Motor unit

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.