about glial cells on slide 18.  We’re going to talk about this more later, but let’s just get an overview of what they do. 

Glial cells first have a lot of different functions. They act as a glue to help hold neurons together.  They help provide nutrients to the particular nerve cells that are in the brain (although this is debated).  They also help regulate brain activity and help balance out particular types of ions in the brain so we don’t go into seizures all the time.  They also help to eliminate waste, help to break down neuro-transmitters and neuro-peptides and finally they also make a substance that increases the speed of signals within the nervous system.  This substance is called myelin. 

Let’s talk about the next major set of structures called the meninges (seen on slide 19).  The meninges are basically a set of membranes that protect the brain and spinal cord.  There are three layers and they are listed here.  The first layer is what is called Dura Matter.  Dura Matter is kind of an outer layer, it’s very thick and it’s very solid.  The next layer is the Arachnoid layer and it’s kind of a middle layer and it overlies the arachnoid space and contains cells that’s going to create cerebral spinal fluid.

The final set of membranes that are within the meninges are what are called Pia Matter. Pia Matter is the inner-most layer.  It goes down into the valleys and fissures of the brain and covers the entire brain and all structures.  We will  come back and talk about the meninges a little bit later as well. 

The next major set of structures are called the ventricles.  The ventricles are in essence spaces in the brain.  They’ve covered by the meninges and they contain cells that are called Chroid Plexus cells.  And each of these cells make the cerebral spinal fluid that’s going to be coating the brain.  A picture of the different ventricles is shown in slide 21.  What we have are two ventricles, which are called the lateral ventricles or ventricles one and two.  There’s one on the left side of the brain and one on the right side of the brain.  Both of these ventricles drain into the third ventricle, which you see shown in the figure.  The third ventricle then drains into the fourth ventricle, via an acqueduct or a channel called the cerebral aqueduct.  From the fourth ventricle, the cerebral spinal fluid and other fluids are going to go out to other structures in the spinal cord and other parts of the brain.  So what is cerebral spinal fluid and what’s it made by?  We show this in slide 22. 

The cerebral spinal fluid is is a fluid that surrounds the brain and does a couple of things.  First, it makes the brain lighter and helps to eliminate wastes.  It’s made from chroid plexus cells that are found in the ventricles of the brain.  Ultimately this fluid, as we see in the previous slide, is going to go from the lateral ventricles into the third and the fourth and then on to the spinal cord surrounding the brain.

Sometimes these ventricles get blocked.  When they get blocked, they begin to develop lots and lots of pressure within the brain.  Usually requires some kind of shunt being placed within the ventricle to help it drain.  In addition, if the person does not have adequate mechanisms to relieve the pressure, the fluid continues accumulate and develop more and more pressure.  This starts to squeeze the brain and brain cells begin to die.  This is a classic example of what happens in diseases such as hydrocephalus. 

Well we’ve covered a variety of different systems and given you an overview of the brain.  In the next topic we are going to use an Excel spreadsheet and walk through structures a little bit differently to kind of show you where the different systems are, how they work together, and ultimately what builds on one or another.  So until we come back and start talking about that, we hope you have a good day.