>>Dr. Ketchum: In this video lecture, we will focus on the efferent division of the peripheral
nervous system, specifically the somatic nervous system.
So one of the things that I’ve found from teaching physiology for so long is that students
don’t understand what it means when we say that a nerve innervates. So when we say
we have a nerve that innervates the eye, for example, that means the nerve is connected
to a certain region in the eye, and the eye sends signals via this nerve to the brain.
So innervate just means that a nerve connects an organ and signals are sent from or delivered
to that organ via a nerve. So in this example, if I take the sympathetic nerve that innervates
the eye, when that sympathetic nerve is stimulated it will cause the pupils to dilate. Let’s
focus on the somatic nervous system.
So when I said somatic, what effector are we talking about? Skeletal muscle. So the
somatic nervous system controls one type of effector, and that is skeletal muscle. Is
skeletal muscle voluntary or involuntary? Voluntary, correct? Since we are discussing
skeletal muscle and the somatic nervous system, the specific name given to the efferent neuron
that innervates the skeletal muscle is the motor neuron. So when you look at the diagram
where it says motor neuron that is a type of efferent neuron. Notice I’m emphasizing
ee-fferent instead of saying e-fferent neuron; it’s ee-fferent neuron. That way you understand
we’re talking about an efferent neuron that takes messages away from the central nervous
system. A few other items that you need to be familiar with to understand this module
include the following: a muscle fiber—that’s the same thing as a muscle cell. Muscle cells
our cylindrical in shape. So when you see that term “muscle fiber,” do not let that
confuse you. It’s just a cell, and because it’s a cell it has organelles.
Next is the motor unit. When you hear the term “motor unit,” you need to think of
a motor neuron and all the muscle fibers it innervates. So with that being said, each
muscle fiber is innervated by only one motor neuron. So let’s look at motor neuron number
one. How many muscle fibers is it innervating? Three different muscle fibers, right? Let’s
take a look at motor unit two. So it’s innervating that muscle fiber—is motor neuron number
two innervating the same muscle fibers as motor neuron number one? No. So when you look
at this diagram, every one of those muscle fibers is only innervated by one neuron.
So here we have a neuromuscular junction. A neuromuscular junction is a junction between
two structures: a motor neuron and a muscle fiber. So here’s the motor neuron. These
are at the end of the motor neuron, so these must be axon terminals. But you have different
names when you start getting into muscle and neuromuscular junctions. You can still call
it an axon terminal, that’s fine, but realize that it’s also called a terminal buoton,
or a terminal button. So different names, but same structure and same function. All
right, so now let’s zoom in on a synapse. What are these red dots inside the axon terminal?
That’s the neurotransmitter, right? In this case, it’s going to be acetylcholine. And
this plate at the bottom is called the motor end plate. It’s part of the sarcolemma,
which is the plasma membrane of the muscle fiber. So the motor end plate is only that
region that sits right below the terminal button, and the motor end plate is just part
of your plasma membrane, which is going to be called, again, the sarcolemma of skeletal
muscle. So here are your synaptic vesicles containing your neurotransmitter acetylcholine.
On the motor end plate we have these nicotinic cholinergic receptors. These are called nicotinic
because nicotine and acetylcholine can bind to them. The general class of receptor here
is called the cholinergic receptor, and since ligands bind to them, they are called ligand
gated channels. So is there any other endogenous neurotransmitter besides acetylcholine that
can bind to cholinergic receptors? The answer is no. Only acetylcholine will bind to cholinergic
receptors. Ligand gated channels are located on postsynaptic cells. We have also talked
about them on cell bodies and dendrites, and now on the motor end plate.
Now let’s focus on the ligand in this neuromuscular junction. Remember the ligand is acetylcholine.
Acetylcholine binds to nicotinic cholinergic receptors. There are two types of cholinergic
receptors: nicotinic and muscarinic cholinergic receptors. When acetylcholine binds to the
nicotinic cholinergic receptors, is that going to generate a fast or slow response? Fast.
How about when acetylcholine binds to muscarinic cholinergic receptors? Slow response. By slow,
does that mean it requires a G protein? Yes.