Anatomy and Physiology Chapter 10 Muscle Tissue. Okay so we're moving into the details of muscle tissue now but before we get started I will go ahead and let you know that this is this is sort of a tough chapter for a lot of people so you're gonna notice whole lot of repetition. I'm gonna go over things multiple times and in different ways.
So if you start to feel like yeah, yeah, yeah, I've already heard this please do fast forward if you get it right away and if not one of the different explanations might help you to get to the point where this makes a lot of sense. Okay, so muscle tissue is a primary tissue. So it's one of the main tissues that we talked about back in chapter four.
But we didn't really go into any detail about it. So this chapter is going to focus in on what muscle tissue looks like and how it functions. But just a reminder first that there are three main types of muscle. We have skeletal muscle, cardiac muscle, and smooth muscle. Skeletal muscle is the one we are going to focus on in chapter 10. It is voluntary and So we're going to talk about how we can make a decision to move our skeletal muscles.
Cardiac muscle is involuntary and found only in the heart. We'll talk about that in anatomy 2 material. And smooth muscle is found lining hollow organs, for example, like the bladder, esophagus, uterus, intestines. And we'll talk more about those as well in anatomy 2 material. So let's focus in on skeletal muscle, the subject of this chapter.
So muscle tissue is made of cells that are specialized for contraction. Skeletal muscle is going to move our bodies by pulling on our bones. Cardiac and smooth, as we mentioned, are going to control movements inside of our body. And that's about all we're going to say about those. for now.
Skeletal muscle is going to allow us to move. In addition, we're going to be able to keep our posture and body position. It supports our soft tissues, so it's going to add a layer of protection, guards entrances and exits to the body, helps maintain our temperature because as we move our muscles we burn energy which will give off heat. And we can store nutrients in our muscles as well. So we'll talk about what that means as we get a little deeper into the chapter.
Skeletal muscles contain muscle tissue, which is the main ingredient. And then we're also going to have some connective tissues throughout. We're going to see that muscle is extremely organized. We've got rich blood supply as well because we need a good source of oxygen to the muscles.
And we're going to have nerves, which we really haven't talked about yet. That's going to come in chapter 12, but nerves are going to help to control and direct those muscles. So let's begin with the anatomy of a skeletal muscle.
So this again is one of the things we're going to be a bit repetitive about, because in order to understand how muscle actually works, we have to be really super comfortable. with what muscle looks like. So I normally recommend to my in-seat classes that we kind of get through half of this material and stop and then really commit these parts of muscle to memory. If we really know those well then we're able to talk about how they work way easier.
If you're not having to go back and look up what the heck do these words mean again, if we know what they mean we will be comfortable with the function which is usually the toughest part of the chapter. So skeletal muscles have three layers of connective tissue that organize them. So first I'm going to give you definitions and then we're going to go back with a visual so that we can really understand what this looks like. So we have an epimysium and this is going to be the outermost covering of the muscle.
Paramysium this is going to be in the middle. And then Indomesium is going to be the deepest of the three. So the epimysium is a layer of collagen fibers that surrounds the muscle. It's connected to deep fascia and separates muscle from surrounding tissues. So it's kind of like a, and it depends on the muscle, but it's kind of like a bag around the muscle to separate it from other tissues.
The paramecium is going to surround muscle fiber bundles. Now we should go ahead and mention that when we see muscle fiber here, muscle fiber means muscle cell. So when we see fiber, think to yourself cell. Muscle cells are large in the world of cells and they're long and thin like fibers. So they're often referred to as fibers.
So in muscle, the fibers are arranged in bundles, very organized, and those are called fascicles. These bundles of cells are further surrounded by the paramecium, and the paramecium is going to have collagen and elastic fibers in it to give it stretch and protection, and then we've got blood vessels and also nerves. The endomysium, this is the deepest, is going to surround individual muscle cells slash fibers and will contain capillary networks. Again, good blood supply.
Stem cells called myosatellite cells, these guys repair damage in the muscle. And you should also note that whenever we see the prefix myo, that should make us think muscle. And nerve. fibers. So remember the nerves are what helps control these skeletal muscles.
So we need them to be connected to the muscle cells. So the collagen from the epimysium, paramysium, and endomysium will all come together at the end of muscles to form a tendon, which is a bundle. those fibers and these tendons will attach the muscles oftentimes to bone or we may have an aponeurosis Which is a tendinous sheet You see a really prominent aponeurosis Going from the forehead across the top of the skull a big tendinous sheet and these will again attach skeletal muscles to bones okay, so we we've read about it, we've given the definition, so now let's try to kind of visualize that with a few pictures and then I've got my own illustration to kind of drive that home and then we'll go a little bit further. So in this picture we can see we've got the humerus bone here which would be your upper arm bone and then coming off of that we're looking at one of the muscles of the forearm so we're coming from this area here.
So here is the whole muscle, all of it, okay? And surrounding that muscle is a thin layer called the epimysium, okay? So the epimysium is the one we defined first. It's a layer of collagen fibers that surround the muscle. So this is the bag around the outside of the muscle that separates it from other tissues, okay?
So epimysium here you can see it being peeled back. Now this muscle has been sliced so that we can see the inside. And inside we should be able to see these neat little packages or bundles of cells.
And we said that those bundles were called fascicles, muscle fascicles. So muscle fascicles are bundles of cells or fibers. So if we...
Extend one muscle fascicle out, which again is a bundle of fibers. It is surrounded by a paramecium. OK, so you can see around it's labeled here around each bundle.
We have this these layers of paramecium in our cross section. It would be what surrounds again the bundle of fibers or muscle fascicle so here's our bundle and so if we look inside the bundle we should see individual cells or fibers so let's pull one of those fibers out So that's an individual cell and it would be surrounded by an endomysium which wraps around each individual cell. So if we look at a cross section of the cell, again up here we see it's wrapped with an endomysium.
So you can see the muscle is super organized and very well covered at each layer. So here's one of my own kind of simplified drawings to sort of revisit what we just talked about. And we're going to revisit one more time, actually, as we go a little further, just to make sure we're really good.
So this, again, whole entire muscle, like the bicep, for example, and it is surrounded by its own protective covering or bag called the epimesium. If we slice the muscle in half, we'll see that it's made up of bundles of cells. Those bundles are called muscle fascicles. So if we extend one bundle out, that's a muscle fascicle, and it is surrounded by a paramecium, its own protective covering. Since the muscle fascicle is a bundle of cells, These would be individual cells or fibers.
So if we pull an individual cell out, that's a muscle fiber or cell and it is surrounded by an endomysium. OK, so skeletal muscles have very extensive vascular network, so lots of blood vessels. They're going to deliver oxygen and nutrients and remove waste. And our muscles are going to contract only when they're stimulated by our central nervous system. So we call them voluntary muscles because we choose when we want our skeletal muscles to work.
Skeletal muscle fibers are large, as we mentioned just a little while ago, compared to other cells. They also are multinucleate. We talked about that.
back in chapter 4 as well. That means that they have more than one nucleus and they are known as striated muscle because they have a striped pattern on them. So this is a blown up view of an individual muscle, fiber, or cell.
So this is one cell and a couple things we can notice about it is there are multiple nuclei on it and in it and we can see that it also has a striped pattern running along it which are called striations. So we refer to this as striated muscle because under a microscope as we can see here it has a striped appearance and those stripes actually have names. and they provide the ability to move so they're very important they're not just a cute pattern.
So a little deeper looking at the individual muscle cell the sarcolemma. The sarcolemma is another name for the cell membrane of a muscle fiber. It surrounds the cytoplasm of the muscle fiber which is called sarcoplasm. So we've got again the cell membrane which we are calling sarcolemma and we've got the cytoplasm which in a muscle fiber is called sarcoplasm. If we have a change in membrane potential in the cell this will start a contraction and a contraction what we mean by that is the muscle will move or shorten.
that pulls on bones and that gives us movement. Now we did mention membrane potential before and we're going to mention it way more heavily in chapter 12 when we get to the nervous system but just to give you a little bit of a reminder of what that means in a nutshell. Membrane potential is the potential difference across the cell membrane that results from the uneven distribution of positive and negative ions across that membrane. This is measured in millivolts or MVs. So again it's the difference across the cell membrane that results from the uneven distribution of positive and negative ions across the membrane.
Measured in millivolts. It's also sometimes called the transmembrane potential. So this difference across the membrane of positive and negative ions is going to be instrumental in helping us to have a contraction. So again we're going to talk about that a little bit in this chapter and get into it really heavy in chapter 12 when we get to the nervous system.
Something else we're also going to find in muscle fibers is transverse or T-tubules. These are tubes that come from the surface of a muscle fiber deep down into the sarcoplasm and their job is to transmit action potentials from the cell membrane, which is the sarcolemma, into the cell interior. And why do we care about action potentials? So that's something we haven't talked about much at all, but again, really going to talk about it a lot in Chapter 12. But action potentials are important because they are what will trigger our contractions in the muscle.
So we do have to talk about those in this chapter. So an action potential is a propagated, which means spread, it's going to spread. It's a propagated change in the membrane potential of an excitable cell, which is initiated by a change in the membrane permeability to sodium. Okay, so that kind of sounds like what the heck did I just say?
I'm going to go over it extensively and draw it out for you. So no worries. But again, it is a propagated which means to spread change in the membrane potential, which we just defined of an excitable cell like a muscle cell, for example, or a neuron. And this will be initiated by a change in membrane permeability to sodium.
So we're going to have some magic happen and sodium is going to get involved and that's going to help us have a contraction. So hold that thought. We will get to it and we'll draw it in and kind of break it all down.
Right now, we just really want to get how everything looks. The sarcoplasmic reticulum, you might guess that that's. the same thing as an endoplasmic reticulum in other cells, but this is what we would call it in a muscle cell. It is a tubular network surrounding myofibrils. Don't worry, I'll tell you what that is too.
Similar to a smooth endoplasmic reticulum, but it has chambers called terminal cisterni that attach to T-tubules. And we're going to look at all this in a picture. Two terminal cisternae plus a t-tubule forms a triad and why we care about this is because they will store and release calcium which will be super critical when we get to our explanation of contraction.
But again right now we're just trying to place where everything is so that we can find it as we talk about how all these things work. in the second half of the lecture. Myofibrils are subdivisions inside of an individual cell responsible for contraction and they're made up of protein filaments.
These protein filaments, also known as myofilaments, can be either thin or thick. And they are not just, it's not just a clever name, they're thin. Filaments because they are literally thinner than the others which are thick filaments because they are thicker.
Thin filaments also known as actin. Thick filaments also known as myosin. Now all that gibberish we just said let's take a look at it in a picture so we can get a visual. Okay so this is a chunk. of a muscle cell.
Okay, so we're down at the cellular level now. This is a muscle cell or fiber. Remember we talked about how there are multiple nuclei. So here are at least three that we can see. and we also said that muscle cells have stripes this is what we call striations or striated inside the muscle cell we can see even further little tiny fibers and if we pull those out those are called myofibrils okay so myofibrils that's about as small as we're going to get as far as these little tubes go So looking at this a little closer what I'm doing here is I'm zooming in right here to this area so that we can see some of the things we just defined.
Okay so again here's our muscle cell this is our individual cell this out here that the arrow is touching this represents the cell membrane which we call the sarcolemma. You can see the stripe pattern that we talked about having. So inside the cell would be filled with sarcoplasm.
That's the cytoplasm. It's not labeled here, but it's it's in here. And then we've got these long protein tubes which are called myofibrils. So here's one extended out and labeled. That's a myofibril and it is made up of the thin and thick filaments that we just talked about and we're going to zoom in on them next.
But around these myofibrils of which there are multiple as you can see here, there are T-tubules which are these yellow tubes. We talked about how they're going to help bring something called an action potential into the cell. If you don't understand what an action potential is that is fine we will get back to that and then this blue thing this is the sarcoplasmic reticulum which is the same thing as an endoplasmic reticulum and we form the sarcoplasmic reticulum forms these things called terminal cisternae and that's what this is here and there and there and where we have T-tubules meeting with the sarcoplasmic reticula like right here, those three things together, that is called a triad. Tri as in three.
Okay, so here two terminal cisterni plus a T-tubule forms a triad. Okay, terminal cisterni, terminal cisterni, and T-tubule. Forms a triad. Okay, we don't know what any of this does yet and that's okay.
We'll get there All right. So here's up close at our myofibril and the myofibril we said had on it thin and thick Filaments and they extend all the way through the myofibril thin and thick filaments the thin and thick filaments One is thinner than the other so again that name stands we can see the thin and thick filaments here as well because the sarcoplasmic reticulum and t-tubule has been removed so let's talk about the thin and thick filaments a little bit more let's kind of zoom in a little bit closer to them this area here with the thin and thick filaments has a name Sarcomere. This is a sarcomere.
So a sarcomere is the smallest functional unit of a muscle fiber and the interaction between those thin and thick filaments we talked about will produce a contraction, which is the whole goal of this chapter. That's what we want. The arrangement of the filaments account for the striped pattern that we see the myofibrils. Dark bands are A bands and the light bands are I bands. So we're going to take a little closer look at that sarcomere and see what these bands are all about.
So the A band, let's start with that. The A band has in it the M line which is in the center of the A band. Okay so I'm going to go back and forth. So here's our sarcomere up close.
Now where you see this zigzag line, that's where the sarcomere begins and if we follow it over to the next zigzag line here, that's where the sarcomere ends. So this is one sarcomere. This would be a neighboring sarcomere. We can't see all of it, we can see part of it.
And this would be another neighboring sarcomere. Okay so they lie end to along the length of that muscle fiber or the myofibril. So here's our A band, which is known as the dark band because it actually shows up dark if we look at a sarcomere under the microscope. A band. So here is what it looks like under the microscope.
A band. And you can see that it does show up quite dark. But we said the A band was made up of a few things. So first the M line which runs vertically in this picture. M line.
The M line is in the center of the A band and the proteins will stabilize the position of the thick filaments. So thick filaments in this illustration are kind of a bluish purple. These are the thick filaments. and you can see the M line is stabilizing those thick filaments. The H band is on either side of the M line and has thick filaments but no thin overlap.
So here's our H band. It's very small, and you can see it's just here on the left of the M line and on the right, and it's only thick filaments in there, also known as myosin. The zone of overlap is a dark region where thick and thin filaments overlap.
Okay so here's the zone of overlap and it is in fact where the thin filaments or actin which is in red and the thick filaments in which is known as myosin in kind of a bluish purple this is where they overlap. So that's the A band made up of the M line, H band, and zone of overlap. So we're going to move now to the I band which is here. The I band contains thin filaments but no thick. It has Z lines which bisect the I band.
and as we mentioned previously marks the boundary between adjacent sarcomeres. You'll also find in it titan. So first let's look at the z lines again and then we'll talk about titan. So the I band which is the light band shows up under a microscope very light compared to the A band.
You can see it's light here. Here's our z line. and the z-line marks the boundary between adjacent sarcomeres. There's one there and one here and then we've got the titan. So the titan is this green stuff, these green squigglies.
Titan is an elastic protein. It extends from the tips of the thick filament to the z-line. So tip of the thick filament to the z-line. It keeps the filaments in proper alignment and helps them restore to their resting lengths.
So when the sarcomere moves or contracts, the titan helps it to stretch back out to its normal resting length. So at this point we have arrived at a summary slide. So this is the last time we'll repeat the layers, but I often recommend to my in-class students once we get to the end of this summary slide and break it down, this is a great place to stop, go back, and become wholly familiar with this first part of the lecture.
Get these words down, become really comfortable with them because if you know all of these things and where they are, it's going to make the second half. exponentially easier for you to understand and follow. Okay so summary slide again.
We have the whole entire muscle like the bicep for example surrounded by an epimysium. If we slice the muscle in half we can see individual bundles of cells. These are bundles called muscle fascicles.
bundles of cells. They are surrounded by a paramecium. So if we take this one muscle fascicle or bundle and blow it up, this is our whole muscle fascicle blown up, which is a bundle of cells. It's surrounded by a paramecium.
If we take one individual cell from the bundle, that's an individual muscle fiber. and blow that up muscle fiber it is surrounded by an endo mesium. If we look inside the muscle fiber we'll see that it's filled with these protein tubes called myofibrils. If we take one myofibril and blow that up this is what we have here a myofibril.
The myofibril is surrounded by sarcoplasmic reticula and t-tubules and on its surface is a stripe pattern. If we take the stripe pattern and blow that up we have the sarcomere and the sarcomere is made of thick filaments which are bluish purple known as myosin and we have thin filaments which are in red known as actin. We went over the parts of the sarcomere, A band and I band, etc.
So individually, skeletal muscle surrounded by epimesium. Take an individual bundle, blow that up, that's a muscle fascicle surrounded by paramesium. Take an individual muscle cell or fiber, blow that up, surrounded by endomysium.
Take an individual myofibril or protein tube, blow that up, surrounded by sarcoplasmic reticulum and t-tubule. On its surface, a striped pattern, remove that and look at it up close. We have a sarcomere.
which is made of thick filaments also known as myosin and thin filaments also known as actin. Okay so we are picking up right after that summary slide on the details of the thin and thick filaments which are also known as myosin and actin. So thin filaments also known as actin contain a few ingredients.
So we're going to list them and then we're going to take a look at a picture to give us a visual of what those ingredients look like. So we're going to have something called F-actin, Nebulin, Tropomyosin, and Troponin. These are proteins. The part of the actin that is filamentous is the F-actin for short. And this is made up of twisted strands of two rows of globular G-actin molecules.
These G-actin molecules have active sites on them, which are going to be a place that the myosin is going to bind during contraction. And the nebulin is going to give us stability. That's going to hold the F-actin strand together.
So let's jump ahead and point out. that list of protein in a picture. Okay, so here's our in our kind of backed up view of a sarcomere.
The red lines remember are the thin filaments or actin. So if we take a thin filament or actin and we blow it up this is what it would look like up close. So to me it looks like two strands of pearls. that have been twisted together.
The pearls are G-actin molecules, these little round things, and that's why we refer to them as globular, nice and round. Those G-actin molecules when they're twisted together in this arrangement we call those two strands twisted together the F-actin strand. And you'll notice a straight strand going right through those twisted pearls and that is the nebulon. That's given us stability.
The green rope going around the outside. that is tropomyosin and the little brown beads those are troponin now underneath troponin we can see a geactin and there is a little indentation there under that troponin that is called the active site and you can see active sites on all these geactins Those active sites are going to be a place for binding which is going to come up when we get to contraction. So again, G-actin are the individual pearls.
The two strands twisted together is the F-actin strand. The green rope, tropomyosin, and the brown beads are troponin which blocks an active site. So tropomyosin, as we just said, covers active sites on the G-actin. This is going to prevent the myosin, which is the thick filament, and the actin from interacting. Troponin is also globular, which means it's round, and it's going to bind tropomyosin, G-actin, and calcium, and we'll see what that's all about soon, so hold that thought.
Okay, so let's look at the thick filaments also known as myosin. Thick filaments contain about 300 myosin molecules. Each molecule consists of a tail and a head. So if we look at that up close, remember the thick filaments were kind of purplish blue and you can see that they're made up of these myosin heads.
So if we look at one myosin unit by itself you can see a double head, a hinge which is going to allow the myosin head to move, and then a tail which attaches it in to the rest of the thick filament. Okay, so that's given us a close-up view of the parts of the sarcomere. So we are ready to transition now into how muscles actually contract because we're familiar with the anatomy at this point.
So the story of how muscles contract is called the sliding filament theory. During a contraction this is what's going to happen and it may not make so much sense as we go through this slide but we are going to break it down into smaller bits so that hopefully we can easily understand it. So during a contraction the H bands and the I bands will narrow.
Zones of overlap will widen and the Z lines will move closer together. The width of the A band will stay the same. So in summary the thin filaments must slide toward the center of the sarcomere. OK so this is a sarcomere again and it is relaxed.
So this would be a sarcomere in a muscle that is not contracted. Above this is the myofibril. Remember the protein tubes that make up the muscle cell.
Remember that the striped pattern or striations is the sarcomere. So what's going to happen when a muscle contracts in short is that the thin filaments. and z-lines are going to slide this way toward the center of the sarcomere.
So we'll slide this way and we'll slide this way simultaneously which will shorten the sarcomere. So that will look like this. So you can see the sarcomere has scrunched in.
So look at the transition relaxed, contracted, relaxed, contracted. Notice the myofibril, which again runs the length of the muscle cell. Notice that it has multiple sarcomeres on it. There's one. 2, 3, 4, 5, 6, 7. If they all scrunch up in contraction then the whole myofibril should scrunch up and you can see that it does.
So contracted myofibril shortens. Now think about it in the big picture. If all the myofibrils in one muscle do this, then it stands to reason that the whole muscle would shrink up or contract which would pull on bones and we would have movement.
This is a myofibril where both ends are free to move so when the sarcomeres contract the myofibril scrunches in towards the center of the fiber. This is a myofibril that has one end fixed in position so only the free end will scrunch in towards the fixed end. Either way, the myofibrils are shortening, which in turn is going to shorten the muscle and we will have contraction.
So how does this all happen? How does the muscle know, because it's voluntary, that it should scrunch up or contract? Well, excitable membranes are found in skeletal muscle fibers and neurons.
Depolarization and repolarization events produce action potentials. So again, we're going to talk a lot more about depolarization and repolarization in chapter 12 when we get to the nervous system. So we're going to do just a little in this chapter, enough to get us through until we get into the details of that. The point is we really want to create an action potential, which in short is an electrical impulse.
That electrical impulse will help stimulate a muscle fiber contraction, which is what we want. In order to do that we must have a motor neuron. Now we haven't talked about neurons yet, which again will come in chapter 12, so this is kind of going to be a sort of a baby view of a neuron for now.
It's not going to give too many details but just enough to get us where we need to go. So a couple things we should be aware of. First is the neuromuscular junction or NMJ. The neuromuscular junction is the synapse or space between a neuron and a skeletal muscle fiber. The connection of the two, the junction, is the neuromuscular junction.
We're going to draw this out. The axon terminal of the motor neuron releases a neurotransmitter into the synaptic cleft. Neurotransmitters are chemical messengers. So again, we're going to draw all this out to help it make sense.
The neurotransmitter we're going to draw out is acetylcholine, abbreviated as ACH. ACH is going to help us to open sodium channels on the muscle fiber. When sodium rushes in, this causes depolarization.
Depolarization when we go from a negative membrane potential that we talked about previously towards zero or positive. When we depolarize, this is going to cause an action potential which will lead to a contraction. Okay, so those are the notes for our overview.
So now we're going to break it down and help it to make sense by looking at this figure and a figure from the text. And then we're going to draw it out just to drive the point home. So this is the basics of a neuron. Very basics. There's a lot more to know about the neuron.
But again, we'll get there in chapter 12. So this part. of the neuron, the top where the nucleus is, is referred to as the soma or cell body. Then we have a long stem coming down called the axon and the axon splits into multiple synaptic terminals. So that's one, that's one, that's one, that's one, and that's one.
Synaptic terminals. These terminals will join up with other cells and then the neuron which carries impulses from the nervous system So if you have a thought I'd like to move my arm that thought that command travels as an action potential also known as an electrical impulse down the neuron to these synaptic terminals and then the synaptic terminal will pass that information on to the cell that it's attached to. And in this case we're focusing in on muscle fibers.
So we're trying to tell the muscle fibers to contract. And the way that we do that is through our body's central nervous system which connects with the peripheral nervous system. So our brain is going to send a command through our neurons down through the neuron to the synaptic terminal.
We will relay that message to the cell we are talking to which in this case is the muscle fiber. So what we're looking at here is a zoomed in view of that synaptic terminal. So blowing it up we have the synaptic terminal.
a space and then the muscle fiber. So when that command comes down for the muscle to contract in the form of an action potential, we can then spread that message across to the muscle fiber. So what we're going to do now is talk about how we get that message across and then we'll talk about how the muscle fiber will react to that message. So zoomed in a little tighter, let's look over here.
So here's our synaptic terminal again. Here's our muscle fiber and you can see there's a gap between the synaptic terminal and the muscle fiber. That is called the synaptic cleft. That's that space, synaptic cleft. Now the part of the muscle fiber that is joined with the synaptic terminal is called the motor end plate.
So this is the part that receives the information. We're going to talk about how in just a minute. And it has folds in it. The folds are called junctional folds and that just increases surface area.
Okay so where the synaptic terminal joins with the muscle is called the neuromuscular junction or NM. So let's go through stepwise how we get a message from the brain down through the neuron to the muscle fiber itself so that the muscle fiber knows it will contract and then it will actually do the contracting. Okay, so we're going to draw out the first steps leading to the sliding filament theory where muscles will contract.
So again, we have an axon of the neuron, a cell body that we went over just a little while ago, and then we end in synaptic terminals. So the synaptic terminal, I have blown that up here. Here's our large synaptic terminal blown up.
Here in red is our muscle fiber, so this would be the inside of the muscle fiber. And then in between... the synaptic terminal and the muscle fiber we have the synaptic cleft which is the gap or space between the synaptic terminal and muscle fiber this is our neuromuscular junction where the neuron and the muscle fiber come together so in order to have contraction what we want to do first is we want the motor unit to send the action potential or the voluntary command to move this muscle fiber. So we are going to show our action potential as a little lightning bolt. Now, clearly, we don't have cute little lightning bolts in our body, but this is going to help us to remember that this is an electrical impulse or a change in membrane potential.
OK, so it's just symbolic, but the action potential travels down the axon to. the synaptic terminal. Now we need this action potential to be in the muscle fiber, but we can't jump this action potential across this synaptic cleft, so we need to use help to get the message across to the muscle fiber.
So we're going to use a neurotransmitter. Neurotransmitters are chemical messengers and it will do a great job of carrying this information across the cleft. So the neurotransmitter we're going to be using in this case is acetylcholine, which is abbreviated as ACH. So when the action potential comes down the axon, this will cause a release of acetylcholine into the synaptic cleft.
Okay. So these are secretory vesicles which will fuse with the membrane and eject the acetylcholine into the cleft, which is called exocytosis if we remember that from chapter 3. So the neurotransmitter is now in the synaptic cleft, and it will bind two receptors on the muscle cell membrane or motor end plate. When the acetylcholine binds to these receptors on the muscle fiber, this will cause what's called depolarization. Depolarization is when the membrane becomes more permeable to sodium. So acetylcholine binds to the receptors.
Sodium will rush into the cell. This is depolarization. Now remember, we're going to talk about the details of depolarization further on in chapter 12. But this... is essentially when our sodium rushes in and makes the membrane potential of the cell more positive. But we'll get back to that.
So sodium rushes in. This will cause our action potential to reform. Now it's going to reform in the membrane of the muscle fiber, and it will spread deep into the cell.
by way of T-tubules. So you remember those T-tubules we talked about in the earlier part of the chapter? Those little passageways? So the action potential will reform and spread through the T-tubules throughout the cell.
This spreading of the action potential through the T-tubules will cause calcium to be released from the sarcoplasmic reticulum, which I'm going to abbreviate as SR. So these are the first steps towards the sliding filament theory. So to recap, we have action potential coming down.
This is the command for the muscles to contract. When we get to the synaptic terminal, acetylcholine will be ejected into the cleft by exocytosis. Acetylcholine binds to receptors on the motor end plate or cell membrane of the muscle fiber or cell.
This causes sodium to rush into the cell. We call that depolarization. The action potential reforms and is carried into the cell and throughout it by the T-tubules and this will cause calcium to be released.
from the sarcoplasmic reticulum. So we're going to see where calcium is going now and that should help us get to where the muscle is actually contracting. Okay so this further goes through those steps but it's going to be zoomed in a little tighter and using the textbook's illustration of how this process works. Okay so what you're looking at here is the synaptic terminal And then this represents the cell membrane of the muscle fiber or motor end plate. Okay, you'll see these little vesicles are filled with acetylcholine.
And in the cleft is something we haven't talked about yet. These little moons, these little crescent moons, they are representative of an enzyme called acetyl. cholinesterase and that's written out right here. It's abbreviated as ACHE. Big A, big C, little h, big E.
And what acetylcholinesterase does is it is an enzyme that breaks down acetylcholine and that's going to become useful when we want to end a contraction. But we haven't had our contraction first so let's get it going before we end it. Okay. So again synaptic cleft is the gap.
So what's happening here as it says the cytoplasm of the axon terminal contains vesicles filled with acetylcholine which are these. Acetylcholine again is a neurotransmitter and when it's released into the cleft that's going to cause the muscle cell membrane to depolarize. So let's see that in action.
So here comes the action potential. Remember in my drawing that was a little lightning bolt coming down. So once the action potential comes in this is going to cause the release of acetylcholine. So you can see acetylcholine is spilling out into the cleft.
Once it spills out into the cleft remember it's going to bind two receptors on the muscle cell membrane which are these little purple things. So it's now bound to receptors and remember that's going to cause the sodium channels to open and sodium will rush in. We call that depolarization. Okay, so sodium rushes in. Then this causes the action potential to reform in the muscle cell membrane.
Okay, you can see it reddening here. So that's our little lightning bolt reforming from our original drawing. And then you can see now that we've passed the action potential into the cell, these little crescents. remember those are acetylcholinesterase, are beginning to break down the acetylcholine because we're done with that acetylcholine. Okay, so we then said that the action potential would travel down the T-tubules to triads.
And remember, that's the sarcoplasmic reticulum and the T-tubule where those meet. Calcium will be released from the sarcoplasmic reticulum. and that's where we left off. So now we're going to talk about what happens to that calcium. So the calcium is actually going to flood into the sarcomeres where the thin and thick filaments are and the calcium will bind to troponin and change its shape.
Now if you remember the troponin and tropomyosin were blocking active sites or binding sites. So this binding of calcium and changing of shape is going to expose the active sites. which will allow contraction to begin. Okay, so let's look at some steps of this and we're also going to see it in a series of pictures.
Okay, so we've already talked about this. Again, sorry for the repetition, but it often is helpful to help us drive it home. Skeletal muscle fiber contracts when stimulated by a motor neuron at the neuromuscular junction.
The stimulus comes in the form of an action potential, that's our lightning bolt. The action potential will allow acetylcholine to be released into the cleft. This gets the muscle fiber all excited, and the action potential reforms in the sarcolemma or membrane.
This travels down the T-tubules and causes the release of calcium from the sarcoplasmic reticulum. The calcium release We'll go into the sarcomere where the calcium will bind to troponin. This will cause the troponin and tropomyosin complex to shift exposing the active sites.
This is going to allow the myosin and actin to bind together which we're going to see in a series of pictures. Okay so this is going to allow the thin and thick filaments to interact with each other pulling the muscle fiber closer together, the ends of it closer together, which is contraction, and we'll pull on bones. So let's take a look at the calcium on in pictures.
So that's the physical contraction. Active sites exposed, and then we form cross bridges. So let's see what cross bridges look like.
Okay, so we're now zoomed in tightly in the sarcomere. Above and below is myosin or thick filament. You can see the myosin heads are cocked back ready to move. We know they're energized because each of the heads have broken an ATP molecule.
Remember ATP is energy. It's adenosine triphosphate. So if we pop off a phosphate we're left with adenosine.
diphosphate. So each of these heads have broken ATP and are energized ready to move. If you look at the center here this is the actin or thin filament and it has the troponin and tropomyosin we talked about.
Tropomyosin being the green rope and the brown beads being troponin. Remember that underneath that are active sites or binding sites. If you notice the myosin heads across from the active sites meaning they would really like to bind to those active sites but right now the active sites are covered.
So remember we said calcium will flood in. When calcium floods in it binds to the troponin and tropomyosin shifting it. So if you see here it was higher calcium bound and shifted it.
and when it did that it exposed the active site so now the active sites are all visible. What will happen is immediately those energized myosin heads will grab on to the active sites and bind with them. That is called a cross bridge when we have connection between the thin and thick filaments. Since the myosin heads are energized, they will pull on that thin filament, which is called a power stroke.
So this is a visual of a power stroke where the myosin heads are pushing or pulling, excuse me, the thin filament. toward the M line. Remember the M line is the center of the sarcomere. So those myosin heads are pulling towards the M line. That's called a power stroke.
Now myosin heads are greedy. They love energy. So they're going to release and grab another ATP molecule and re-cock back. When they do this, they will be ready again and energized.
to grab on once again and pull in another power stroke. They will then grab an ATP, re-cock back. If the active sites are still exposed, they will reattach with a cross bridge and continue with another power stroke. Grab an ATP, re-cock back, and so on.
Okay? So this is a resting sarcomere. You can see the myosin heads here.
They are not grabbing on to the thin filaments. But as soon as those active sites are exposed, those myosin heads will grab on to that thin filament and yank these thin filaments on both sides toward the M-line, which will scrunch up the sarcomere like so. Okay, so again, myosin heads are energized, calcium comes in, binds to the troponin and tropomyosin, shifting it, exposing the active sites.
We grab on. This is a cross bridge. The power stroke pulls the thin filament toward the M-line.
We grab a new ATP molecule. Re-cock back. We are now ready for another cross bridge and this will continue as long as we want to keep the sarcomere contracted.
So when muscle cells contract, they produce tension. To produce movement, the tension must overcome the load. The entire muscle shortens at the same rate because all sarcomeres contract together. The duration of a contraction will depend on the duration of the neural stimulus, the presence of calcium ions in the cytosol, and the availability of energy. As calcium is pumped back into the sarcoplasmic reticulum and calcium concentration in the cytosol falls, the calcium will detach from the troponin.
Troponin will return to its original position which means the active sites are recovered by tropomyosin and the contraction will be over. So these are two great summary pictures in the text that go over when we initiate a muscle contraction and when we stop a muscle contraction. So further repetition here again, but it's a great study slide.
So acetylcholine is released at the neuromuscular junction and binds to receptors on the sarcolemma. An action potential will be generated. Remember, sodium will rush in.
An action potential will be generated across the membrane surface, and then it will spread using T-tubules. The sarcoplasmic reticulum, which is this, is filled with green circles, which represent calcium. Sarcoplasmic reticulum will release calcium. Calcium binds to the troponin, exposing the active sites on the thin filaments. Cross bridges form when myosin heads bind to the active sites.
Contraction cycle begins as repeated cycles of cross bridge binding, pivoting, which is our power stroke, and detachment occur. These are all powered by ATP. So this is how we contract in a nutshell. These are the steps that end.
a muscle contraction. So we started with talking about acetylcholinesterase. So acetylcholine will be broken down.
In the cleft you can see there's none here by acetylcholinesterase ending the action potential generation. As calcium is reabsorbed, the concentration in the cytosol goes down. So if there's not much calcium, then it can't bind to the troponin, so the active sites will be recovered. If the active sites are recovered, the myosin heads are unable to bind.
So the contraction will end and the muscle will passively return to its resting length. OK, so that's the end of the sliding filament theory and just a few more things in the chapter before we can be finished. So tension is produced by skeletal muscles and depends on the number of stimulated muscle fibers.
A motor unit is a motor neuron and all the muscle fibers it controls. This could be a few muscle fibers or thousands. All the fibers in a motor unit contract at the same time. So this is a view of what the motor unit looks like. Here's your spinal cord and you can see three motor neurons attached and their axons coming down and attaching to the muscle.
and it's color coded to demonstrate the motor unit. So purple motor unit is a neuron and its muscle fibers that it controls which are all the purple ones. A red one attached to all these red muscle fibers.
So the point is we don't need an individual motor neuron for every single muscle fiber. You can have one motor neuron controlling multiple muscle fibers at once. and that is called a motor unit.
Muscle tone is the normal tension and firmness of your muscles when you are resting. Without causing movement, these motor units can stabilize your joints and your bones and help you to keep your posture. Contractions are classified based on their pattern of tension production. We have two main kinds, isotonic and isometric.
Isotonic contractions are when the skeletal muscle changes in length, which will result in a motion. There are two kinds, isotonic concentric and isotonic eccentric. So the isotonic concentric, let's start with that. So the muscle tension is greater than the resistance or load and the muscle will shorten.
So this would be like if you were holding a dumbbell beside you and you curled it up towards your chest. You're flexing using that dumbbell. Your muscles are, the tension is greater than the load, so you're able to move the load up to your chest and your muscle will bulge or flex.
So that would be like this. So here's our muscle and here's the weight. When we lift that weight, the muscle bulges. Isotonic eccentric is when the muscle tension is less than the load, the muscle elongates. So this would be like if you take that dumbbell from your chest where you just flexed it and you slowly controlled movement, slowly lower that dumbbell back down towards your waistline.
The muscle will elongate. You're still tensing the muscle but it will elongate. So that would be like this.
Okay, next we have isometric contraction. This is when skeletal muscle will develop tension that never exceeds the load. So the muscle will not change in length.
This is like when we carry a bag of groceries alongside our body or just simply holding your head up. The muscle will not change in length, but it still develops tension. Okay. Muscle relaxation.
So when muscle relaxes it returns to its resting length and it does that by three things that help it. Elastic forces. So tendons can recoil after a contraction and help return muscle fibers to their original length.
Opposing muscle groups can return a muscle to its resting length quickly. So if you were to use your bicep to flex your arm upward, you can use your triceps on the back of your arm to straighten your arm back out again. That would be an opposing muscle group.
Gravity also helps us to assist opposing muscles to lower, like in an arm example, to lower our arm back down. We're using opposing muscle groups and gravity. Adenosine triphosphate or ATP is the only energy source used for muscle contraction directly.
Contracting muscles use a lot of ATP, so they have to store ATP to start a contraction. More ATP has to be generated to keep the contraction going. Oxygen debt, also called excess post-exercise oxygen consumption, that's a mouthful, after exercise or other exertion your body needs more oxygen than usual to normalize your metabolism.
So your breathing rate and depth will be increased. So I'm sure you've noticed this after you've done something athletic or vigorous you have to breathe a little more often and deeper breaths to try to restore your metabolic activity. Active skeletal muscles also produce heat.
They release up to 85% of the heat needed to maintain normal body temperature. And we notice that too when we do something very active. We tend to start feeling really, really warm.
Several hormones can also increase metabolic activities in your skeletal muscle. Growth hormone, testosterone, thyroid hormone, and also epinephrine. Muscle performance. Force is the maximum amount of tension you produce. Endurance is the amount of time that you can sustain that activity.
Force and endurance depend on the type of muscle fibers and the physical conditioning. So there are three types of muscle fibers, fast, slow, and intermediate. Fast fibers make up the majority of skeletal muscle.
They are fast to contract, hence their name. They have a large diameter, large glycogen reserves, which is an energy reserve, very few mitochondria. So they produce strong contractions, but they get tired really fast. And that makes sense because there are not many mitochondria in this type of fiber, so they're not going to be able to sustain the energy needed to keep the contraction going. Slow fibers are slower to contract, but they're slower to fatigue, so they have better endurance.
Small diameter, a lot of mitochondria. very high oxygen supply because we have lots of capillaries in slow fibers. They contain myoglobin, which is a red pigment that binds oxygen. So white muscle is pale and made up mostly of fast fibers. An example of this would be chicken breast.
So if you look at A chicken, raw, you'll notice that the breast and even cooked is very pale compared to the legs. So this would be fast fibers. And we know chickens are flightless birds. And this makes a lot of sense because those white fibers do not allow the chicken to use its wings enough to be able to actually fly.
So pale fibers can contract quickly, but they do not have any endurance. And we can see in this picture this is white muscle also known as fast fibers and it is a lot more pale because it doesn't have a rich myoglobin supply and myoglobin gives muscle a red color. Okay, slow fibers are also known as red muscle or dark muscle.
So this would be like chicken legs or thighs or salmon and these muscles show up darker. This is red muscle here, slow fibers. Lots of myoglobin, which makes them look darker. And these have lots and lots and lots of endurance.
So chickens, what are they doing? Since they're not flying, they're walking around and walking around and walking around. So they've got lots of endurance in those little eggs. Intermediate fibers are mid-sized, have little myoglobin, and are slower to fatigue than fast fibers, so they're kind of in the middle.
Most human muscles contain a mixture of fiber types and are pink. Hypertrophy is muscle growth from heavy training. This will result in a diameter the increased diameter in muscle fibers, number of myofibrils, number of mitochondria, and glycogen reserves. Muscle atrophy is a reduction in muscle size, tone, and power due to lack of activity.
Changes in muscle tissue as we age. So skeletal muscle fibers will become smaller, less elastic, and they will increase in fibrous connective tissue which is called fibrosis. Tolerance for exercise goes down and your ability to recover from muscular injuries also goes down.
Physical conditioning can improve the power and endurance of your muscles. Anaerobic endurance, so this would be without adequate breathing and this is what People often do when they do really hard or vigorous activities like a 50 meter dash or weightlifting. This is when we use fast fibers and this is going to stimulate hypertrophy. Improved by frequent, brief, intensive workouts.
So without oxygen. So this is not with steady measured breathing. Short bursts of activity.
Aerobic endurance prolonged activity supported by mitochondria, does not stimulate muscle hypertrophy. Training involves sustained low levels of activity. The effects of training improvements in aerobic endurance result from alterations in the characteristics of muscle fibers and improvement in cardiovascular performance.
So a reminder about cardiac muscle tissue and smooth, just a very light reminder because we'll get into those in anatomy 2 content. Cardiac muscle cells found only in the heart have very excitable membranes and they too are striated just like skeletal muscle. Unlike skeletal muscle, cardiac muscle cells are small, branched with a single nucleus, Short, wide T-tubules, sarcoplasmic reticula with no terminal cisternae, and are almost totally dependent on aerobic metabolism. They have lots of myoglobin and a lot of mitochondria. So here's a picture of a cardiac muscle cell, which is branched, can see the striations, and one nucleus.
Cardiac muscle tissue exhibits automaticity, which means contraction without neural stimulation, so involuntary. It's controlled by a pacemaker, so we do not have to think about it. And the nervous system, though, is able to alter the pace and tension of the contraction. So your nervous system can speed up or slow down your heart rate. But again, we don't have to think about our heart rate.
It's pacemaker cells that make it happen. The nervous system can alter the heart rate. Smooth muscle tissue is found in places like the integumentary system where they are found connected to hairs. We hopefully remember the erector pili muscles that cause the hair to stand on end. Involuntary cardiovascular and respiratory systems we find Smooth muscle, so in the cardiovascular we would be talking about lining vessels.
And this is going to help regulate our blood pressure and air flow and it's part in the respiratory system. Digestive and urinary systems lining hollow organs, forming sphincters, it's going to move materials along and out of the body. And then reproductive system It's going to transport gametes and help expel the fetus when we're talking about the uterus.
Smooth muscle is going to be made up of long, slender, spindle-shaped cells. They have one central nucleus, no t-tubules, myofibrils, or sarcomeres, so this makes them non-striated so they look nice and smooth. This concludes chapter 10, muscle tissue anatomy 1, and we'll begin again in chapter 12 over nervous tissue.