A myofibril is a long, thread-like structure found within muscle cells, specifically in skeletal and cardiac muscles. These myofibrils are composed of repeating units called sarcomeres, which are the basic functional units of muscle contraction. Each sarcomere contains proteins like actin and myosin that interact to produce muscle movement. Myofibrils are essential for muscle function, as they enable the contraction and relaxation of muscles during physical activities. When a muscle receives a signal from the nervous system, the myofibrils shorten, leading to muscle contraction. This process is crucial for various bodily movements, from walking to lifting objects.