How muscles contract, from nerve signal to sliding filaments

Every muscle cell (also called a muscle fiber) can contract, but the body builds three different kinds of muscle tissue. The fastest way to tell them apart is to ask three questions: Does it have stripes? How many nuclei? Can you control it?

The stripes come from actin and myosin lined up in a very regular pattern. Smooth muscle has actin and myosin too, but they are not organized into sarcomeres, so it has no stripes. Intercalated discs link cardiac cells so the heart contracts as one coordinated unit.

Picture a set of nesting bundles. A muscle is made of many long muscle fibers. Each fiber is packed with myofibrils, and each myofibril is a chain of repeating units called sarcomeres.

The sarcomere runs from one Z-disc to the next and is the functional unit of contraction. Inside it are two kinds of filaments:

• Thin filaments are mostly actin, anchored to the Z-discs. Two helper proteins ride along. Tropomyosin covers the spots on actin where myosin wants to bind. Troponin holds tropomyosin in place and has a binding site for calcium.
• Thick filaments are myosin, anchored at the center of the sarcomere (the M-line). Each myosin molecule has heads that can grab actin.

The fiber's cell membrane is the sarcolemma, and its specialized endoplasmic reticulum, the sarcoplasmic reticulum (SR), stores calcium.

Skeletal muscle only contracts when a nerve tells it to. The handoff happens at the neuromuscular junction (NMJ), where a motor neuron meets the muscle fiber.

1. An action potential travels down the motor neuron to its axon terminal.
2. The terminal releases the neurotransmitter acetylcholine (ACh).
3. ACh crosses the small gap (synaptic cleft) and binds receptors on the muscle fiber, and the fiber's membrane depolarizes.
4. An action potential sweeps along the sarcolemma and down the T-tubules into the fiber.
5. The signal makes the SR release calcium (Ca2+) into the cell.

Meanwhile, the enzyme acetylcholinesterase breaks down ACh in the cleft so the signal does not keep firing.

This is the sliding filament model. Calcium is the switch, and ATP is the fuel.

1. Ca2+ binds troponin. Troponin shifts, pulling tropomyosin off the binding sites on actin.
2. Myosin heads attach to the exposed sites, forming cross-bridges.
3. Power stroke: each head pivots and pulls the thin filament toward the center of the sarcomere.
4. ATP binds the myosin head, and the head lets go of actin.
5. The head splits the ATP and re-cocks, ready to grab a new spot further along.

This cycle repeats as long as calcium and ATP are present. The thin filaments slide inward past the thick filaments, so the Z-discs move closer together and the sarcomere shortens. Under a microscope, the I band gets narrower while the A band stays the same width. The filaments slide past each other; they do not shrink.

When the nerve signal stops, calcium is pumped back into the SR. Tropomyosin slides back over actin's binding sites, cross-bridges can no longer form, and the fiber relaxes.

ATP is needed for myosin to release actin, not just to pull it. After death, ATP production stops, so the myosin heads stay locked onto actin and the muscles become rigid. This is rigor mortis. Working muscle makes ATP three ways: from creatine phosphate (about the first 15 seconds), from anaerobic glycolysis, and from aerobic respiration, which needs oxygen.

A skeletal muscle moves the skeleton by pulling on a bone. The muscle's connective tissue blends into a tendon, and the tendon fuses with the covering of the bone, so the pull is transferred to the bone. The end attached to the bone that stays fixed is the origin; the end attached to the bone that moves is the insertion.

Muscles work in teams. The prime mover (agonist) does most of the work for a movement. A synergist helps it, and a fixator steadies the bone the prime mover pulls from. The antagonist produces the opposite movement. When you straighten your knee, the quadriceps femoris are the agonists and the hamstrings are the antagonists; when you bend it, the roles switch.