THE SLIDING-FILAMENT MODEL OF MUSCULAR CONTRACTION
The contractions of vertebrate skeletal muscle, through its pulling on bones, are responsible for all voluntary movements. What we generally refer to as muscle, for example the biceps brachii, is actually the largest member of a hierarchy of smaller and smaller organizational units. Muscles are composed of bundles of muscle fibers, which in turn are made of bundles of myofibrils. Within the myofibrils are the actual moving parts that result in contraction, the myofilaments actin and myosin, which are organized into contractile units called sarcomeres.

Contraction of skeletal muscle is initiated whan an action potential traveling down a motor neuron reaches the neuromuscular junction. The motor neuron releases acetylcholine, which binds with receptors on the muscle fiber and depolarizes the cytoplasmic membrane of the muscle fiber. This action potential travels down the inward-projecting  T tubules that reach deep into the muscle fiber, and actual muscular contraction begins.