Muscles, Part 2 - Organismal Level: Crash Course Anatomy & Physiology #22

Summary

This script from Crash Course explores the intricacies of skeletal muscles, which are essential for movement and force. It explains how muscles pull rather than push, the roles of agonist, antagonist, and synergist muscles, and the concept of motor units. The video delves into muscle contractions, including the three phases of a muscle twitch and the processes of temporal and spatial summation. It also distinguishes between isotonic and isometric contractions, highlighting the importance of muscle function in everyday activities.

Takeaways

• 💪 The human body has 640 skeletal muscles that perform a variety of tasks, from heavy lifting to delicate movements.
• 🏋️‍♀️ Muscles always pull, never push, during movement, as they contract to bring the insertion point closer to the origin.
• 🤸‍♂️ Skeletal muscles are categorized into four functional groups: prime movers (agonists), antagonists, synergists, and stabilizers.
• 🔁 Antagonist muscles work in opposition to prime movers, relaxing or stretching to prevent over-extension during movement.
• 🤲 Synergist muscles assist prime movers by either adding extra force or stabilizing joints to prevent dislocation.
• 🏃‍♀️ Motor units, consisting of muscle fibers controlled by a single motor neuron, are responsible for muscle contractions.
• 🔬 The strength and duration of muscle contractions are regulated by the frequency and strength of neural stimulation.
• 🏋️‍♂️ Graded muscle responses allow for a range of forces through temporal summation and recruitment of motor units.
• 🏋️‍♀️ Isotonic movements involve a change in muscle length, while isometric contractions produce tension without changing muscle length.
• 🏁 Muscle fatigue occurs when ATP reserves are depleted, leading to a decrease in muscle tension and the inability to sustain contractions.

Q & A

What is the term used to describe the difficult tasks that require a lot of physical effort?

The term used to describe difficult tasks that require a lot of physical effort is 'the heavy lifting'.

How many skeletal muscles does the human body have, and what are some examples of their variety?

The human body has 640 skeletal muscles, which come in various shapes and sizes. Examples include the longest muscle, the sartorius in the upper thigh, the biggest muscle, the gluteus maximus in the buttocks, and the tiniest muscle, the stapedius in the middle ear.

How do muscles create movement in the body, and what is the fundamental principle behind their action?

Muscles create movement by contracting and pulling on bones across joints. The fundamental principle behind their action is that muscles always pull, never push, and they do so by contracting and bringing the insertion point closer to the origin point.

What is the term for the muscle that is primarily responsible for producing a certain movement?

The muscle that is primarily responsible for producing a certain movement is called the prime mover or agonist muscle.

What is the role of antagonist muscles in relation to prime movers during movement?

Antagonist muscles work in reverse of the prime movers to prevent over-extension by staying relaxed, stretching, or contracting just enough to control the movement.

What are synergists in the context of muscle function, and how do they assist prime movers?

Synergists are muscles that assist prime movers, either by providing extra force or by stabilizing joints to prevent dislocation during movement.

What is a motor unit, and how does it relate to muscle contraction and control?

A motor unit is a group of muscle fibers that are innervated by a single motor neuron. These fibers contract and relax together in response to the signals from the neuron, allowing for fine control of muscle contractions.

Describe the three distinct phases of a muscle twitch as per the sliding filament model.

The three distinct phases of a muscle twitch are the latent period, where calcium ions flood the sarcomeres but no force is produced; the contraction phase, where myosin heads bind and pull, causing the muscle fibers to contract; and the relaxation period, where calcium is pumped back into the sarcoplasmic reticulum and the muscle relaxes.

How does the frequency of nerve impulses affect the strength of muscle contractions?

The frequency of nerve impulses affects the strength of muscle contractions by increasing the force with more frequent firing of motor neurons. This leads to temporal summation, where successive twitches become stronger as they occur more closely together in time.

What is the term for the maximum level of muscle contraction where all actin binding sites are exposed and no further increase in force is possible?

The term for the maximum level of muscle contraction where all actin binding sites are exposed and no further increase in force is possible is tetanus.

Explain the size principle in the context of muscle recruitment.

The size principle in muscle recruitment refers to the order in which motor units are activated, starting with the smallest motor units with the smallest fibers and least excitable neurons, and progressing to the largest motor units with the largest fibers and least excitable neurons as more force is required.

What is the difference between isotonic and isometric muscle contractions?

Isotonic contractions are when the muscle tension is sufficient to overcome the resistance and change the muscle's length, as in lifting a weight. Isometric contractions occur when the muscle develops tension without changing its length, as in pushing against an immovable object.

Outlines

00:00

💪 The Versatility of Skeletal Muscles

This paragraph introduces the concept of 'heavy lifting' as a metaphor for difficult tasks and discusses the role of skeletal muscles in performing various tasks. It highlights that skeletal muscles are not just for heavy work but also for delicate movements. The paragraph describes the diversity of skeletal muscles, from the longest sartorius to the tiniest stapedius, and their ability to perform a range of movements with varying power and subtlety. It explains the mechanics of muscle contraction, emphasizing that muscles always pull and never push, and how they work across joints to create movement. The concept of prime movers and antagonist muscles is introduced, illustrating how muscles work in opposition to each other to create and control movement.

05:04

🏋️‍♀️ Muscle Contraction and Motor Units

This paragraph delves into the functional groups of skeletal muscles and their roles in movement. It explains the concept of agonist and antagonist muscles, synergists, and how they contribute to various movements. The paragraph then focuses on the motor units, which are groups of muscle fibers controlled by a single motor neuron. It describes the process of muscle contraction at the level of motor units, including the phases of a muscle twitch: latent period, contraction, and relaxation. The paragraph also discusses how the frequency and strength of nerve impulses affect muscle force, leading to concepts like temporal summation, tetanus, and the role of ATP in muscle fatigue. It explains the recruitment of motor units and the size principle, which governs the order in which motor units are activated during muscle contraction.

10:08

🎬 Behind the Scenes of Muscle Movements

This paragraph provides a behind-the-scenes look at the production of the video, acknowledging the contributions of various team members. It mentions the scriptwriter, editor, consultant, director, editor, sound designer, and graphics team, as well as the performers. The paragraph concludes with a note on the filming location and a humorous remark about the treatment of corgis during the production, ensuring that no animals were harmed.

Keywords

💡Skeletal Muscles

Skeletal muscles are the muscles attached to bones by tendons and are responsible for body movement. In the video, it is mentioned that there are 640 skeletal muscles in the human body, which vary in size and shape, and are capable of a wide range of movements from delicate to powerful. The video uses the example of the sartorius, the longest muscle, and the gluteus maximus, the largest muscle, to illustrate the diversity of skeletal muscles.

💡Muscle Contraction

Muscle contraction is the process by which muscles generate force and move the body. The video explains that muscles always pull and never push, as they contract by drawing the insertion point closer to the origin point across a joint. This concept is crucial for understanding how muscles facilitate movement, such as in the push-up example where the pectoralis major contracts to move the body.

💡Prime Movers (Agonist Muscles)

Prime movers, or agonist muscles, are the muscles that are primarily responsible for producing a specific movement. The video uses the example of jumping jacks, where the pectorals and latissimus dorsi are the prime movers for the arm adduction movement, illustrating how these muscles work to bring about the desired action.

💡Antagonist Muscles

Antagonist muscles are those that work in opposition to the prime movers to control and reverse the movement. They are essential for preventing over-extension and maintaining balance. In the context of the video, the antagonists of the jumping jacks would be the deltoids, which help control the speed of arm movement to prevent injury.

💡Synergists

Synergists are muscles that assist the prime movers in performing a movement, either by providing additional force or by stabilizing joints. The video mentions that during arm movements, the rotator cuff muscles act as synergists, helping with the overall movement while also ensuring joint stability.

💡Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates. The video explains that the size of motor units varies, with larger units found in muscles that perform less delicate movements, while smaller units are found in muscles that require fine control, such as those in the eyes and fingers. Motor units are key to understanding how muscles can produce a range of forces.

💡Muscle Twitch

A muscle twitch is a brief contraction of a muscle fiber in response to a single action potential. The video describes the three phases of a twitch: the latent period, the contraction period, and the relaxation period. These phases are part of the sliding filament model and are essential for understanding how muscles generate force.

💡Temporal Summation

Temporal summation refers to the increase in muscle force that occurs when action potentials arrive at a frequency that does not allow the muscle to fully relax between contractions. The video uses the analogy of increasing the volume of a sound to explain how increasing the frequency of nerve impulses can lead to stronger muscle contractions.

💡Tetanus

Tetanus is a state of sustained muscle contraction that occurs when muscle twitches are so rapid and close together that they blend into a single, continuous contraction. The video explains that this is the point at which a muscle can generate the maximum force, and it is limited by the availability of ATP within the muscle cells.

💡Recruitment

Recruitment is the process by which the nervous system activates additional motor units to increase the force of a muscle contraction. The video describes this as a strategic process that follows the size principle, starting with the smallest motor units and progressing to the largest as more force is required, allowing for a nuanced control of muscle force.

💡Isotonic and Isometric Contractions

Isotonic contractions are those in which the muscle generates enough force to overcome a load and change its length, such as lifting a weight. Isometric contractions, on the other hand, involve generating tension without changing muscle length, such as pushing against an immovable object. The video uses the example of picking up a mug versus trying to lift a building to illustrate the difference between these two types of muscle contractions.

Highlights

Metaphors involving skeletal muscles are often used to describe difficult tasks.

Skeletal muscles are essential for performing necessary but sometimes unpleasant exertions.

There are 640 skeletal muscles in the human body, varying in size and function.

Skeletal muscles can perform a range of power and duration, as well as delicate subtlety.

Muscles always pull, never push, due to their anatomical structure over joints.

The movement of bones is described by the terms 'insertion' and 'origin' in relation to muscle contractions.

Muscles are classified into four functional groups: prime movers, antagonists, synergists, and fixators.

Prime movers are responsible for producing a certain movement, while antagonists work in reverse.

Synergists assist prime movers by providing extra force or stabilizing joints.

Motor units, consisting of muscle fibers innervated by a single motor neuron, are the basic functional units of muscle contraction.

The strength and duration of muscle contractions are regulated by the frequency and strength of neural stimulation.

Muscle twitches have three distinct phases: latent period, contraction, and relaxation.

Graded muscle responses allow for a variation of smooth forces through temporal summation.

Tetanus is the state where muscle contractions become a sustained maximum tension.

Muscle fatigue occurs when ATP supply is depleted, leading to a drop in muscle tension.

Recruitment of motor units allows for fine control over muscle force, following the size principle.

Isotonic movements involve a change in muscle length during contraction, while isometric movements do not.

Crash Course is made possible by the support of patrons and a team of experts in various fields.

Transcripts

You’ve probably heard somebody refer to a really difficult, onerous task as “the heavy lifting.”

Or maybe when someone else tells you that you have to do hard work on your own, they’ll

say: “You can’t have somebody else do your pushups for you.”

So, yeah, often when we’re talking about hard work that we just don’t want to do,

we use metaphors that involve the skeletal muscles.

And yeah that’s their reputation. They're what you use to perform all of the necessary-but-sometimes-unpleasant,

brute-force exertions that life requires of us.

But they do a lot more than just heavy lifting.

Your skeletal muscles, 640 in all, come in all different shapes and sizes, from the longest

(the sartorius in your upper thigh) to the biggest (the gluteus maximus in your butt),

to the tiniest (the stapedius in your middle ear -- which I’ve been doing my best to

work out lately, but I just can’t get any definition).

These organs are capable of a whole range of power and duration, as well as surprising and delicate subtlety.

The same muscles you might use to pluck an eyebrow, or catch a firefly, or cuddle a kitten,

can, in other circumstances, crush cans, punch holes in walls, or do a bunch of push-ups.

Which, by the way, are not really a thing, and she’s gonna prove it.

That’s right -- I’m gonna have somebody do my pushups for me.

Now when you look at how the muscular system moves, you gotta keep two things in mind:

First, muscles never push. They always pull.

Now, how can that be, since Claire here is obviously pushing herself up?

Well, remember that most skeletal muscles extend over joints to connect to at least

two different bones. That’s why they’re skeletal.

When a muscle contracts, the bone that moves is called the muscle’s insertion point.

And the muscle brings the insertion closer to the bone that doesn’t move -- or at least

moves less -- and that’s called the muscle’s origin.

And that movement is always a pull -- with the insertion bone being drawn toward the origin bone.

And when you think about it, it has to be this way. Muscles can’t, like, extend themselves

beyond their resting state to push a bone away from it.

So even though Claire’s pushing herself up off the ground in an exercise we call push ups,

her muscles are actually pulling their insertions toward their origins.

When she pushes herself up, her pectoralis major is contracting, pulling its insertion

point -- which in this case is the top of her humerus -- toward the immobile origin, which is her sternum.

Every single movement that your skeleton makes uses the very same principle -- whether you’re

hammering on on anvil or lifting your pinky to sip a cup of tea.

So that’s the first thing. The second big thing to remember about skeletal muscles is

that whatever one muscle does, another muscle can undo.

You can generally classify skeletal muscles into four functional groups depending on the movement being

performed. For example, the muscles that are mainly responsible for producing a certain movement are

called that motion’s prime movers, or agonist muscles.

So, when Claire does jumping jacks, she’s using those pectorals in her chest and latissimus

dorsi on her back to adduct her arms back down to her sides.

Put another way, those are her prime mover muscles for adduction. At the same time,

there are antagonist muscles that are working in reverse of that particular movement, by staying

relaxed, or stretching, or contracting just enough to keep those prime movers from over-extending.

So, in this case, the antagonists of the jumping jacks would include the deltoids on top of her

shoulders, which among other things help her slow her down arms so that she doesn’t slap her thighs too hard.

But when it’s time to start abducting her arms from her side to over her head, those

deltoids now become the primary movers, while the pecs and lats switch to being antagonists.

The third functional muscle group is your synergists, and they help the prime movers

usually by either lending them a little extra oomph, or by stabilizing joints against dislocation.

With all these arm movements, most of the rotator cuff muscles -- like the teres minor

or the infraspinatus -- are acting like synergists.

So this is how skeletal muscles are functionally grouped. But what about their actual functions?

As individual organs, how do they contract to create both quick and sustained movements,

and to regulate force?

How can Claire’s hands gently pet this corgi in one moment, and then crush a can in another?

I’ve got two words for you: motor units.

A motor unit is a group of muscle fibers that all get their signals from the same, single motor neuron.

Since all those fibers listen to only one neuron, they act together as a unit.

In a big power-generating muscle like your rectus femoris in your quad, each of a thousand

or so motor neurons may synapse with, and innervate, a thousand muscle fibers.

Those thousand fibers together form a large motor unit. And big units are typically found in muscles that

perform big, not-very-delicate movements, like walking, and squatting, and drop-kicking.

But other muscles -- like the ones that control your eyes and fingers, which exert fine motor

control -- may have just a handful of muscle fibers connected to a single motor neuron.

Those relationships are small motor units.

And when a motor unit, no matter how large or small, responds to a single action potential,

those fibers quickly contract and release, in what we call a twitch. And every tiny twitch

has three distinct phases.

To understand which happens when, we gotta go back to the sliding filament model.

Immediately after a muscle fiber is stimulated by a nerve -- when calcium ions are flooding

into the sarcomeres to pull away those two protein bodyguards of tropomyosin and troponin

from the actin -- that’s called the latent period.

The stimulus has arrived, but no force is being produced. That’s when the action is just starting.

Then comes a brief period of contraction, when the myosin heads are binding, and pulling,

and releasing, over and over, and the muscle fibers contract.

But soon the fiber slides back down into the relaxation period, when the calcium gets pumped

back into the sarcoplasmic reticulum, and the actin and myosin stop the binding cycle,

and the muscle relaxes.

Each phase consists of a lot of little steps, and while you couldn’t tell by watching

my brother dance, the fact is that our muscular movements are pretty smooth.

That’s because one muscle can produce a variation of smooth forces, called graded muscle

responses. And they’re generally affected by both the frequency and strength with which they’re stimulated.

So say Claire’s trying to lift something heavy, like a paint can.

Just as the volume of a sound corresponds to the frequency of action potentials from

your ear to your brain, her brain gets her muscles to increase their force, by increasing

the frequency with which her motor neurons are firing -- it’s like pushing a button

over and over again really fast.

Lift! You can do it! Feel the burn .. or whatever!

And the faster these nerve impulses fire, the stronger each successive twitch gets,

since the muscle doesn’t get a chance to relax in between.

Because, remember, the relaxation period of a twitch is when all the calcium is being

pumped back into the sarcoplasmic reticulum.

If another action potential travels down before that can happen, even more calcium gets released,

which ends up exposing more actin for myosin to bind to, and that means more force in that fiber.

In this way, twitches end up adding to each other as they get closer together in time.

And that’s what we call that temporal summation.

At some point, though, almost all actin binding sites are exposed, so all of the myosin heads

can work through their cycles of ATP and ADP, and the muscle force can’t increase any

more, even with faster action potentials and more calcium.

When all those little twitches blend together until they feel like one gigantic contraction, that’s called tetanus.

At that point, any person on the planet will hit a ceiling of maximum tension.

That tension means myosin and the calcium pumps are burning up the muscle cells’ ATP,

and the finite supply of ATP is what makes it impossible to maintain vigorous muscle activity indefinitely.

Prolonged contraction leads to muscle fatigue, and when your muscles just can’t take it

anymore all that tension crashes to zero.

And remember, all of this twitching happens in individual motor units.

Since twitches are driven by action potentials, and action potentials only have one intensity,

frequency is the only way to create a grade of force.

But when we zoom out to the complete muscle of maybe a thousand motor units, we can increase

the strength of the stimulus by sending action potentials to more motor units.

If amping up frequency is like hitting a button again and again, then increasing the signal

strength is like smashing the whole keyboard … with your forehead.

Since multiple action potentials don’t travel down all the motor neurons at exactly the

same time, each motor unit twitches at slightly different times, which helps smooth out the … twitchiness.

So, contractions intensify as your motor neurons stimulate more and more muscle fibers. This

is a process called recruitment, or multiple motor unit summation. And it’s where some

of your muscles’ more nuanced abilities come in.

So let’s say Claire is holding Abby. She wants to hold onto her tight, so that Abby

doesn’t fall, but you know not too tight, right?

So to increase the contraction force and tighten her grip, she can recruit another motor unit.

Recruiting one with 20 fibers will firm her grasp, but calling on one with 1000 fibers

might … well, let’s not think about that too much.

Lucky for our corgi friend, this recruitment doesn’t escalate at random -- it follows

what’s known as a size principle. It starts

when the smallest motor units with the smallest fibers are activated by your most excitable neurons.

Then some larger motor units with larger fibers are enlisted, increasing the strength of contraction.

And finally, if you want to give it all you’ve got -- which you don’t in Abby’s case

-- your largest motor units, with your biggest muscle fibers will get involved.

These big guns are the last to join up, in part because they’re controlled by your

largest and least excitable motor neurons. But when they’re in, they are all in -- packing

fifty times the force of those smaller fibers.

So the basic rule is: the more motor units recruited, the greater the force that’s generated.

Now that we know how muscle contractions happen, let’s look at our two main flavors:

isotonic and isometric.

Say I want to pick up my Crash Course mug. I can do this workout myself.

If the temporal and recruitment summation triggers enough muscle tension in my arm to

overcome the weight of the load and lift the mug, changing the length of the muscles involved

during contraction, than that is an isotonic movement.

Now if I want to pick up a building, I could contract my muscles all I wanted, and develop

a lot of tension without actually changing the muscle’s length -- in which case, I’d

be experiencing isometric contractions.

And possibly a hernia.

Which is why I asked Claire to do all the heavy lifting in this episode.

Today you learned how skeletal muscles work together to create and reverse movements.

We also talked about the role size plays in motor units, the three phase cycle of muscle

twitches, and how the strength and frequency of an impulse affects the strength and duration

of a contraction. Finally, we discussed twitch summation versus tetanus, and isotonic vs.

isometric movements. No corgis were harmed in the making of this video.

Thank you to our Headmaster of Learning, Thomas Frank, and all of our Patreon patrons who

help make Crash Course possible through their monthly contributions. If you like Crash Course

and want to help us keep making videos like this, you can go to patreon.com/crashcourse.

Crash Course is filmed in the Doctor Cheryl C. Kinney Crash Course Studio. This episode was

written by Kathleen Yale, edited by Blake de Pastino, and our consultant, is Dr. Brandon Jackson.

It was directed by Nicholas Jenkins, the editor is Nicole Sweeney, our sound designer is Michael Aranda,

our demonstrations were performed by Claire Grosvenor, and the graphics team is Thought Café.