myofibrillum

A myofibril is a long, filamentous organelle found within muscle cells. These structures are the fundamental units of striated muscle, responsible for the contraction of muscle tissue. Each myofibril is composed of repeating units called sarcomeres, which are the smallest contractile elements of the muscle fiber. Within a sarcomere, there are two main types of protein filaments: actin and myosin. Actin filaments are thin and typically anchored at the Z-lines, while myosin filaments are thicker and located in the center of the sarcomere.

The arrangement of these actin and myosin filaments creates the characteristic striated or striped appearance of skeletal and cardiac muscle. The interaction between actin and myosin, driven by the hydrolysis of ATP, causes the sarcomeres to shorten, leading to the overall contraction of the myofibril and subsequently the muscle cell. Myofibrils are densely packed within the sarcoplasm of muscle cells, often occupying a significant portion of the cellular volume. The precise arrangement and sliding of these protein filaments during muscle contraction is known as the sliding filament theory. Understanding the structure and function of myofibrils is crucial for comprehending muscle physiology and the mechanisms behind movement and force generation in the body.