Voiceover: In this video I'm going to talk about the motor unit. The motor unit is made
up of a couple of parts. The first part are what are
called lower motor neurons. These are efferent neurons of the peripheral nervous system, meaning that they're
carrying information away from the central nervous system. These efferent neurons synapse on and control skeletal muscle. Let me just draw a red outline here for some of the skeletal
muscle in the thigh. Skeletal muscle is the main
muscle type of our body. It's all over our body, and mostly connected to our
skeleton, to move us around. The neurons of the nervous system that tell skeletal muscle when to contract are the lower motor neurons. The term motor unit refers to one lower motor neuron. Let me just draw a soma, and an axon coming out of this lower motor neuron. I'm going to draw this one just having two axon terminals, although they can have
lots of axon terminals, but this one we'll just say has two. We'll say that this motor
neuron is contacting just two skeletal muscle cells. Let me just draw these
two little red tubes to represent two skeletal muscle cells, that are being contacted
by this lower motor neuron, so that these skeletal muscle cells- Let me just write that out. These skeletal muscle cells are the other part of the motor unit. So the motor unit is
one lower motor neuron, and all the skeletal muscle cells that it contacts and controls. The place where a neuron
contacts it's target cell is called a synapse, but this synapse between a lower motor neuron and a skeletal muscle
cell has a special name. That special name is the
neuromuscular junction. "Neuro" for the neuron, and "muscular" for the muscle cell. So, neuromuscular junction is the synapse between a lower motor neuron and a skeletal muscle cell,
and lower motor neurons will usually synapse with
multiple skeletal muscle cells so they'll have multiple
neuromuscular junctions. All of this is the motor unit. The reason we call it
a unit is that usually, when a lower motor neuron
fires an action potential, it causes all of the skeletal
muscle cells in its unit to contract, so that
instead of these cells doing different things at different times, usually they function as a unit. All of them are activated together. The somas of the motor
neurons are in the spinal cord like I've drawn here, or they're up in the brain stem, and then their axons will pass
out in the cranial nerves, if they pass out through the
skull, or the spinal nerves if they pass out through the spine. The axons will continue through
little branches of nerves in the peripheral nervous system, until they reach and synapse on all of the skeletal muscle
cells in their motor unit. So, lower motor neurons
in the cranial nerves, primarily control the skeletal muscles of the head and the neck, and the lower motor
neurons of the spinal cord, primarily control all of
the skeletal muscle cells in the limbs and the trunk. Small muscles that need
rapid precise control, like those that move the eyes, or those that move the fingers- Let me just draw a little
muscle here in the hand, to represent muscles
that move the fingers- These muscles tend to
have small motor units. They're more like what I've drawn here, where a lower motor neuron is synapsing on just a small number
of skeletal muscle cells. Large muscles, that do not
need rapid precise control, like those muscles in the trunk, and those muscles in the limbs, like these big muscles in the thigh here, usually have large motor units, with each lower motor neuron synapsing on a large number of skeletal muscle cells. Let me just draw a little bigger group of skeletal muscle cells here. I'll put them right next to each other. There could actually be many hundreds of individual skeletal muscle cells in a single muscle unit, in some of these big muscles
of the limb or the trunk. Then the lower motor
neuron for that motor unit would have lots more axon terminals, that'll form neuromuscular junctions, these connections with all of these skeletal muscle cells, in the large motor unit
of a larger muscle. A number of things can happen, with any kind of abnormality
of the motor unit. One abnormality that we
could see is weakness, or loss of strength of
contraction of skeletal muscle. Problems of other parts
of the nervous system can also cause weakness, and we'll get into some
of that in later videos. Abnormalities of the lower
motor neuron specifically, in addition to potentially
causing weakness, can cause several other changes, that are called the
lower motor neuron signs. I'll just write LMN for
lower motor neurons. The lower motor neuron signs can happen in addition to weakness, if there's some abnormality of these lower motor neurons. The first lower motor neuron sign is atrophy of skeletal muscle. Atrophy means decreased bulk of skeletal muscle, so decreased size. Here's a photograph of a person who has a lower motor neuron
abnormality causing atrophy. In this person, they
had lower motor neurons coming down here through the wrist that were enervating skeletal myocytes in this part of the hand. They had it on both sides,
just like we all do, but then they had some kind of abnormality here in the wrist that caused a problem with these nerves passing through here, and injured these lower motor neurons heading towards these
muscles in the hands. If you look at these particular muscles in this part of the
hand, they have shrunk, they have shriveled up
and kind of wasted away. We call that atrophy of
those skeletal muscles. The next lower motor neuron
sign is called fasciculations. I can't actually draw these, because what these are are twitches, involuntary twitches of skeletal muscle, that can occur after some problem of the lower motor neurons. So like in this person, if
we looked at these areas with atrophy of skeletal muscle, we would see little twitches of the muscle that we could see through the skin. The occasional fasciculation is normal, everybody gets a little
bit of a muscle twitch here and there, now and then. But with abnormalities of
the lower motor neurons, whichever muscles are affected will often have lots of twitching going on for a very long period of time, just in those muscles that are affected. It's not moving around all
sorts of different muscles, unless there's problems
in lower motor neurons, all over their body. Fasciculations aren't specific to problems with the lower motor neurons, but if we see a lot of them in one spot, than that suggests
there could be a problem with those lower motor neurons. The next lower motor neuron
sign is called hypotonia which means a decrease in
the tone of skeletal muscle. The tone of skeletal
muscle refers to how much the muscle is contracted when a person is trying to relax it, because our muscles are always just a little bit contracted, even when we're not
trying to contract them. So for example, let's
say that this doctor here tells this patient to relax their leg, to go as relaxed as they can, and go relaxed and
floppy, like a wet noodle. Then the doctor here started moving the patient's leg for them, they started bending and
unbending their knee. The doctor will feel a
little bit of resistance, a little bit of tone of
the muscles of the leg, even if this person is trying to relax as best they can. But if there's a problem
with the lower motor neurons so that that the lower motor neurons aren't telling the skeletal muscles cells to contract as much, then there won't be as much tone. There'll be hypotonia, and the doctor will be able to feel that the leg is kind of floppy,
there isn't as much tone when a person is trying to relax it. Another lower motor neuron
sign is called hyporeflexia This refers to decreased
muscle stretch reflexes. I'm just going to write MSR for muscle stretch reflexes. This is a reflex that happens if you rapidly stretch a skeletal muscle, like if you hit the tendon of the muscle with a little rubber hammer, like this doctor is doing
to this patient right here. I'm going to do a different video on the muscle stretch reflexes, so I'll come back to that, and in that video, we'll talk about why the reflexes can decrease with problems of the lower motor neurons, because that one we
understand pretty well. These other three, we don't understand why they happen quite as well. We don't know why you get atrophy if there's problems with
the lower motor neurons, but for some reason, if
the skeletal muscle cells aren't getting periodically stimulated by lower motor neurons, these
muscle cells degenerate, they actually shrink, or they're lost we actually can lose
skeletal muscle cells, if we lose the lower motor neurons. We also don't understand
why fasciculations occur, but apparently with loss of periodic input from lower motor neurons,
some skeletal muscle cells will just start contracting on their own, without being told to do so. Hypotonia is probably just because less skeletal muscle cells are being told to contract in general, but we're not totally
sure about that either.