Physiology of Muscle Relaxation

In the resting (relaxed) muscle, the active (myosin-binding) sites on actin remain covered by the troponin-tropomyosin complex, so myosin cannot interact with actin and contraction cannot occur. When an action potential (nerve impulse) reaches the muscle at the motor end plate, it spreads across the sarcolemma of the myofibril. This signal travels down the transverse (T) tubules of the sarcoplasmic reticulum, which then releases a large number of calcium ions into the sarcoplasm.

These calcium ions interact with the troponin molecules, inactivating the troponin-tropomyosin complex and changing tropomyosin's structure so that it detaches from the active sites on the F-actin filament — uncovering those sites. The myosin head, which has already cleaved ATP to derive energy, now attaches to the newly uncovered active site of actin, forming an actomyosin complex. The myosin heads then tilt backward, pulling the attached actin filament inward. This pull, repeated across many cross bridges, results in the actin filaments sliding inward over the myosin filaments an …