A muscle is made of bundles of muscle fibers. Each fiber contains parallely-arranged contractile myofibrils, which are made by repetitive units (each indicated between the two blue lines in the figure). These are relaxed when the muscle is resting and contracted when it is activated. Muscle contraction results from the relative sliding of molecular components of the contractile unit on each other reducing their lengths. The final shortening of a contracted muscle is the summation of the contractile units shortenings. It is noteworthy that Galvani, knowing almost nothing about the microscopic structure of the muscle, had suggested that muscle contraction could be the result of increased reciprocal contact among its particles. The bottom left corner of the figure shows that the beginning of the movement is similar to the transmission between central neurons. A motor nerve-ending contacts a muscle fiber. Upon its activation, a chemical messenger is released, which causes the opening of channels in the membrane of the muscle fiber and the consequent flow of ions. An electric impulse travels along the membrane and reaches the inside of the muscular fiber, where it causes the liberation of the calcium ions from cellular stores. The increased availability of free calcium is the starting signal for the myofibrils to contract.
