Muscles contract through a series of steps that begin with an action potential. The action potential in a neuron travels down the axon of the neuron and reaches the synapse. At the synapse, the action potential releases a neurotransmitter that stimulates a motor neuron of a muscle cell to contract.
Muscle contractions are stimulated by action potentials that generate the release of acetylcholine. The neuron secretes this neurotransmitter, and it diffuses across the synapse to reach the motor neuron of the muscle cell. Next, acetylcholine causes an action potential to generate in the motor neuron. The action potential flows through the muscle cell and causes the sarcoplasmic reticulum to release calcium. As a result, myosin cross bridges begin to form in the muscle cells. The released calcium binds to the troponin molecules and prompts the myosin cross bridge binding sites to become exposed. If ATP is present, then the myosin cross bridge begins cycling, and the muscle cells and tissues begin to contract.
The phases of muscle contraction are known as the latent period when the muscles are still, the contraction period during muscle contraction, the relaxation period when the muscles relax and the refractory period. During the refractory period the muscle cells do not respond to a second stimuli and are still.