Skeletal Muscle: Definition, Function, Structure, Location | Biology Dictionary
Found where neurons meet skeletal muscles b. Because Endorphins are among the brain chemicals known as neurotransmitters, which. That is, some systems in the brain or body with defined natural functions are made it is found in the more easily studied neurons located outside the brain. Acetylcholine resides in the axon terminals of neurons that activate the skeletal muscles. At sites where nerves meet muscles, there is a space similar to the synapse. Meeting point between neuron is synapse Neurons are separated by synaptic Uploaded By TyTreas; Pages 8; Ratings 50% (2) 1 out of 2 people found this it is the messenger at every junction between a motor neuron and skeletal muscle. Endorphins is the body's natural form of morphine • If the brain is flooded with.
And then the the whole thing happens. You have potassium that can then take it out, but by the time this comes in, this positive charge, it can trigger another channel and it could trigger another sodium channel if there's other sodium channels further down, but near the end of the axon there are actually calcium channels. I'll do that in pink. So this is a calcium channel that is traditionally closed.
So this is a calcium ion channel. Calcium has a plus 2 charge. It tends to be closed, but it's also voltage gated. When the voltage gets high enough, it's very similar to a sodium voltage gated channel is that if it becomes positive enough near the gate, it will open up and when it opens up, it allows calcium ions to flood into the cell. So the calcium ions, their plus 2 charge, to flood into the cells. Now you're saying, hey Sal, why are calcium ions flooding into the cells?
These have positive charge. I just thought you said that the cell is becoming positive because of all the sodium flowing in. Why would this calcium want to flow in? And the reason why it wants to flow in is because the cell also-- just like it pumps out sodium and pumps in potassium, the cell also has calcium ion pumps and the mechanism is nearly identical to what I showed you on the sodium potassium pump, but it just deals with calcium.
So you literally have these proteins that are sitting across the membrane. This is a phospobilipid layer membrane.
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Maybe I'll draw two layers here just so you realize it's a bi-layer membrane. Let me draw it like that. That makes it look a little bit more realistic, although the whole thing is not very realistic. And this is also going to be a bilipid membrane.
You get the idea, but let me just do it to make the point clear. So there are also these calcium ion pumps that are also subsets of ATPases, which they're just like the sodium potassium pumps. You give them one ATP and a calcium will bond someplace else and it'll pull apart the phosphate from the ATP and that'll be enough energy to change the confirmation of this protein and it'll push the calcium out.
Essentially, what was the calcium will bond and then it'll open up so the calcium can only exit the cell. It's just like the sodium potassium pumps, but it's good to know in the resting state, you have a high concentration of calcium ions out here and it's all driven by ATP. A much higher concentration on the outside than you have on the inside and it's driven by those ion pumps.
So once you have this action potential, instead of triggering another sodium gate, it starts triggering calcium gates and these calcium ions flood into the terminal end of this axon.
Neuronal synapses (chemical)
Now, these calcium ions, they bond to other proteins. And before I go to those other proteins, we have to keep in mind what's going on near this junction right here. And I've used the word synapse already-- actually, maybe I haven't.
The place where this axon is meeting with this dendrite, this is the synapse. Or you can kind of view it as the touching point or the communication point or the connection point.
And this neuron right here, this is called the presynaptic neuron. Let me write that down. It's good to have a little terminology under our belt.
This is the post-synaptic neuron. And the space between the two neurons, between this axon and this dendrite, this is called the synaptic cleft. It's a really small space in the terms of-- so what we're going to deal with in this video is a chemical synapse. In general, when people talk about synapses, they're talking about chemical synapses. There also are electrical synapses, but I won't go into detail on those. This is kind of the most traditional one that people talk about.
So your synaptic cleft in chemical synapses is about 20 nanometers, which is really small. If you think about the average width of a cell as about 10 to microns-- this micron is 10 to the minus 6. This is 20 times 10 to the minus 9 meters. So this is a very small distance and it makes sense because look how big the cells look next to this small distance. So it's a very small distance and you have-- on the presynaptic neuron near the terminal end, you have these vesicles.
Remember what vesicles were. These are just membrane bound things inside of the cell. So you have these vesicles. They also have their phospobilipid layers, their little membranes. So you have these vesicles so these are just-- you can kind of view them as containers. I'll just draw one more just like that. And they can train these molecules called neurotransmitters and I'll draw the neurotransmitters in green.
So they have these molecules called neurotransmitters in them. You've probably heard the word before. In fact, a lot of drugs that people use for depression or other things related to our mental state, they affect neurotransmitters. I won't go into detail there, but they contain these neurotransmitters. And when the calcium channels-- they're voltage gated-- when it becomes a little more positive, they open calcium floods in and what the calcium does is, it bonds to these proteins that have docked these vesciles.
So these little vesicles, they're docked to the presynpatic membrane or to this axon terminal membrane right there. It's an acronym, but it's also a good word because they've literally snared the vesicles to this membrane.
So that's what these proteins are. And when these calcium ions flood in, they bond to these proteins, they attach to these proteins, and they change the confirmation of the proteins just enough that these proteins bring these vesicles closer to the membrane and also kind of pull apart the two membranes so that the membranes merge.
Let me do a zoom in of that just to make it clear what's going on. So after they've bonded-- this is kind of before the calcium comes in, bonds to those SNARE proteins, then the SNARE protein will bring the vesicle ultra-close to the presynaptic membrane.
So that's the vesicle and then the presynaptic membrane will look like this and then you have your SNARE proteins.
Skeletal muscle structure and function | Musculoskeletal Genetics
And I'm not obviously drawing it exactly how it looks in the cell, but it'll give you the idea of what's going on. Your SNARE proteins have essentially pulled the things together and have pulled them apart so that these two membranes merge.
And then the main side effect-- the reason why all this is happening-- is it allows those neurotransmitters to be dumped into the synaptic cleft.
So those neurotransmitters that were inside of our vesicle then get dumped into the synaptic cleft. This process right here is called exocytosis. It's exiting the cytoplasm, you could say, of the presynaptic neuron.
These neurotransmitters-- and you've probably heard the specific names of many of these-- serotonin, dopamine, epinephrine-- which is also adrenaline, but that's also a hormone, but it also acts as a neurotransmitter. It consists of elongated multinuclear cells called the myocytes or myofibers. The muscle cells can be anything from 1 mm to 30 cm in length. The longest muscle cell in our bodies can be found in the sartorius muscle and is 30 cm nearly 12 inches!
This is due to the highly organsied structure of the muscle fibers where actin and myosin myofilaments are stacked and overlapped in regular repeating arrays to form sarcomeres. Actin and myosin filaments slide against each other and are responsible for the muscle contraction. To see how the muscle contracts and works, have a look at the video here. The energy for muscle function comes from intracellular organelles called the mitochondria.
Mitochondria are the powerhouses of every cell in our bodies and responsible for delivering energy that the cells need to function. Muscles are ennervated by motor neurons. A motor neuron and the muscle fibers ennervated by it form a motor unit. Size of motor units varies in the body, depending on the function of the muscle. Fine movements eyes have fewer muscle fibers per neuron to allow for fine movement.