Muscle contraction

Contracting, in physiology, does not mean shortening. Muscle contraction is defined by two variables: the length of the muscle and the tension it generates. A muscle can generate tension without changing length. Holding a sleeping child in your arms is contraction too: tension without shortening.

The command starts outside the muscle. An action potential, which is an electrical impulse, travels down the motor nerve to its terminal, and there acetylcholine, a neurotransmitter, is released. The action potential depolarizes the membrane of the muscle fiber, meaning it changes its electrical charge, and that makes the sarcoplasmic reticulum, a storage compartment inside the fiber, release calcium.

Calcium is the final signal. In its presence, attractive forces appear between two kinds of filaments inside the fiber, actin and myosin, and those filaments slide past each other.