AP Biology Musculoskeletal System

Welcome to Educator.com.0000

In today's lesson, we are going to be discussing the musculoskeletal system.0002

And the musculoskeletal system functions in the support and motility of an animal.0006

In addition to things like allowing us to walk or run, move around, the muscles also play an important role in various organ systems of the body.0013

The heart of course is a muscle, and so, we will be discussing that, as well.0025

We are going to start out by taking about some of the functions and different types of skeletal systems before we go on to talk about muscles.0031

So, focusing right now just on the skeletal system, and then, we will talk about muscles. Of course, the two do work closely together.0043

The skeletal system functions in support, protection and in mobility- movement.0051

The support function is more obvious. A skeleton is what holds us out, gives us structure, gives us shape.0060

Now, for movement, what the skeletal system does is provides an attachment point for the muscles.0068

And although, we are going to be focusing mostly on mammalian and specifically, human skeletal systems,0077

what I am going to talk about right now is just the two major types: exoskeletons and endoskeletons.0085

And exoskeletons are not found in vertebrates.0091

So, we talked about this earlier under the diversity of life, but just to review, an exoskeleton is found on the outside of the body.0095

One example is in arthropods such as insects. They have an exoskeleton that is made of chitin.0112

And since the exoskeleton does not grow,0124

what an insect needs to do or an arthropod needs to do is actually shed the exoskeleton or molt in order to allow for growth.0127

Some molluscs such as snails have exoskeletons, chitons.0143

There are other molluscs that have shells. Those are exoskeletons.0149

Endoskeletons are what all vertebrates have, and endoskeletons are found on the inside of the body.0153

Now, we are going to go on and talk specifically about mammalian and specifically, human skeletons.0169

The components of the skeletal system are bone, cartilage and then, we are also going to talk about ligaments and tendons.0177

Bone is actually a connective tissue, and it is composed of collagen that is mineralized; so bone is composed of mineralized collagen.0187

And the mineralization gives bone strength, so bone contains calcium and phosphorus, which makes it much stronger than it would otherwise be.0200

The result, though, is a decrease in flexibility.0212

Now, not all endoskeletons are made of bone. For example, sharks have a cartilaginous skeleton.0216

Bone is produced by cells called osteoblast, and it is remodeled or broken down by a type of cells called osteoclast.0227

And recall that when we talked about parathyroid hormone, for example,0244

and an increase in the level of calcium in the blood or calcitonin, decrease in the level of calcium in the blood,0253

that all is closely related to bone because bone functions as a reservoir for calcium.0258

So, bone is a storage area, reservoir for calcium. Let me just put here "stores calcium".0265

And therefore, when osteoblast lay down more bone, calcium is stored in the bone, and then, the osteoclast break down the bone, the calcium is released.0277

Another function of bone is that the bone marrow is the site of production of both red blood cells and white blood cells.0290

And we will talk more about this when we discuss actually the immune system when we talk about leukocytes or white blood cells.0297

Bones meet up at joints, and a material called cartilage is also found at joints spaces.0306

Cartilage is also a connective tissue. It contains collagen, but it is more flexible than bone because it is not mineralized.0318

In addition to being found in the joint spaces, it is found in various areas of the body such as ears, in the nose, in the airway.0327

As I said, some animals like sharks have a skeleton that is composed of cartilage. Cartilage is produced by cells called chondrocytes.0338

An important difference between bone and cartilage is that bone is vascularized, and it is also innervated, so it has vessels and nerves.0350

So, it is vascularized and innervated. By contrast, cartilage does not have vessels running through it or nerves.0366

Tendons are made of a type of connective tissue called fibrous connective tissue.0380

And they attach muscle to bone, whereas, ligaments attach bone to bone, so bones are the attachment point for muscle.0388

And what we are going to talk about now is muscle starting out with describing the fact that there are three types of muscle.0409

Skeletal muscle, which is under voluntary control, so there is skeletal muscle. There is also smooth muscle.0419

We will talk in more depth about this later, but smooth muscle is under involuntary control; and it is found in places like the blood vessels and the GI tract.0427

Third, there is cardiac muscle, which is found only in the heart, and is under involuntary control.0441

So, starting out with skeletal muscle, it is attached to bones via tendons as I just mentioned.0446

And a contraction of one muscle group is accompanied by relaxation of the opposing group.0455

For example, when you bend your elbow, your biceps contracts at the same time that the opposing muscle group, the triceps relax.0466

So, those two work in...they have to work in concert, or movement will not occur.0476

A skeletal muscle is composed of bundles of muscle fibers, and here, this is showing a bundle, so here is the muscle.0484

You can see it attaching to the bone, and here is a muscle fiber.0494

So, a single muscle fiber is a muscle cell, and muscle cells are very large cells. They are long.0498

They are multinucleated, so muscle cells are large, and they are multinucleated- skeletal muscle. I am just talking about skeletal muscle right now.0505

Smooth muscle and cardiac, it is a different structure.0516

If you went and looked very closely at a single muscle cell, what you would see within the muscle fiber or muscle cell is that it contains myofibrils.0523

There are many, many myofibrils, and each of these myofibrils is composed of thick filaments and thin filaments.0540

Thick filaments are made of myosin.0552

Thick filaments - or excuse me - thin filaments are made of actin, and these are the materials that allow for contractility of muscle.0559

In skeletal muscle, the actin and the myosin are arranged in a very regular pattern.0570

Because of that, when you look at skeletal muscle, it almost looks like it is striped or striated.0576

So, skeletal muscle, the cells are large and multinucleated, and overall, skeletal muscle has striations. It is striated.0582

The sarcomere is the contractile unit of the muscle, so here is a sarcomere.0593

And if you took one muscle fiber, and you looked at it, and you looked at the way the actin and myosin were arranged,0601

what you would see is that they are arranged very regularly in these sarcomeres,0611

and that the muscle fiber is divided up into sarcomeres by what is called Z-lines, so this is a Z-line.0614

A single sarcomere runs from one Z-line to the next Z-line.0628

Right here are the thin filaments, and then, running across in blue are the thick filaments.0638

This is actin, the actin fibers, and then, here is the myosin.0648

So within a sarcomere, we have this regular pattern of overlapping actin and myosin, and the Z-lines mark the borders of a single sarcomere.0656

Here, running down the middle, is what is called the M-line, and this is where the myosin attaches.0665

There are some other structures and terms that you should be familiar with.0674

The H-zone contains thick filaments only- only thick.0679

If you look, the only filaments in this region are thick. There is not going to be any thin.0689

Actually, this should go along here. No thin within this region, so that is thick filaments only.0696

The A-band actually contains the entire thick filament including regions that overlapped with the thin filaments.0705

H-zone is only thick filament, whereas, A-band is thick. The whole thick even parts that overlapped with thin.0725

OK, the I-band contains thin filament only, so the I-band would be thin filament only, which if you look is right here.0735

I will make that a different color.0751

So, this is thin only, and this is I-band; and just remember that H is thick only.0753

I is thin only, and then, A is the whole thick even overlapping parts with thin.0765

What we are going to talk about next is what is called the sliding filament theory.0778

And what the sliding filament theory does is it describes how muscle contraction occurs at the level of the sarcomere.0783

OK, so in the sliding filament theory, what it says is that during muscle contraction, the thin and thick filaments slide past one another.0791

And this causes the sarcomere to shorten.0811

What I am saying is that the sarcomere is a whole shortened.0816

If you measured from one Z-line to the next Z-line, those two Z-lines move closer together.0820

So look at it this way. The Z-lines move closer together, but the actual actin and myosin, those do not become shorter.0826

What they do is they increase the amount of overlap, so thick and... so, this is during contraction.0836

During the contraction of a muscle, there is the thick and thin filaments increase in overlap.0843

The way this occurs is that myosin has a globular head region, and that region of myosin binds ATP.0861

So, myosin binds ATP, and it hydrolyzes the ATP to ADP plus inorganic phosphate.0869

This provides energy, and the myosin uses the energy, so myosin, then, forms cross bridges with actin.0881

So, it forms a cross bridge with the actin, and you could think of it as it is actually pulling the actin. It is pulling it towards the center.0896

And then, these cross bridges breakdown during relaxation.0906

So, ATP is hydrolyzed to ADP. That gives energy for really the myosin to grab the actin.0911

The actin and the myosin slide past through each other, and then, during relaxation, the myosin lets go.0918

The cross bridges break, and the myosin lets go; and then, we are back to where we started.0924

Now, to give you an idea of what exactly this means. After contraction, what you are going to have is...0931

You are still going to have myosin that is the same length. Actually, make this blue so that it will coordinate.0942

So, here, we have myosin just as we did in the beginning.0950

OK, now, the actin is now going to overlap with the myosin more and even maybe overlap with itself a little bit.0966

What you are going to see here is that the sarcomere is now shorter.0987

The sarcomere from Z-line to Z-line is not as large, but the actual myosin filaments are still the same length.0994

But what has happened is that the actin, those have moved closer inward. There is more overlap between the actin and the myosin.1005

The actin and the myosin have slid past each other, and that is what occurs during contraction.1012

During relaxation, the cross bridges between the actin and the myosin break, and it will go back to its relaxed state.1016

And this explains why exercising muscle requires a lot of energy.1025

ATP needs to be used. It is hydrolyzed by the myosin to allow for a contraction of a muscle.1030

Next, we are going to look at what happens at the neuromuscular junction to trigger muscle contraction in the first place.1039

The neuromuscular junction is a type of synapse.1047

So, it is a connection between the motor neuron, and its affector cell to the muscle cell- the muscle fiber.1049

As you know, the post synaptic cell could be another neuron, so neurons frequently synapse with other neurons.1062

Or a neuron can connect to have a synapse with an affector cell like a muscle cell or an endocrine gland.1071

So, here at the neuromuscular junction, what we have is the neuron, and then, just downstream, we have this muscle cell.1079

One motor neuron controls a group of muscle fibers, and what a motor unit is, is one neuron plus all the muscle fibers it controls.1104

This one neuron will synapse on a bunch of different muscle cells, and this neuron plus these various muscle cells all constitute a motor unit.1127

Recall that the neurotransmitter acetylcholine is a neurotransmitter that is used at the neuromuscular junction.1145

What is going to happen is an action potential will travel down this motor neuron until it reaches the synaptic terminal.1151

An action potential will cause an influx of calcium.1165

And the vesicles containing acetylcholine will fuse with the cell membrane at the synaptic terminal, and exocytosis will occur.1168

Now, what we have is a bunch of acetylcholine here in the synaptic terminal.1181

The muscle cell is going to have receptors for the acetylcholine, so how does this trigger a muscle contraction?1188

Well, what happens is that binding of this acetylcholine to the muscle cell causes an action potential to occur in the muscle cells.1196

So, we have acetylcholine is released by a motor neuron. Acetylcholine binds receptors on the muscle cell.1206

Now, what happens? Well, an action potential is triggered.1229

And here is what you need to know a little bit about the structure of the muscle cell because the structure of the muscle cell is very specialized for its function.1234

So, there are various structures that are analogous to typical structures in cells, but they have different names and some specialization.1248

The plasma membrane in a muscle cell is called the sarcolemma.1254

The sarcolemma propagates the action potential, so it is a specialized cell membrane that will propagate this action potential.1262

There are also structures called T-tubules.1276

And what T-tubules are, are infoldings in the cell membrane that allow the action potential to be propagated into the cell.1280

The sarcolemma propagates the action potential, and then, the action potential will travel along the T-tubules.1291

AP travels along this T-tubule system into the cell.1298

Another structure that is modified in a muscle cell is the endoplasmic reticulum.1309

So, the sarcolemma is modified plasma membrane, and the sarcoplasmic reticulum or SR is modified endoplasmic reticulum.1314

And what it does is it stores calcium, so this is modified endoplasmic reticulum; and it stores large amounts of calcium.1336

The motor neuron has released its acetylcholine, which has travelled over, diffused over, binds through the receptor in the muscle cell that causes1349

an action potential, which is propagated along the sarcolemma, travels via T-tubules down into the cell to near the sarcoplasmic reticulum.1362

When the AP, the action potential, reaches the sarcoplasmic reticulum, the SR is stimulated, so it releases calcium.1372

Here, the action potential is propagated, and in the SR, there is all this calcium. Now, it is going to be released into the cytoplasm.1389

How does this allow for a contraction? How does this allow the sarcomere to contract?1398

Well, at rest, there is a type of protein called tropomyosin, and tropomyosin covers the myosin-binding sites.1403

Tropomyosin is a protein that covers the myosin-binding sites on actin.1417

There is a second type of protein called troponin that is associated with tropomyosin, and it helps to regulate the tropomyosin.1432

What happens is the calcium is released into the cytoplasm, and the calcium will bind to troponin.1449

Binding of calcium to troponin causes some movement that results in the tropomyosin uncovering the myosin-binding sites.1465

So, finally, myosin-binding sites are revealed.1478

So, we start out with the myosin-binding sites and the actin being covered by tropomyosin.1488

But because of the motor neuron stimulation of the muscle cell, calcium is released.1495

And when calcium binds troponin then, the tropomyosin moves off those myosin-binding sites.1500

And now, myosin can bind to actin, and that is what triggers the formation of the cross bridges that we talked about1509

between actin and myosin and the sliding of the actin and myosin past each other in contraction of a muscle.1517

So, an action potential does cause a muscle fiber to contract, and the contraction is very brief and then, there is relaxation.1529

And this contraction is called a twitch. So, a contraction of a muscle cell, which is very brief - just milliseconds - is shown here.1538

And this is the tension increasing here, and time is down here.1547

So, a single twitch occurs and then, relaxation occurs.1557

If a second action potential arrives before the first one has dissipated, then those two twitches will...1562

Actually, if two action potentials arrive quickly enough, that the muscle is not fully relaxed, so let me correct that.1583

If two action potentials arrived one after the other, then, what has not dissipated is the original twitch.1590

So, the muscle is still undergoing some contraction, and then, the two twitches are what will sum; and this is called summation.1597

And as you can see, this twitch is not totally done, and then, another one occurs; so here, we have higher tension due to summation.1605

With enough stimulation from the motor neuron, action potentials are arriving one after another after another.1619

What can eventually happen is what is called tetanus and in tetanus,1625

the twitches combine to form just this single long sustained smooth contraction, and that is what tetanus is.1631

Now, there are a couple types of muscle fibers you should be familiar with. One is fast twitch, and the other is slow twitch.1645

Fast twitch fibers, as their name suggests, create rapid contractions, and they are also strong contractions.1654

So, this is rapid and strong contractions.1663

Slow twitch fibers are good when you need a muscle to stay contracted for a while. Endurance is more important versus speed.1671

Most muscles contain both type of fibers, but some only contain a single type depending on the function of the muscle.1684

Now, what accounts for this is that slow twitch fibers use aerobic respiration to generate ATP.1692

Fast twitch a lot of times can use aerobic, but they also use anaerobic respiration. They use glycolysis to generate ATP.1702

They fatigue more quickly, but they also can act very quickly, whereas, endurance will not fatigue as quickly would be a slow twitch fiber.1713

Alright, so we talked a lot about skeletal muscle.1727

And now, I would like to go on and talk about the other two types of muscle that I introduced, which are smooth and cardiac, starting with smooth muscle.1731

Smooth muscle is found in the walls of blood vessels for example. It is found in the airways in the respiratory tract.1739

It is found in the reproductive tract such us in the uterus, the urinary tract, the bladder.1749

It is found in the GI tract, and these are all areas of the body that are not under conscious control. They are involuntary.1756

The controls are involuntary.1763

They also lack the striation of skeletal muscle has because they do not have that very regular arrangement of the thick and thin filaments.1765

Another difference between skeletal and smooth muscle is that smooth muscle cells have only one nucleus each.1776

Now, when I say that smooth muscle is under involuntary control, this means that the autonomic nervous system can regulate it.1785

However, not all smooth muscle relies on nervous system stimulation to cause contraction. For example, hormones like oxytocin.1796

Recall that oxytocin causes uterine contractions during child birth, so hormones.1807

Involuntary control could include the autonomic nervous system. It can also include things like hormones that can stimulate smooth muscle.1813

Another thing to note is that smooth muscle contracts more slowly than skeletal muscle.1825

Alright, so the next type of muscle that we are going to cover is cardiac.1837

The only place you will find cardiac muscle is in the heart, so the heart is composed of cardiac muscle.1843

Cardiac muscle is striated, but it only has one nucleus per cell; so you can see there is a similarity with the skeletal muscle, and there are differences as well.1848

Something that is very important about cardiac muscle that relates to this function is that it is inherently contractile.1861

If you took some cardiac muscle cells, and put them in a dish, put some saline in the dish and then, watch them.1866

You would see that they contract, and they contract on their own because they do not rely on the nervous system to general that action potential.1873

So, they are inherently able to contract.1883

Action potentials are communicated or transmitted from one cardiac cell to another via gap junctions.1889

And this is very important because the contraction of the heart needs to be coordinated.1909

So, remember that the electrical signal for contraction in the heart originates at the SA node, sinoatrial node.1913

And then, it is transmitted throughout the heart, and then, the heart contracts in a very organized, highly coordinated manner.1920

OK, skeletal muscle, smooth muscle and cardiac muscle are the three types of muscle.1928

Skeletal muscle is found attached to bone. It has multiple nuclei per cell, and it is under the control of the voluntary nervous system.1935

Because of the regular arrangement of actin and myosin, thin and thick filaments, this muscle appears striated.1947

Smooth muscle is found in the GI tracts, reproductive, urinary, respiratory, in the blood vessels.1956

And it is under involuntary control including control by the autonomic nervous system or by hormones.1965

There is only one nucleus per cell in smooth muscle, and it lacks striations, which is why it is in the muscle.1972

Finally, cardiac muscles found in the heart, it has one nucleus per cell, and it is under involuntary control. Like skeletal muscle, cardiac muscle is striated.1981

Alright, we are now going to review what we have covered beginning with example one.1995

Describe the series of events that occur in a skeletal muscle cell that result in a contraction.1999

So, what it is asking us to describe how a contraction occurs in a skeletal muscle cell.2006

Begin with the release of acetylcholine form the motor neuron controlling the muscle cell, so we are starting out with neuron releases acetylcholine.2012

What is going to happen is that the acetylcholine binds receptors on the muscle cell.2029

What that is going to do is trigger a response from the muscle cell. It is going to trigger an action potential.2040

So, we have the neuron, we have the muscle cell, and the acetylcholine has been released, and it binds to the muscle cell.2054

That triggers an action potential, and that action potential is propagated along the sarcolemma and then, into the T-tubules.2069

Eventually, this action potential reaches the sarcoplasmic reticulum, and then, calcium will be released into the cytoplasm.2098

When calcium is released into the cytoplasm, the calcium will bind troponin- calcium binds troponin.2122

The result is that the myosin-binding sites on actin are uncovered, and myosin will, then, hydrolyze ATP to ADP and forms cross bridges with actin.2134

That formation of the cross bridges is what allows actin and myosin to slide past each other, and contraction results.2167

Here are the steps that occur from the time of the release of the neurotransmitter to the actual contraction of the muscle.2175

Example two: list three differences between skeletal muscle and smooth muscle. Well, let’s do it this way- skeletal versus smooth.2184

One difference is that a skeletal muscle is striated. Smooth muscle is not striated.2195

A second difference is that skeletal muscle is multinucleated, whereas, smooth muscle has only one nucleus per cell.2206

Skeletal muscle is under voluntary control, whereas, smooth muscle is under involuntary control.2219

That gives you three. You also could have said skeletal muscle has a faster response, whereas, smooth has a smaller response.2230

OK, one more question on here. What material are thin filaments composed of?2241

And the correct answer is actin. Thick filaments are composed of myosin.2247

Example three: a sustained muscle contraction resulting from a rapid series of action potentials is called?2254

Remember that is tetanus, and it results in just a smooth long sustained contraction.2263

Blank are the cells that produce bone. Remember that bone is produced by osteoblast, and it is broken down or remodelled by osteoclast.2272

Finally, blank is non-vascularized connective tissue found between joints.2285

Recall that cartilage is found between joints, and it is a non-vascularized type of connective tissue.2290

Finally, label the following structures in the diagram of the sarcomere below.2298

So, the first one we are asked is Z-line, and there is a Z-line here; and there is also a Z-line right here, so that is taken care of.2304

M-line: M-line runs down the middle, so it is right here.2318

H zone: H-zone, recall, is thick filament only, so I do not want any overlapping thin filaments; so that would be right here, H-zone.2324

The A-band contains the entire thick filament and then, finally, the I-band.2338

Thick filament only is A-band. Thin filament only is I-band.2352

So, these are the different structures of the organization of the sarcomere.2358

That concludes this discussion of the musculoskeletal system at Educator.com.2363

Thank you for watching.2368