Chapter 21: Movement and Muscle: Plasticity in Response to Use and Disuse

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Welcome to the Deep Dive.

Today we're plunging into the really remarkable world of animal physiology, specifically muscle.

How does this incredible tissue adapt to everything an animal, or even us, throws at it?

We're going deep into a chapter from Animal Physiology, the fourth edition by Hill Wise and Anderson, and honestly it's packed with some real aha moments.

This topic, it really hits home how fundamental muscle actually is.

Just think about it.

An animal's survival, its whole life really, hinges on its muscles,

finding food, growth, making babies.

Escaping predators.

Escaping predators,

absolutely critical.

Muscles are the engine, right?

And get this, invertebrates, these striated muscles, they can make up nearly half of the body mass.

The half.

The half.

And they're not just sitting there, they're incredibly dynamic, constantly breaking down, rebuilding, synthesizing proteins, assembling, disassembling, it's nonstop.

Even your heart, beating away constantly, it's a prime example of this plasticity.

Okay, so our mission today, let's lay it out.

We want to uncover the surprising facts, the core ideas about how muscles change, adapt,

and sometimes, yeah, how they fail.

We'll look at some amazing real world examples in animals doing incredible things and also crucially the experiments that helped us figure all this out.

Prepare to be amazed by your own body and some seriously cool creatures.

Let's do it.

Alright, let's jump right in.

So one of the first big ideas is concept of muscle being so dynamic, always changing size, changing its characteristics.

But here's the kicker.

Adult muscle cells, the fibers, they usually don't divide, they're postmitotic, right?

That's right.

So, how does a muscle get bigger or smaller if it's not really adding or losing cells in the way we normally think about growth?

That's the million dollar question, isn't it?

And it really gets to the core of muscle plasticity.

It's all about changes within the existing cells, the fibers themselves.

Ah, okay.

So when muscles change size, we're generally talking about two main things.

First, you've got hypertrophy.

That's the muscle getting bigger, bulking up.

Right, lifting weights.

Exactly.

This happens because the individual muscle cells start packing in more structural proteins, actin and myosin, the contractile bits.

So the cell itself gets fatter.

It's not about making new cells.

Okay.

And to handle that bigger size, the fiber can actually fuse with these special nearby cells called satellite cells.

Think of them like little helpers donating their nuclei.

More control centers for the bigger factory.

Precisely.

More nuclei mean the cell can produce more protein to maintain that larger volume.

Okay, so that's getting bigger.

What about shrinking atrophy?

Right, atrophy is the flip side.

That's a reduction in mass.

And it mostly happens because those same proteins, actin and myosin, are lost from the fibers.

They shrink down.

So the components get removed.

Exactly.

Now, in some more severe situations, like certain diseases or advanced aging, you might see an actual loss of the muscle cells too.

Okay.

But typical disuse atrophy.

It's mainly about the existing cells getting smaller.

Got it.

Bigger cells, not more cells.

But are all the fibers within one muscle the same, or is it more like a team with different players?

Does that affect how they adapt?

It's definitely a team.

A highly specialized team.

Our source points out three main types of muscle fibers, each with its own sort of personality.

Speed, endurance, they're different.

Okay.

You've got your slow twitch fibers,

technically slow oxidative or type I.

Then there are two kinds of fast twitch.

Fast oxidative glycolytic, type CS,

and fast glycolytic, type PIS -X.

Slow, fast oxidative, fast glycolytic.

Right.

What makes them tick differently?

Like, down at the molecular level, is it like different kinds of engines?

That's a perfect way to put it.

It really comes down to subtle differences in key proteins.

Think about the myosin heavy chain that's a core part of the muscle's engine.

Different versions or isoforms of this protein cycle, the cross bridges at different speeds, that directly sets the contraction speed.

Faster isoform, faster contraction.

Simple as that.

And there's more to it.

Yep.

There's another piece.

A specialized pump in the circoplasmic reticulum, the K2 plus ATPase pump.

It's like the muscle's reset button.

Ah, clearing out the calcium after a contraction.

Exactly.

A faster pump clears calcium quicker, lets the muscle relax faster, and get ready for the next twitch sooner.

Essential for rapid movements.

So let's break down that team then.

Type I.

Type I, the slow twitch,

slowest myosin, slow calcium pump.

And this is key, they're packed with mitochondria, those little energy factories.

Making them fatigue resistant.

Incredibly fatigue resistant.

These are your marathon runners.

Great for holding a posture or for repetitive, low intensity contractions.

Very economical.

Okay.

And type Ia.

Type CyA, fast oxidative glycolytic, faster myosin, fast calcium pump.

They've still got a decent amount of mitochondria, so they're relatively fatigue resistant, but definitely faster and more powerful than type I.

Kind of your middle distance runners.

Versatile.

And the last one, type I is the sprinters.

Exactly.

Type III, fast glycolytic, these are the speed demons.

Fastest myosin, fastest calcium pump.

They can contract up to ten times faster than type I.

Ten times?

Yeah.

Usually the largest diameter fibers too, but they have very few mitochondria.

They burn through energy anaerobically, produce lactate and fatigue really quickly.

So short bursts of intense power, like lifting something heavy or jumping.

Spot on.

Crucial for resistance exercises, those power pursuits.

That's a fantastic breakdown.

Marathoners, middle distance and sprinters, all in one muscle.

And how they're controlled matters too, right?

The motor unit.

Absolutely.

A motor unit that's one motor neuron and all the muscle fibers it connects to is always made up of the same fiber type.

All type I or all type Cs or all type Ia.

Okay, so the signal determines the fiber type it talks to.

Well, actually the neuron influences the fiber type it innervates, and the recruitment order is usually fixed.

When you need a bit of force, the brain recruits the type I motor units first.

Need more power?

It adds the type pre -units.

Need maximum explosive force, then brings in the big guns, the type I units.

So it's efficient.

Start with endurance, add speed, then max power, managing fatigue along the way.

Clever.

Very clever.

And this ties into power output.

Power isn't just force, it's force times the velocity.

How strong and how fast.

And interestingly,

muscle power peaks not at maximum speed, but when it's shortened it may be 20 -40 % of its top speed against a low that's about 30 -40 % of its max force.

Counterintuitive.

Not just max speed or max force.

Exactly.

And those fast -twitch fibers, Ia and especially Ix, they generate much more power simply because they're so much faster, even if the force per cross -sectional area is pretty similar across all types.

Which loops us right back.

Changes in fiber size, the hypertrophy and atrophy we talked about, and changes in the type of fiber,

those directly impact how much power a muscle can produce and how it performs overall.

It's this beautifully tuned system.

It really is.

Designed for whatever demands life throws at it.

Okay, so we've got the nuts and bolts.

Different fiber types, how they're controlled, how size and type affect power.

But let's get practical.

How does this play out in real life, like specific activities?

Do they really sculpt muscles in specific ways?

Oh, absolutely.

The body is incredibly adaptive.

Let's take endurance training.

Yeah.

Take long -distance running, cycling, swimming, that kind of sustained effort.

Yeah.

What happens there?

Well, studies show it tends to increase the proportion of those versatile type TIE fibers,

often at the expense of the super -fast type E.

The type I marathoners often say about the same.

So shifting towards more fatigue -resistant fast fibers.

Exactly.

But here's something really cool.

Endurance training triggers angiogenesis.

Angiogenesis.

Blood vessels.

Yep.

The growth of new capillaries, tiny blood vessels sprouting off existing ones.

They wrap around the muscle fibers, creating a denser network.

Think more highways for oxygen delivery.

Makes sense.

Endurance needs oxygen.

Totally.

And related to that, it also causes massive mitochondrial biogenesis.

More power plants.

More and bigger power plants inside the cells.

We're talking potentially a 40 % increase in total mitochondrial volume per fiber.

40%.

Wow.

This lets the muscles get much better at using fatty acids for fuel, which spares glycogen stores and reduces lactic acid buildup.

Huge advantage for endurance.

Is there like a master switch for this?

There are key signals.

One really important one is VEGF, vascular endothelial growth factor.

Exercise ramps up VEGF production, and that's the signal to build more capillaries.

Interestingly, just one bound of exercise triggers a rapid VEGF increase.

So the body responds almost immediately.

It does.

Though, trained muscles actually show less of a VEGF spike after exercise they're already adapted.

And the real conductor of this whole endurance orchestra seems to be a molecule called PGC1I1.

PGC1 alpha 1.

Okay.

It's a transcriptional co -activator.

It drives the mitochondrial growth, pushes fibers towards more oxidative types like IAA, and stimulates that VEGF -driven angiogenesis.

Experiments in mice clearly show you need PGC1I1 for exercise to trigger new capillary growth.

That's incredible coordination.

How fast does this happen?

In mice studies, they see VEGF production boosting within three days, measurable increases in capillaries after a week, and that shift from the fastest fibers towards type Thea after about two weeks.

It's a pretty rapid adaptation.

Endurance is about oxygen, mitochondria, capillaries, shifting towards fatigue resistance.

Now, flip side.

Resistance training.

Heavy lifting.

What's the goal there and how's it different?

Different goal, different signals, different outcome.

Resistance training is primarily about hypertrophy, making those muscle fibers bigger and increasing maximum strength.

More actin and myosin?

Exactly.

It stimulates the individual fibers to synthesize more of those contractile proteins, adding myofibers, literally bulking them up.

There's also a fiber type shift, similar to endurance in one way.

The super fast type Thea myofibers tend to convert into the slightly more durable type B fibers.

Type B usually doesn't change much.

But the big difference from endurance.

The big difference is resistance training doesn't typically boost aerobic capacity much.

You don't see the same big increases in capillary density or mitochondria.

It's all geared towards those short,

intense bursts of force production, not sustained aerobic effort.

And a different master switch, I bet.

You got it.

Remember, PGC 101 for endurance.

Resistance exercise triggers the production of a different isoform, PGC 104.

Alpha 4.

And PGC 104 specifically drives hypertrophy.

It does this partly by dialing down myostatin, remember that brake pedal on muscle growth?

Yeah, the bullywhipid gene.

Right.

And dials up IGF 1, which is a potent growth signal.

So different exercise, different PGC 1 alpha, different outcome,

endurance versus strength and size.

That's incredibly specific.

And this connects to muscle memory, right?

That feeling that it's easier to regain muscle than build it the first time.

Those extra nuclei donated by satellite cells during hypertrophy, they seem to stick around even if the muscle later atrophies from disuse.

So when you start training again, those persistent nuclei are already there, ready to ramp up protein synthesis quickly.

It gives you a head start on rebuilding.

Your muscles literally remember being bigger.

That explains a lot.

OK, what about tapering?

Athletes cutting back training before a competition to improve performance seems backward.

It does seem counterintuitive, but it works.

Often gives a 2 -4 % performance boost.

It's about letting the muscle fully recover and fine -tuning for peak power.

How?

Well, remember how training shifts type I's to type II?

One study on resistance -trained men found that during detraining or tapering, the proportion of the myosin associated with type I fibers actually increased, sometimes even beyond the levels they started with before training.

More of the super -fast fibers right before the event.

Exactly.

For a sprinter, or someone needing maximum power, that temporary surge in the fastest, most powerful fiber type could provide a real competitive edge.

The trade -off is they fatigue faster, so it's a delicate balance.

Fascinating.

And can you mix training types, like resistance and endurance?

You can.

And sometimes it's beneficial.

For example, a study on elite male cyclists showed that adding resistance training to their endurance routine improved both their endurance capacity and their muscle strength.

How did that work?

Did they get huge muscles?

No, that's the interesting part.

They saw the typical fiber shift, more type quia, less type VIII, but without significant hypertrophy or changes in capillary density.

They basically added strength while preserving their lean endurance -adaptive physique.

It's all about tailoring the training to the specific demands of the sport.

The specificity is just amazing.

Okay, we've spent a lot of time on skeletal muscle, the ones moving our limbs.

But what about the heart?

Cardiac muscle.

Does it play by the same rules?

Can it grow and shrink too?

And are there animal examples that push this to the extreme?

Great questions.

And yes, cardiac muscle hypertrophy, the heart getting bigger, shares principles with skeletal muscle.

In mammals, heart muscle cells also get bigger.

Hypertrophy.

They generally don't multiply.

So, the existing cells enlarge.

Exactly.

And that happens normally during growth,

with regular endurance exercise, and notably during pregnancy,

the heart adapts the walls, especially the left ventricle, might thicken, the internal chamber might get slightly larger, and yes, new capillaries grow to support the increased tissue.

Why does it do this?

It's adaptive.

A bigger, stronger heart consumes more oxygen, but can contract with more force and speed, pumping a larger volume of blood with each beat.

Crucial for meeting increased demands.

Is it permanent?

Physiological hypertrophy.

Usually not.

It's remarkably reversible.

A pregnant woman's heart grows to handle the extra blood volume, then typically returns to its pre -pregnancy size after birth.

Okay.

Well, here's a wild example, grizzly bears.

Their hearts atrophy shrink by about 26 % during their long winter hibernation, when their heart rate drops drastically,

maybe 84 beats per minute in summer, down to 19 in winter.

Then when they wake up, it grows back.

This physiological adaptation is very different from pathological hypertrophy, the kind you see with chronic high blood pressure or after heart attacks.

That type is often linked to cell damage, metabolic problems, and worse outcomes.

Understanding the difference is vital.

Which brings us to the Burmese python.

This is an incredible model for physiological hypertrophy, right?

Absolutely incredible.

These snakes, they live pretty quiet lives most of the time.

But then they eat a huge meal, sometimes up to half their body weight.

Unbelievable.

And their metabolism just skyrockets.

Oxygen consumption can increase sevenfold.

To handle this massive digestive load, something amazing happens to their heart.

What?

Within just 48 hours of feeding, their ventricular mass, the main pumping chamber, increases by 40%.

40 % in two days.

40%.

And it's true hypertrophy.

They measure increased protein synthesis markers, like cardiac myosin mRNA.

It's not just swelling or new cells.

And just like the bear, once digestion is done, the heart shrinks back down.

It's completely reversible.

That's stunning adaptation.

What did experiments show?

Could they trigger it?

This is where it gets even more fascinating.

Researchers took blood plasma from pythons that had just eaten and infused it into pythons that were fasting.

It triggered the same kind of cardiac hypertrophy in the fasted snakes.

They even identified a specific mix of three common fatty acids.

Muuristic, palmitic, and palmitoleic acid found in the fed python plasma.

Infusing just that fatty acid mix could also stimulate heart growth, not only in fasted pythons, but even in baby mice.

So wait, in mice too?

In rice too.

It strongly suggests there's a conserved molecular pathway triggered by these circulating factors after a meal that promotes beneficial heart growth.

It's a huge clue from comparative physiology that could maybe help us understand how to prevent harmful heart growth in humans.

It's just remarkable.

Truly remarkable.

Okay, from extreme growth to the opposite, disuse atrophy in humans.

We know it happens fast, like losing 20 % muscle mass in weeks if your arm's in a cast.

What's going on inside the cell when muscle wastes away?

It's a rapid and depressing process.

Besides casts, think bed rest, spinal injuries, nerve damage, aging, starvation, chronic illnesses.

Many triggers.

The core mechanism.

While protein synthesis might decrease a bit, the main driver is usually a big increase in the enzymatic breakdown of existing muscle proteins.

So the demolition crew gets hyperactive?

That's a good way to put it.

You lose actin and myosin, the myofibrils shrink, the fiber diameter decreases, you even lose mitochondria and nuclei over time.

It's a systematic dismantling.

And this happens in space too, microgravity.

Dramatically so.

Astronauts' studies are stark.

Those type I soleus fibers, the slow twitch ones in the calf, crucial for posture, their diameter can shrink measurably after just 17 days in space.

After several months on the ISS, even with exercise protocols, type I fibers can lose 20 % of their diameter.

20%, despite exercise.

Despite exercise.

And they also see a shift away from slow twitch towards faster fiber types, which aren't ideal for endurance or sustained work.

This significantly impacts strength and power, which is a real concern for long missions, especially emergencies.

And it sounds worryingly similar to what happens with aging, sarcopenia, right?

It is similar in outcome, though the causes are more complex.

Sarcopenia, the age -related loss of muscle mass and function.

After 40, you can lose maybe 1 -2 % of muscle mass per year, and it often accelerates after 65.

Even if you stay active?

Even in fit, older individuals, contractile performance declines.

And the decline is often steeper for power things requiring speed and force than for pure endurance.

Sarcopenia seems to involve not just disuse, but actual loss of muscle fibers, potentially due to problems with the motor neurons connecting to them, and a reduced ability of the muscle to repair and regenerate itself, even though the basic molecular tools are still there.

It's like the repair crew isn't getting the right signals or resources.

Something like that.

And there's an interesting idea that type times, the fastest fiber, might be the default state.

If a fiber isn't actively contracting or feeling mechanical stretch, it tends to revert to expressing type times genes.

Activity, on the other hand, specifically turns on type I and kinase genes and suppresses IX.

It really hammers home that use -it -or -lose -it principle at a molecular level.

A sobering thought.

But it makes you wonder, are there animals that just don't have this problem?

Are some immune to disuse atrophy?

It certainly seems that way.

And studying these animals gives us incredible insights into resistance to atrophy.

Take hibernating bears again.

The black bears.

American black bears, yeah.

They're basically inactive for five to seven months, curled up nearly 247.

You'd expect massive muscle loss.

But their tibialis anterior, a key leg muscle, only loses about 25 -30 % of its strength after over 100 days of inactivity.

Humans would lose way more, way faster.

How on earth do they manage that?

It seems they maintain protein balance incredibly well.

They actually recycle nitrogen from urea, a waste product, back into amino acids to build proteins.

So protein synthesis roughly equals protein breakdown throughout the winter.

No net loss.

Recycling waste into building blocks.

That's genius.

Isn't it?

And they might also use subtle shivering, essentially isometric contractions, to provide some minimal mechanical stimulus to the muscles.

Wow.

Okay, any other atrophy -resistant champs?

How about the Australian green -striped burrowing frog?

This little guy can estivate Godormin buried underground for months, sometimes years, waiting for rain.

Years.

Years.

And shows remarkably little muscle atrophy, especially in its powerful jumping muscles like the gastrocnemius.

It seems to prioritize breaking down proteins from other, less essential muscles to spare the jumping ones.

Plus, its overall metabolic rate plummets, and it seems to produce fewer damaging reactive oxygen species, ROS, in its mitochondria, which might also protect against atrophy pathways.

So metabolic slowdown, targeted protein use, maybe less cellular damage.

Nature's found multiple ways to beat atrophy.

Exactly.

These animals are living libraries of potential strategies to combat muscle wasting in humans.

These comparative examples are just gold mines.

So drilling down further, what are the molecular master switches controlling this whole balance?

The signals telling muscles to grow or shrink.

Okay, let's get into the key regulators.

A huge player discovered back in 97 is myostatin.

It's basically a stop signal for muscle growth, a negative regulator.

The brake pedal.

The brake pedal, exactly.

Remember the bully whippets.

Those dogs have a mutation that inactivates the myostatin gene.

Result, way more muscle mass, double muscle.

They look like little canine Schwarzeneggers.

Pretty much.

Myostatin is a protein that binds to receptors on muscle cells and triggers signals that put the brakes on growth.

And this gene is incredibly conserved, basically the same across mice, humans, chickens, you name it.

Which means it's fundamental.

Absolutely.

Yeah.

And it's a major target for research.

Could blocking myostatin help treat muscle wasting diseases?

Could it boost meat production and livestock?

Does natural variation in myostatin explain why some people build muscle easier than others?

Lots of interest there.

Okay, if myostatin is the brake, what's the accelerator?

A major accelerator pathway involves a protein called act T1, also known as protein kinase B, or PKB.

Think of act T1 as a central hub managing the balance.

Act T1.

When muscles work, they exert force.

This stimulates the muscle cells to release IGF -1, insulin -like growth factor 1.

IGF -1 binds to receptors on the muscle cell surface, and this triggers a chain reaction involving PA -3K, which then activates act T1.

So force, act GF -1, act T1, activation.

Right.

An activated act T1 does two crucial things simultaneously.

It ramps up protein synthesis pathways, telling the cell machinery to build more protein, and it actively inhibits protein degradation pathways.

Some act T1 even moves into the nucleus to stop the cell from making the machinery needed for breakdown.

So it pushes build and blocks destroy.

Precisely.

It tells the balance heavily towards growth, towards hypertrophy.

Insulin, by the way, also stimulates muscle protein synthesis through this same act T1 pathway.

And remember those satellite cells donating nuclei for hypertrophy?

IGF -1 helps stimulate them too.

So inactivity means less force, less IGF -1, less act T1 activation.

And the balance tips the other way.

Less stimulation for synthesis, less inhibition of degradation.

Breakdown starts to outpace synthesis, and you get atrophy.

Makes sense.

Any other players?

Hormones.

Definitely.

We mentioned glucocorticoids, stress hormones like cortisol or prednisone.

They actively inhibit the act T1 pathway.

That's why chronic stress or high dose steroid medication can cause muscle wasting.

They're interfering with the build signal and letting the destroy pathways run wild.

On the flip side, growth hormone and androgens like testosterone are known promoters of protein synthesis, adding more layers to the control system.

It's this complex network of signals, checks, and balances.

It really is.

And pulling it all together, you see this incredible malleability, this plasticity of muscle tissue.

It's constantly listening to signals, use, disuse, the type of activity, hormones, growth factors and responding by adjusting its size, its fiber type composition, its metabolic machinery, its blood supply,

all orchestrated by these intricate molecular pathways.

Beautiful example of biological adaptation.

And while the environment like exercise drives a lot of this,

our individual genetics play a role too, right?

Like our baseline fiber type mix or how strongly our muscles respond to training.

Absolutely.

There's definitely a genetic component influencing things like your natural fiber type proportions or your inherent potential for hypertrophy versus endurance adaptation.

That's another reason why studying a wide range of animals from elite human athletes to hibernating bears and feasting pythons is so valuable.

It shows us the full spectrum of what muscle tissue is capable of, the sheer range of its plastic potential.

So, okay, bringing it back to the listener, what's the big takeaway?

Why does understanding all this muscle plasticity matter for you?

Well, the applications are huge, aren't they?

I mean, optimizing training for athletes is obvious.

But beyond that,

managing age -related muscle loss, circopenia, helping people recover from injuries, combating muscle wasting and diseases like cancer or muscular dystrophy, designing better exercise programs for astronauts on long space flights, promoting healthy aging for everyone through targeted exercise and maybe even nutritional strategies based on these molecular pathways.

It really empowers us to potentially live stronger, more mobile lives.

We're taking these fundamental biological lessons and figuring out how to apply them.

Which really leads to the next big question, doesn't it?

Given this incredible adaptability we see across the animal kingdom, and as we keep unraveling the precise molecular signals, the VEGFs, the PGC -1 alphas, the ACTI -1 pathways, what breakthroughs are next?

What new ways might we find to harness this plasticity, learning from nature to keep our own muscles healthy and functional throughout our entire lives?

That's the exciting frontier.

Thank you for joining us on this deep dive into the truly fascinating world of muscle plasticity.

And as always, thank you for being part of the Last Minute Lecture family.