myofilamenta

Myofilaments are the protein filaments that are responsible for muscle contraction. They are the main structural component of muscle fibers, also known as myocytes. There are two main types of myofilaments: thick filaments and thin filaments.

Thick filaments are primarily composed of the protein myosin. Myosin molecules are arranged in a staggered fashion, with their heads extending outwards. These heads are capable of binding to actin, a protein found in thin filaments.

Thin filaments are mainly composed of the protein actin. Actin molecules are arranged in a double helix structure. Associated with actin are two regulatory proteins: tropomyosin and troponin. Tropomyosin is a long, rope-like protein that covers the binding sites on actin, preventing myosin from attaching. Troponin is a complex of three proteins that binds to calcium ions.

During muscle contraction, a signal from a nerve causes an influx of calcium ions into the muscle cell. These calcium ions bind to troponin, causing a conformational change. This change in troponin shifts tropomyosin away from the actin binding sites. Once exposed, the myosin heads can bind to actin, forming cross-bridges. The myosin heads then pivot, pulling the actin filaments closer together. This sliding of filaments past each other shortens the muscle fiber and generates force. This process is known as the sliding filament theory. The coordinated action of many sarcomeres, the basic contractile units of muscle, results in the overall contraction of the muscle.