Ultrastructure of the contractile system of striated skeletal muscle and the processes of muscular contraction. II. Releasing system and mechanisms of muscular contraction.

The tropomyosin-troponin system of the thin filaments acts as a regulator of the interaction of myosin with actin by a process of inhibition. This inhibition is released by Ca2+ ions. Muscle contraction occurs when Ca+2 ions are released from the sarcoplasmic reticulum into the myofibril, where they interact with the troponin of the thin filaments. Muscular contraction is due to the sliding of the two kinds of filaments past each other in the hexagonal network of the myofibril. Many hypotheses have been formulated concerning the intimate molecular mechanisms responsible for the sliding of the filaments past each other, but none of these can be considered to be entirely satisfactory.