When muscle is in the rest condition or being relax, there will not be any interaction between thick and thin myofilaments. This happens because the active sites of both myofilaments are blocked by tropomyosin compound.
The muscle contraction occur if the stimulation coming from motorist nervous system releases transmitter substance (acetylcholine) that causes permeability change in sarcolemma (muscle cell membrane ) The Na+ion ouside sarcolemma will flow fast into cell and cause Na+ion will compound with troponine, so that the active site of myofilament is freed from the tropomyosin blockage. The release of tropomyosin causes interaction between actins and myosin. The myosin fiber will drive action fiber, so that both fibers slide and become closer, and the cell shortens or contracts. The I band and H zone disappear as the Z lines come closer together.
As long as there is nervous stimulation, the concentration of ion becomes high, and this condition will occur continuously. In turn, If the stimulation stops, the concentration of ion becomes low, so that tropomyosin will move back to the active site of myofilament to block it.