We move our bodies by shortening, or contracting, our muscles. Let's take a closer look. A muscle consists of parallel muscle fibers. Each fiber is a single cell in close contact with a motor neuron. Each muscle cell contains bundles of parallel myofibrils, shown in red, surrounded by endoplasmic reticulum, or ER. An action potential traveling down a motor neuron initiates an action potential in the muscle cell. The action potential spreads along the membrane and down tubules that extend into the cytoplasm, causing the myofibrils to contract. Each myofibril consists of a series of sarcomeres arranged end to end. The sarcomere is the contractile unit of muscle. Each sarcomere consists of thick filaments of myosin, shown in purple, and thin filaments of actin, shown in orange. A sarcomere contracts when its actin filaments slide past its myosin filaments. Contraction shortens the sarcomere but does not change the length of the actin or myosin filaments. The myosin filaments have heads that bind and pull actin repeatedly, shortening the sarcomere.