In the relaxed state, what prevents myosin from binding actin?

Tropomyosin covers myosin binding sites on actin and prevents myosin to bind to actin. When a neurotransmitter stimulates the muscle, an action potential travels down the T tubuals to the sarcomere reticulum and opens the calcium voltage-gated channels on sarcomere reticulum and calcium (Ca2+) rushes out and binds to troponin complex and that releases the tropmyosin form myosin binding sites so myosin binds to actin and contracts the muscle.