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Motor unit

Created by Matthew Barry Jensen.

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  • blobby green style avatar for user Bartosz Laczek
    Hello at you mention atrophy as being the decrease in size of muscle cells. In my studys we call decrease of size for hypotrophy, while the decrease in muscle cell numbers is termed atrophy. Can you clarify on the termanology?
    (10 votes)
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  • duskpin ultimate style avatar for user Vanessa Angelica
    Would athletes who have repeatedly used their muscles precisely and quickly (such as a professional soccer player) have more motor units?
    (6 votes)
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    • piceratops ultimate style avatar for user John Hogue
      In weight lifting, the initial gain in strength a person experiences is due to neurological changes that lead the an increased number of motor units becoming activated rather than an increase in muscle mass. The total number of motor units, however, doesn't change (as I understand it).
      (6 votes)
  • piceratops ultimate style avatar for user courtneyliston
    So the soma of the nerves is attached to the spinal cord and then the axon is super long and branches to where it needs to go?
    (5 votes)
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  • winston default style avatar for user Wayne Otash
    At , Matthew describes fascinations; which are twitches. Sometimes above my eye feels twitchy. Is that part of the fascinations?
    (4 votes)
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    • orange juice squid orange style avatar for user handles 8/12
      Hi Fox Otash,
      Did you mean fasciculations? Technically, the are involuntary muscle tremors. So, if the muscles on someone's arms were wriggling like a bucketful of worms, that would be a fasciculation. The symptoms you described with your eye are actually very common, and usually occur when people are tired or stressed. I really don't think that the eye twitches completely meet the requirements for a true fasciculation. Hope this helps.
      (4 votes)
  • leaf green style avatar for user Bkocsis3
    How does a signal know what "path" to follow? For example, if you want to move your hand, how does that thought travel down to the muscle in your hand instead of your leg?
    (2 votes)
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    • leaf green style avatar for user Joanne
      The spinal nerves are really dedicated pathways or roads to specific regions of the body. The motor command from the brain goes down the specific nerve pathway to motor or move the arm. The sensory information from the arm goes up a different but parallel pathway to the brain. The brain interprets that sensory information as touch, tickle, pain,etc. An analogy that many can relate to is a highway, one road is dedicated to going north (to the brain) and one road is dedicated to going south (from the brain). Each areas have dedicated connected highways or spinal nerves. The signal does not jump from one road to another so the signal to the arm can not go to your leg. https://en.wikipedia.org/wiki/Spinal_nerve
      (3 votes)
  • duskpin tree style avatar for user Donut Slam
    what does atrophy mean?
    (1 vote)
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    • leafers tree style avatar for user bear
      It is a decrease of muscle size. The issues with lower motor neurons cause the affected muscles to shrivel up. That's why the picture in the video shows portion of the hand have indents in them.
      (3 votes)
  • piceratops sapling style avatar for user azzopapj
    Are all lower motor neurons under somatic/conscious control? Do they always innervate skeletal muscle?
    (1 vote)
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  • blobby green style avatar for user mdnslvrj
    is there a reason why it is called 'LOWER motor neurons' ? is it with respect to portion of the body that it controls?
    (2 votes)
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    • piceratops ultimate style avatar for user Girl on Fire
      He divides the nervous system into two parts, depending on what their function is. The Higher generally comes from the brain and the Lower generally comes from the spinal cord. Each of them do different things. He also called it Basic and Higher. You might want to watch the previous video for more information. I hope I was of some help!
      (45 votes)
  • female robot amelia style avatar for user beauZ
    from my understanding, if there was a damage to the cranial nerve, neck and/or head can't be moved. and a damage to spinal nerve will disable movement of the limbs.

    I know somebody who had brain cancer and was not able to move his left side of the body.
    he did not have cancer in spinal cord, so i'm wondering how was it so.

    if somebody could help, i would appreciate it.
    (1 vote)
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    • female robot ada style avatar for user Lidiya
      The motor cortex of the brain as well as its association area sends signals to the muscles of the body to activate their workings. More precisely, left hemisphere of the motor cortex controls the right side of the body and the right hemisphere controls the left side; the signals to the limbs are sent through the spinal cord but nerve fibers cross in the medulla oblongata of the brain stem. If there are damages to the brain tissue in the right half of the cortex, then the result is inability to move the left side of the body even when the spinal cord is intact. I hope this helps.
      (3 votes)
  • leaf yellow style avatar for user Jacob Winters
    Do large and small motor units have anything to do with fast-twitch versus slow-twitch muscles in, say, the legs? The reason I ask is because at he implied that muscles with small motor units that synapse at a low number of muscles move faster. So does that mean that fast-twitch leg muscles just have smaller motor units?
    (2 votes)
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Video transcript

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.