The Molecular Machine: How Muscles Contract

Claire Brooks

When you flex your arm, what actually shortens? Not the muscle as a lump. Something far smaller. Today we go from a whole muscle down to a single protein, and then we watch that protein move.

Marcus Hayes

I like starting from the outside and working in, like unwrapping a gift with many layers. The whole muscle is wrapped in epimysium, and a tougher sheet called fascia wraps a group of muscles together. Inside, the perimysium wraps bundles called fascicles. Then the endomysium wraps each single muscle fiber. It's like a cable: an outer jacket, then bundles, then individual wires.

Claire Brooks

And each wire is one muscle cell. Let's open it up, because the membrane is the first surprise. It's called the sarcolemma, and it doesn't just sit on the surface. It dives into the cell through channels called transverse tubules, or T tubules. Why would a cell pull its skin inward like that?

Marcus Hayes

Speed. An electrical impulse has to reach the whole fiber at once. If it only crawled along the surface, the middle would lag behind. The T tubules carry the signal deep inside, so the entire cell gets the message together.

Claire Brooks

Inside is the sarcoplasm, which is the cytoplasm. It's packed with glycogen, which is stored energy, and myoglobin, which stores oxygen. Then there's the sarcoplasmic reticulum, a specialized smooth endoplasmic reticulum that works as a calcium reservoir. Its ends, called terminal cisternae, sit right beside the T tubules.

Marcus Hayes

So the signal comes down a T tubule, and right next to it is a tank of calcium. That's the setup. Now the real machinery. Filling the fiber are myofibrils, and those are made of myofilaments, thick and thin.

Claire Brooks

Thick filaments are bundles of hundreds of myosin molecules. Each myosin has an intertwined tail and two globular heads that stick out on flexible hinges. The head has two jobs: a binding site for actin, and a binding site for ATP, the energy.

Marcus Hayes

The thin filament is a bit like a beaded necklace twisted into a double strand. Each bead is a globular actin, and the whole strand is fibrous actin. Every bead has an active site where a myosin head can grab. But at rest, those sites are covered. Tropomyosin strands lie over them like yarn. Then troponin, a complex of three parts, binds tropomyosin, actin, and calcium.

Claire Brooks

Two more players. Titin, also called connectin, is a huge elastic protein that anchors the thick filaments and resists overstretching. And dystrophin links actin to the sarcolemma, so the force generated inside the fiber is carried outward to the connective tissue around it. Hold onto dystrophin. We'll come back to it.

Marcus Hayes

Now arrange those filaments in a pattern and you get the striations. The basic unit is the sarcomere, running from one Z disc to the next. Think of it as one repeating link in a chain, and a myofibril is millions of these links end to end.

Claire Brooks

The bands are easier if you remember what each one is made of. The A band is the dark region, the full length of the thick myosin filaments, including where they overlap the thin ones. The I band is the light region with only thin and elastic filaments, and the Z disc cuts right through its middle. The H zone is the lighter center of the A band, where at rest there's only myosin. And the M line is in the exact middle, holding the thick filaments in place.

Marcus Hayes

Here's the part that trips people up. When the muscle contracts, the filaments don't shrink. They slide. The A band stays the same length, because the myosin isn't getting shorter. But the I band narrows, the H zone disappears, and the Z discs move closer together. Actin gets pulled toward the M line.

Claire Brooks

Walk us through it, step by step. It starts at the neuromuscular junction. A motor neuron's synaptic knob releases acetylcholine, ACh, into the synaptic cleft. It binds to receptors on the junctional folds of the muscle membrane, and that starts an action potential.

Marcus Hayes

That impulse sweeps over the sarcolemma and plunges down the T tubules. At the terminal cisternae, calcium channels open, and calcium floods out of the sarcoplasmic reticulum into the sarcoplasm. Calcium binds troponin.

Claire Brooks

And that's where I'll quote the Crash Course Anatomy and Physiology episode on muscle cells. Host Hank Green gives a rule of thumb: "Proteins like to change shape when stuff binds to them." That is exactly what happens. Calcium lands on troponin, troponin changes shape, and it pulls tropomyosin off the active sites.

Marcus Hayes

The covers are off. A myosin head, already energized, carrying ADP and a phosphate, attaches to actin. That's the cross bridge. Then it releases the ADP and phosphate and bends at its hinge. That's the power stroke, and it pulls the thin filament toward the center.

Claire Brooks

Here's what I find most interesting. The head can't let go on its own. A fresh ATP has to bind the myosin head to make it detach. Then myosin's ATPase splits that ATP into ADP and phosphate, which re-cocks the head, the recovery stroke, ready to grab again.

Marcus Hayes

So ATP is not what powers the pull. It's what releases the grip and resets the head. And that explains rigor mortis, which the course material ties to this exact step. Without ATP, the heads stay locked in cross bridges.

Claire Brooks

Heads grab, pull, release, and reattach farther along, over and over, until the Z discs reach the myosin or the load is too heavy. Then relaxation: acetylcholinesterase breaks down ACh in the cleft, pumps powered by ATP return calcium to the sarcoplasmic reticulum, and tropomyosin slides back over the sites. The bridges break.

Marcus Hayes

Zoom back out. You don't fire every fiber every time, and that's thanks to the motor unit: one motor neuron plus every muscle fiber it controls. The fibers of one unit are scattered through the muscle belly, so the force is spread out smoothly rather than a single spot twitching.

Claire Brooks

And size matters. Muscles that need fine control, like those moving the eye, have small motor units, around three to five fibers per neuron. A big force muscle like the gastrocnemius has hundreds to thousands per neuron.

Marcus Hayes

All that work needs fuel. At rest, muscle gets about one quarter of the heart's output. In vigorous exercise it's more than three quarters, of a larger total output.

Claire Brooks

Does exercise make muscle by adding cells? No. Resistance training produces more myofilaments, so larger myofibrils, plus more mitochondria, myoglobin, glycogen, and blood vessels. That's hypertrophy. Mature muscle fibers don't divide by mitosis, so there's no hyperplasia. Satellite cells help with repair.

Marcus Hayes

Aging shows the flip side. Lean mass drops, and fat and fibrous tissue build up. Fast glycolytic fibers atrophy first and worst. The slides show cross sections of two thighs, both belonging to 74 year olds. A triathlete's thigh is dense with muscle. The sedentary one has shrunken muscle and far more fat.

Claire Brooks

Now the two diseases that tie it together. Remember dystrophin? Muscular dystrophy comes from genetic defects in it, where it's missing, minimal, or breaks down. It's hereditary, sex linked, and incurable. Without that support the sarcolemma tears, the fiber dies, and muscle gets replaced by fat and scar tissue.

Marcus Hayes

The second is myasthenia gravis, and it hits the junction instead. It's an autoimmune disease where antibodies destroy ACh receptors at the neuromuscular junction. Fibers become less sensitive to ACh, so you get drooping eyelids, trouble swallowing, weak limbs, and poor endurance.

Claire Brooks

The slide has a lovely illustration. A person gazes upward. At the start both eyes are fine. At 60 seconds the left eyelid is half closed, and at 90 seconds it's fully shut while the right eye stays open. The muscle simply can't keep up.

Marcus Hayes

One disease breaks the structure of the fiber, and the other breaks the signal that reaches it. Everything we covered today, from the wrapping to the receptors to the sliding filaments, is a place where a single failure becomes something you can see.