Excitation-contraction coupling is a physiological process by which an electrical response is converted into a mechanical response. In this instance, a motor neuron has an action potential, the electrical response, resulting in a depolarization of the muscle cell membrane, the sarcolemma, and contraction of the muscle, the mechanical response.
The action potential induces release of the neurotransmitter acetylcholine from the presynaptic terminals of the motor neuron into the synaptic cleft. Binding of acetylcholine to the sarcolemma triggers calcium release from the sarcoplasmic reticulum inside the muscle cell. The calcium binds to troponin-C in the thin filaments producing a conformational change that exposes a myosin-ATPase site on an actin molecule. ATP is hydrolyzed and produces a ratcheting effect in the filaments responsible for the muscle contraction.