Motor unit

in this video I'm going to talk about the motor unit motor unit and the motor unit is made up of a couple of parts the first part are what are called lower motor neurons lower motor neurons and these are efferent neurons of the peripheral nervous system meaning that they're carrying information away from the central nervous system and these efferent neurons synapse on and control skeletal muscle let me just draw a red outline here for some of the skeletal muscle and the thigh and skeletal muscle is the main muscle type of our body that's all over our body and mostly connected to our skeleton to move us around and the neurons of the nervous system that tells skeletal muscle when to contract are the lower motor neurons now the term motor unit refers to one lower motor neuron so let me just draw soma and an axon coming out of this lower motor neuron and then I'm going to draw this one just having two axon terminals although they can have lots of axon terminals but this one will just say has two so we'll say that this lower 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 skeletal muscle cells 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 contacts and controls now the place where a neuron contacts its target cell is called a synapse but this synapse between a lower motor neuron and a skeletal muscle cell has a special name and 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 and all of this is the motor unit and 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 now the sohma's of the lower motor neurons are in the spinal cord like I've drawn here or they're up in the brainstem 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 up through the spine and then 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 and so the lower motor neurons and the cranial nerves primarily control the skeletal muscles of the head and the neck and the lower motor neurons in the spinal cord primarily control all 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 you just draw a little muscle here in the hand represent muscles that move the fingers these muscles tend to have small motor units so 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 and the thigh here usually have large motor units with each lower motor neuron synapsing on a large number of skeletal muscle cells so let me just draw a little bigger group of skeletal muscle cells here I'll put them right next to each other and there could actually be many hundreds of individual skeletal muscle cells and a single muscle unit in some of these big muscles of the limb or the trunk and 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 so I'll just write lmn for lower motor neurons and 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 and 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 so in this person they have lower motor neurons coming down here through the wrist they were innervating skeletal myocytes in this part of the hand I added 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 toward these muscles in the hands and 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 and we call that atrophy of those skeletal muscles the next lower motor neuron sign is called fasciculations as sic you lations and 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 an trophy of skeletal muscle we would see little twitches of the muscle that we could see through the skin now 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 did 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 kind of moving around all sorts of different muscles unless there's problems in lower motor neurons all over their body so fasciculations aren't specific to problems with the lower motor neurons but if we see a lot of them in one spot then that suggests there could be a problem with those lower motor neurons the next lower motor neuron sign is called hypotonia hypotonia which means a decrease in the tone of skeletal muscle now 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 a relaxed as they can and go relaxed and floppy like a wet noodle and 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 the lower motor neurons aren't telling them skeletal muscle cells to contract as much then there won't be as much tone they'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's trying to relax it another lower motor neuron sign is called hyporeflexia hypo reflects iya and this refers to decreased muscle stretch reflexes just going to write ms r for muscle stretch reflexes and 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 and 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 neuron because that one we understand pretty well these other three we don't understand why they happen as well we don't know why you get an trophy 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 actually shrink or they're lost we actually can lose skeletal muscle cells if we lose the lower motor neurons and 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 hi photo Nia is probably just because less skeletal muscle cells are being told to contract in general but we're not totally sure about that either