Big Guns: The Muscular System - CrashCourse Biology #31

Summary

This Crash Course Biology episode delves into the intricate world of muscle function, explaining how muscles contract and relax through the sliding filament model involving myosin and actin proteins. It covers the three types of muscles in the human body—cardiac, smooth, and skeletal—and highlights the importance of ATP in muscle movement. The episode also explores the historical discovery of muscle contraction mechanisms and the role of calcium ions in the process.

Takeaways

• 🏋️ Muscles are essential for various body movements, including walking, running, and playing sports.
• 🔬 There are three types of muscles in the human body: cardiac, smooth, and skeletal muscles, each with distinct functions.
• 📚 Skeletal muscles, like the gluteus maximus and abductor pollicis brevis, are the most familiar and are responsible for voluntary movements.
• 🌌 Muscles can only contract and relax, which allows for a wide range of complex movements.
• 🔍 Muscle structure includes the muscle belly and tendons, with tendons made mostly of collagen and connecting muscles to bones.
• 🧬 Muscle cells, or fibers, are unique due to their multiple nuclei and are formed by the fusion of progenitor cells.
• 🏗 Myofibrils, composed of sarcomeres, are the structural units within muscle cells that enable contraction and relaxation.
• 🔄 The sliding filament model explains muscle contraction as a result of the interaction between actin and myosin filaments.
• ⚡ The release of calcium ions from the sarcoplasmic reticulum and the binding of myosin to actin initiate muscle contraction.
• 🔋 ATP is crucial for muscle movement, providing the energy required for the contraction and relaxation process.
• 🔄 Rigor mortis occurs when ATP is depleted, causing muscles to remain in a contracted state due to the lack of energy to relax.

Q & A

What is cellular respiration and why is it important for muscle function?

Cellular respiration is the process by which cells generate energy from food. It is crucial for muscle function because it provides the ATP (adenosine triphosphate) needed for muscles to contract and relax.

How many types of muscles are mentioned in the script, and what are they?

The script mentions three types of muscles: cardiac muscle, which is unique to the heart; smooth muscle, responsible for involuntary movements like digestion and blood flow; and skeletal muscle, which is attached to bones and used for voluntary movements.

What is the primary function of skeletal muscles?

Skeletal muscles are primarily responsible for voluntary body movements, such as walking, lifting, and playing sports.

What is the role of the muscle belly and tendons in a muscle?

The muscle belly is the thick middle part of a muscle where the muscle fibers are located. Tendons are fibrous connectors at each end of the muscle that attach it to bones, allowing for movement across joints.

What is the sliding filament model of muscle contraction?

The sliding filament model is the mechanism by which muscles contract. It involves the sliding of actin filaments over myosin filaments, causing the sarcomere to shorten and the muscle to contract.

Who discovered the sliding filament model of muscle contraction, and when?

The sliding filament model was discovered independently by two teams of researchers in 1954. One team included Andrew Huxley and Rolf Niedergerke, and the other included Jean Hanson and Hugh Esmor Huxley.

What role does ATP play in muscle contraction?

ATP provides the energy required for muscle contraction. It fuels the movement of myosin heads along actin filaments, which leads to the shortening of sarcomeres and muscle contraction.

What are tropomyosin and troponin, and how do they affect muscle contraction?

Tropomyosin and troponin are proteins that regulate muscle contraction by blocking the binding sites on actin filaments when the muscle is at rest. During contraction, calcium ions bind to troponin, causing a change in position that exposes the binding sites, allowing myosin to attach and pull the actin filaments.

What is the sarcoplasmic reticulum and how does it contribute to muscle contraction?

The sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum found in muscle cells. It stores calcium ions and releases them into the sarcomere when stimulated, triggering muscle contraction.

How does the process of rigor mortis relate to muscle contraction and relaxation?

Rigor mortis is the stiffening of the body after death due to the depletion of ATP. Without ATP, muscles cannot relax, causing them to remain in a contracted state.

What happens during muscle relaxation after contraction?

After contraction, ATP binds to the myosin head, which breaks the bond with actin and lowers the head. The sarcoplasmic reticulum then pumps calcium ions back inside, allowing troponin and tropomyosin to cover the actin binding sites again, resetting the sarcomere for the next contraction.

Outlines

00:00

💪 Introduction to Muscles and Cellular Respiration

The paragraph starts with a humorous introduction to the topic of muscles, mentioning the speaker's previous discussion on cellular respiration during a workout. It then delves into the importance of cellular respiration in providing energy for muscle movement. The speaker explains that muscles are essential for various activities, including walking and playing video games, and highlights the role of adenosine triphosphate (ATP) in muscle function. The paragraph introduces three types of muscles in the human body: cardiac, smooth, and skeletal muscles, each with distinct functions. It emphasizes the complexity and importance of the muscular system, setting the stage for a deeper exploration of muscle anatomy and function.

05:01

🔍 The Sliding Filament Model of Muscle Contraction

This paragraph provides a detailed explanation of how muscles contract, focusing on the sliding filament model. It describes the structure of muscle fibers, including the muscle belly and tendons, and the role of ligaments. The speaker explains the layered structure of skeletal muscles, comparing them to a rope made of smaller ropes and threads. The paragraph delves into the microscopic structure of muscles, discussing muscle fascicles, fibers, and cells. It introduces the concept of myofibrils and sarcomeres, which are critical for muscle contraction. The speaker explains the interaction between actin and myosin, two types of protein strands, and how they contribute to muscle movement. The paragraph also touches on the historical discovery of the sliding filament model by two teams of researchers, including the Huxleys, and the technological advancements that enabled this discovery.

10:02

🏋️‍♂️ Muscle Contraction and Calcium Ion Dynamics

The final paragraph explores the biochemical processes that lead to muscle contraction and relaxation. It discusses the role of ATP in providing energy for muscle movement and the importance of calcium ions in the contraction process. The speaker explains how calcium ions are stored in the sarcoplasmic reticulum and released to initiate muscle contraction. The paragraph describes the role of tropomyosin and troponin in preventing myosin from binding to actin when the muscle is at rest. It also details the process of muscle contraction, including the binding of myosin to actin, the release of energy from ATP, and the subsequent shortening of the sarcomere. The paragraph concludes with an explanation of how muscle relaxation occurs and the role of the sarcoplasmic reticulum in repumping calcium ions, resetting the muscle for the next contraction. The speaker wraps up by emphasizing the importance of chemistry in making muscle movement possible.

Keywords

💡Cellular Respiration

Cellular respiration is a set of metabolic reactions that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. In the video, it is highlighted as the process that makes muscle movement possible by providing the necessary energy from the food we eat.

💡Adenosine Triphosphate (ATP)

ATP is the primary energy currency of the cell, used to power various cellular functions. In the context of the video, ATP is described as the molecule that enables muscle cells to perform their tasks, including contraction and relaxation, which are essential for movement.

💡Muscle Types

The video discusses three types of muscles in the human body: cardiac muscle, which is unique to the heart; smooth muscle, responsible for involuntary movements like peristalsis in the digestive tract; and skeletal muscle, which is attached to bones and facilitates voluntary movement. Understanding these types is crucial for grasping how the muscular system contributes to various bodily functions.

💡Skeletal Muscles

Skeletal muscles are the muscles that are most commonly associated with movement and are under voluntary control. The video uses examples like the gluteus maximus and the abductor pollicis brevis to illustrate the role of these muscles in everyday activities, emphasizing their importance in the body's movement.

💡Muscle Contraction

Muscle contraction is the process by which muscle fibers shorten, generating force and causing movement. The video explains that all muscles, including skeletal muscles, are capable of only contracting and relaxing, which is fundamental to the body's ability to move.

💡Tendons

Tendons are fibrous connective tissues that attach muscles to bones. In the video, tendons are mentioned as the structures that, along with muscles, form muscle-tendon units that facilitate movement across joints, such as the elbow.

💡Sarcomere

A sarcomere is the smallest functional unit of a muscle fiber that can generate force. The video describes how sarcomeres contract and relax, which is the basis for muscle movement. The sliding filament model of muscle contraction, which occurs within sarcomeres, is a key concept in understanding how muscles work.

💡Myofilaments

Myofilaments are the protein filaments that make up the contractile elements of muscle cells. The video explains that there are two types of myofilaments: actin (thin filaments) and myosin (thick filaments). Their interaction is central to the sliding filament model of muscle contraction.

💡Sliding Filament Model

The sliding filament model is a theory that explains how muscle contraction occurs through the sliding of actin filaments over myosin filaments. The video provides a detailed explanation of this model, which was a significant discovery in 1954 and is essential for understanding muscle mechanics.

💡ATP and Muscle Relaxation

The video explains that ATP not only fuels muscle contraction but also plays a role in muscle relaxation. When muscles are not stimulated, ATP is used to detach myosin heads from actin, allowing the muscle to return to its resting state. This process is essential for preventing continuous contraction and illustrates the dual role of ATP in muscle function.

💡Sarcoplasmic Reticulum

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum found in muscle cells. It stores and releases calcium ions, which are necessary for muscle contraction. The video describes how the SR manages calcium ions, which is critical for the initiation and termination of muscle contractions.

Highlights

Cellular respiration is essential for muscle function.

Muscles are involved in various activities like walking and playing sports.

Adenosine triphosphate (ATP) is crucial for muscle movement.

There are three types of muscles in the human body: cardiac, smooth, and skeletal.

Skeletal muscles are responsible for voluntary movements.

Muscles can only contract and relax.

Muscles are composed of tendons and muscle bellies.

Tendons are made mostly of collagen and connect muscles to bones.

Muscles are made up of layers of long, thin strands.

Muscle cells are unique due to their multiple nuclei.

Muscle cells are formed by the fusion of progenitor cells.

Myofibrils are the protein strands that make up a muscle cell.

Sarcomeres are the segments of myofibrils that contract to create movement.

Myofilaments, actin, and myosin, interact to cause muscle contraction.

The sliding filament model explains how muscles contract.

ATP is required for muscle contraction and relaxation.

Calcium ions play a role in triggering muscle contraction.

Tropomyosin and troponin regulate muscle contraction by binding to actin.

The sarcoplasmic reticulum stores calcium ions for muscle contraction.

Muscle contraction is initiated by a signal from a motor neuron.

Muscle relaxation is also facilitated by ATP.

Rigor mortis is a result of the lack of ATP in muscles after death.

Chemistry is fundamental to muscle function and movement.

Transcripts

Well hello there!

You caught me while I was working out.

Last time I was lifting weights during a Crash Course episode,

also...the last time I was lifting weights.

We were talking about how all of this is possible

because of cellular respiration, the process our cells use

to get and store energy from the food that we eat.

Remember that?

Good times.

As it happens, a lot of what we learned then

is also really helpful in understanding the organ system

that we use to do our gun-blasting and walking

and fork-and-knife operating, and parkour

and playing Assassin's Creed and you know, like, moving around.

I'm talking about your muscles of course,

and you wouldn't be able to move them without the help

of the same molecule that your cells use to get all their jobs done.

Good old adenosine triphosphate.

Now, your muscles may be your body's most obvious moving parts,

but as with all things that are truly worth learning about,

this system is both way more complex and way more awesome

than it first appears.

YEAH!

Why? Because of chemistry.

When you think of muscles, your mind usually goes straight

to the guns there. But you really have three different types

of muscle in your body. You have the cardiac muscle.

Your heart muscle, which is different from all the other

sorts of muscle in your body.

And then you have smooth muscle, which is responsible

for carrying out most of your involuntary processes,

like pushing food through your digestive tract

and pushing blood through your arteries.

Important stuff there.

And then there's the muscles that you're most familiar with:

the skeletal muscles.

Your gluteus maximus.

Your masseter, which is important for chewing your hot pockets.

And your abductor pollicis brevis, right at the base of your thumb

aka your "video game muscles"

That's important for the Assassin's Creed.

Just some of the 640 skeletal muscles you have.

Those muscles, like all of your muscles,

are only good at two things: contracting to become shorter

and relaxing back out to their resting length.

That's all muscles do, they contract, and they relax,

it's pretty amazing that you can make a ballerina out of that...

If you were to peel back my skin and take a look at one of my muscles

Please don't do that, but if you did...

You'd see that it thickens in the middle,

at what's called the muscle belly,

and then tapers off on either end into a tendon.

Tendons are made of fibrous proteins, mostly collagen,

that connect the muscle to the bone.

Just a side note, ligaments are similar to tendons,

but instead they connect bones to other bones.

These muscle-tendon combos stretch across one or more joints

in this case, it stretches across my elbow

so that one bone can move in relation to the other bone.

So I just moved my arm and now I'm moving my mouth,

and I'm basically moving my whole body right now,

and the question is: how am I doing this?

How am I moving all of these things in all of these amazing, fluid ways?

How am I able to do that at all?

Unfortunately, it's kind of complicated, but it's wonderful

and amazing so it will be worth it in the end.

First we need to understand the anatomy of a skeletal muscle,

which includes many, many layers of long, thin strands.

Think of one of your skeletal muscles as a rope.

It's made of smaller ropes that are bundled together,

and those ropes are made of bundles of thread,

and those threads are made of tiny, tiny filaments.

This structure is what makes meat stringy,

because after all, meat is just muscle.

This chicken breast is, or was, the pectoralis major

muscle of a chicken.

It connected the bird's sternum or breastbone to the humerus

in its wing, and sometimes I feel like chickens have

bigger pecs than I do.

This is crazy.

When you peel this muscle apart, you see that it's really

made up of layers of thin strings.

These are muscle fascicles, and each fascicle is made up of

lots and lots of much smaller strands, these we can't see.

They're called muscle fibers and these are the actual muscle cells.

Now, because muscle cells perform such a specialized job,

they're not like your run-of-the-mill somatic cells.

For starters, they each have multiple nuclei.

That's because each muscle cell is actually formed

by a bunch of cells, somewhat like stem-cells,

called progenitor cells, fusing together.

Muscle cells are basically just bundles of complex protein strands,

and since nuclei are essential for the protein-making process,

muscle cells need lots of nuclei to make all the protein they need.

From here on you'll notice, by the way, that a lot of the stuff

I'm talking about start with the prefixes myo- and sarco-,

from the Greek words for muscle or flesh, respectively.

Whenever you see those terms in biology,

you know you're probably in muscle country.

For instance, those protein strands that I just mentioned

that make up a muscle cell are called myofibrils.

And each one is divided lengthwise into segments called sarcomeres.

This is where the action happens, my friends,

because it's the sarcomere that will actually do the contracting

and relaxing to create the muscle movement.

Each muscle cell has tens of thousands of these guys,

and they all contract together to make you do stuff.

And this contracting and relaxing occurs through this really cool

and complex interaction between two different kinds of protein

strands called myofilaments.

One myofilament is the protein actin, which are skinny strands

that attach to either one of the two ends of the sarcomere.

And the other is myosin, which is thicker and studded

with these little golf-club shaped knobs along it called heads.

Inside a sarcomere, these proteins occur in layers,

with the thick strand of myosin

floating between several strands of actin.

Just how many strands of actin depends on the muscle

we're talking about.

In this case, let's just say that there are four:

two sitting on top, and two sitting on the bottom.

Now, when the muscle cell is at rest,

none of these strands are touching each other,

but they really, desperately want to!

They're like middle school students at a formal dance.

The myosin in particular wants nothing more than to reach

its little heads up and do some heavy petting with the actin.

The chemical dance that allows this to happen

is one of the sexiest things that goes on in your body

other than, like, sex

and it's known as the sliding filament model of muscle contraction.

Which reminds me of an interesting story ...

I mentioned last week that we didn't really have even

a passing understanding of the human skeleton until the 1500s,

which seems kind of tardy to the party to me.

But that's nothing compared with this:

we didn't figure out how muscles worked until 1954!

In 1954, two teams of researchers independently discovered

that the sliding filament model is how muscles contract.

And, as luck would have it, two of the four scientists

who made this discovery were named Huxley.

We've already discussed Thomas Henry Huxley,

the father of comparative anatomy, and Darwin's Bulldog.

Well, his grandkids were all awesome at something, too,

like Aldous Huxley, who wrote the novel Brave New World;

Julian Huxley, who was central to the development of

modern evolutionary theory; and Andrew Fielding Huxley.

Andrew Huxley was a physiologist who

with colleague Rolf Niedergerke set out to solve

the muscle-contracting mystery.

Until the early 1950's all we knew was that myofibrils

were full of protein strands.

At the time, most people thought that these strands

simply changed shape and shortened,

like how a spring recoils after its been stretched out.

And by the '50s, we'd learned pretty much everything

we could about muscle cells by using conventional microscopes.

So Huxley and Niedergerke actually designed and built

a new microscope.

A tricked out kind of an interference microscope,

which uses two separate beams of light.

And with that, they found that during contraction,

some protein strands kept their lengths the same,

while others around them contracted.

But at the very same time, British biophysicist Jean Hanson,

and Hugh Esmor Huxley, an American biologist who had no relation

to the famous British Huxleys,

were using another new-fangled tool, the electron microscope.

Using that, they observed that muscle fiber

was composed of those thick and thin filaments

the myosin and the actin

and that the filaments were arranged in such a way

that they could slide across each other to shorten the sarcomere.

So in two separate papers published the same day in the same journal,

the two teams proposed that muscle contractions were caused

by the movement of one protein over another.

I guess, an idea whose time had come.

Except it's not that simple.

To understand how the sliding filament model works,

the first thing to keep in mind is that,

in addition to needing a bunch of protein,

muscle cells need to make lots of ATP.

ATP, you remember, creates the energy for almost everything

your body does. Yes, that goes for muscle movement as well.

Another thing to remember is that some proteins can change shape

when they come into contact with certain ions

like we've seen that with the sodium potassium pumps, for instance.

Those pumps are proteins that can accept sodium ions outside a cell

and then they change shape to release them inside a cell,

and also suddenly at the same time become able to accept potassium ions.

These shape-changers are how cells get a lot of the

day-to-day job of living done.

In a sarcomere, it's calcium ions that change the shape

of some of the proteins, so that the myosin can finally

have its way and grope the actin strands all around it.

Then it'll drag those actin strands toward each other,

causing the sarcomere to contract.

But when the muscle cell is at rest,

there are a couple of things that keep this groping from happening.

The first is a set of two proteins wrapped around the actin.

They're called tropomyosin and troponin,

and together they act as a kind of insulation.

Let's just continue our middle school metaphor.

They're the chaperones that protect the actin from groping.

At this point, each little head on the myosin strand

has the wreckage of a spent ATP molecule stuck to it

that's an ADP and a phosphate

and the energy from that broken ATP is already stored

inside the head.

So yeah, the myosin has a lot of pent-up...frustration.

While the muscle cell is resting,

it's preparing a stockpile of calcium ions that it will use

as a trigger when it's go-time.

This is done by a specialized version of the smooth endoplasmic

reticulum, called the sarcoplasmic reticulum or SR.

It's wrapped around each sarcomere

and it's studded with calcium pumps.

These pumps are constantly burning up ATP to create

a high concentration of calcium inside the SR.

And of course, whenever you create a concentration gradient,

You know it's gonna get used.

So now we're ready for a muscle contraction to start,

but what starts it?

Well, stimulus, of course, from a neuron.

Muscles are activated by motor neurons,

and each sarcomere has a motor neuron nearby.

When a signal travels down the neuron to the neuron's synapse

with the muscle cell, it triggers a release of neurotransmitters,

which in turn set off another action potential

inside the muscle cell.

That action potential continues along the muscle cell's membrane,

and then flows inside it along special folds

in the membrane called t-tubules.

When that signal reaches the SR inside the cell, bingo.

The SR's channels open wide and let all the calcium ions

diffuse down that concentration gradient.

The calcium ions bind with one of the chaperones to the troponin

which causes the troponin to rotate around the actin

and drag the tropomyosin out of the way,

revealing all of those super-hot binding sites on the actin.

With our chaperones distracted, the myosin...

it totally goes to town.

It reaches all of those little tiny heads along its length

to bind up with the actin, and the excitement of that long-awaited,

precious contact finally releases the energy

that came from breaking that ATP molecule.

This burst of energy causes the heads to suddenly bend

toward the center of the sarcomere, pulling the actin strands

together, and shrinking the sarcomere.

In millions of sarcomeres in hundreds of thousands of muscle cells,

this is what allows me to, like, lift my arms.

You wouldn't think it would be so complicated.

Now, in order for the contraction to stop,

you're gonna have to tear those two proteins apart.

Because each myosin head is really comfortable here,

snuggling with its beloved actin.

It'll take another passing ATP molecule to attach to the head,

which breaks off one of the phosphates to release its energy

as soon as they touch.

That energy breaks the myosin's bond with the actin

and lowers the head, leaving it alone and frustrated once more.

So, it's weird, that the energy from the Atp

is actually used to make the muscle relax.

But in fact, that's why we get rigor mortis.

When you're dead there's no more ATP to make the muscle relax

and all the calcium ions diffuse out of the sarcoplasmic reticulum

causing the muscles enter their resting state...which is contracted.

But, you're not dead yet, so let's wrap this up.

While the myosin and actin are being separated,

the sarcoplasmic reticulum is hard at work pumping all of the calcium

ions back inside it and storing them up for next time.

That lets our chaperones come back,

the troponin and tropomyosin retake their positions

around the actin strands, and resets the sarcomere

for the next impulse to come along.

Chemistry makes it all possible!

From blasting your guns to, my awesome dance moves.

Thank you for watching this episode of Crash Course Biology.

If you want to go back and look at some stuff,

because it was a confusing episode today: table of contents!

And thanks to everyone who helped put this together,

this one was a doozy.

So thanks to our head writer, Blake de Pastino.

And of course, Amber, as always, for doing our amazing graphics.

If you have any questions, please leave them down below

in the comments, or get in touch with us on Facebook or Twitter.

We will endeavor to answer you, as will all of those

extraordinarily helpful people who are not affiliated with us at all,

but are quite smart and helpful.

So thank you to them.

And we'll see you next time.