myofibrilsthe

Myofibrils are the fundamental structural and functional units of muscle cells, also known as muscle fibers. These elongated, rod-like structures are packed tightly within the sarcoplasm of a muscle cell and are responsible for muscle contraction. Each myofibril is composed of numerous smaller protein filaments called myofilaments. There are two main types of myofilaments: thick filaments, primarily made of myosin, and thin filaments, composed mainly of actin, along with regulatory proteins like tropomyosin and troponin.

The arrangement of these myofilaments within the myofibril creates a repeating pattern of light and dark bands, which gives skeletal muscle its striated appearance. The basic contractile unit of a myofibril is called a sarcomere, extending from one Z-line to the next. Within the sarcomere, the thick and thin filaments overlap in a specific manner. Muscle contraction, known as the sliding filament theory, occurs when the myosin heads of the thick filaments bind to actin on the thin filaments, pulling them closer together, thereby shortening the sarcomere and, consequently, the entire myofibril and muscle. This process requires energy in the form of ATP and is regulated by calcium ions.