A contractile unit, also known as a sarcomere, is the basic functional unit of muscle tissue. It is the segment of a muscle fiber that contracts to produce movement. Each sarcomere is made up of overlapping protein filaments, primarily actin and myosin, which slide past each other during muscle contraction, shortening the overall length of the muscle. The arrangement of sarcomeres in a series forms a myofibril, which is a long, thread-like structure within muscle cells. When a muscle receives a signal from the nervous system, the contractile units work together to generate force, allowing for various movements in the body.