Muscles, Part 2 – Organismal Level: Crash Course A&P #22


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.

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