myofilamentens

Myofilaments are the protein filaments that are responsible for muscle contraction. They are the main structural component of myofibrils, the long, cylindrical structures found within muscle cells. There are two primary types of myofilaments: thick filaments and thin filaments. Thick filaments are primarily composed of the protein myosin, which has a globular head region that can bind to actin. Thin filaments are mainly composed of the protein actin, which forms a helical structure. Associated with actin are regulatory proteins called tropomyosin and troponin.

During muscle contraction, these myofilaments interact in a sliding mechanism. The myosin heads in the thick filaments bind to actin in the thin filaments and pull them towards the center of the sarcomere, the basic contractile unit of a muscle fiber. This process, known as the sliding filament theory, shortens the sarcomere and thus the entire muscle. The interaction between myosin and actin is regulated by calcium ions and the proteins tropomyosin and troponin, which control the binding sites on actin. When calcium levels are low, tropomyosin covers the actin binding sites, preventing myosin from attaching. Upon nerve stimulation, calcium is released, causing troponin to shift tropomyosin, exposing the binding sites and allowing contraction to occur.