Organisms respond to external stimuli.
If the stimulus is stong enough, a nerve impulse occurs. When the synaptic cleft separates a nerve cell from a muscle cell it is referred to a neuromuscular junction. The acetylcholine then diffuses across the synaptic cleft to the acetylcholine receptors on the postsynaptic membrane of the muscle cell. The opening of these ion channels on the postsynaptic membrane of the muscle cell then stimulates the muscle cell to contract.