Excitation-contraction coupling is a series of events that occur between generating an action potential and initiating a muscle contraction. It occurs at the triad, a structure found in skeletal muscle fibers that comprise a T-tubule and terminal cisternae of the sarcoplasmic reticulum on each side. These triads are visible in longitudinally sectioned muscle fibers. They are typically located at the A-I junction — the junction between the A and I bands of the sarcomere.
When an action potential reaches a triad, it triggers calcium release from the sarcoplasmic reticulum. This temporary change in calcium permeability lasts for about 0.03 seconds, significantly increasing calcium concentration in and around the sarcomere. Troponin acts as a lock, preventing interaction between thick and thin filaments within a sarcomere, the smallest functional unit of muscle cells. Upon calcium binding, troponin undergoes a conformational change, allowing tropomyosin to move away from the active sites on actin filaments. This change marks the beginning of the contraction cycle.
Once the active sites are exposed, the myosin heads attach to them, creating cross-bridges between thick and thin filaments. The connection between the head and tail of the myosin molecule acts as a hinge, allowing the heads to pivot. This pivoting motion, fueled by the energy released from ATP hydrolysis, is called the power stroke and is a crucial step in muscle contraction. The myosin heads slide the thin filaments from both ends of a sarcomere toward the M line. This inward movement causes the filaments to move towards the center of the sarcomere. The filaments may often overlap, causing the I band and H zone to narrow and disappear when the muscle fully contracts. However, the A band and lengths of the thick and thin filaments do not change. Since the thin filaments at each side of the sarcomere attach to Z discs, their inward movement results in the shortening of the sarcomere and, as a result, the entire muscle fiber.
