A sarcomere is the basic functional unit of a muscle fiber, responsible for muscle contraction. It is composed of long protein filaments called actin and myosin, which slide past each other to shorten the muscle. Sarcomeres are arranged end-to-end within a muscle cell, creating a structure that allows for coordinated contraction. Each sarcomere is bordered by structures called Z-discs, which anchor the actin filaments. When a muscle receives a signal to contract, the myosin heads attach to the actin filaments and pull them inward, leading to the overall shortening of the muscle. This process is essential for movement and various bodily functions.