Muscle cell contraction begins when an excitatory neurotransmitter is released from the motor neuron in the cell. The neurotransmitter causes an action potential to propagate through the muscle cells, and contraction occurs.
Excitatory neurotransmitters, including acetylcholine, diffuse through the motor neuron to generate an action potential in a muscle fiber. This action potential propagates through the connective tissue sheaths of each muscle fiber. As it travels through the muscle fibers, calcium, sodium and potassium ions are released and absorbed to keep the muscle cell membrane electrically excited. Once the ion concentration in the muscle cells return to normal, the action potential slows and muscles relax.