In the neuromuscular junction, each muscle fiber’s sarcolemma is innervated by a motor neuron. The presynaptic neuron has vesicle that releases a neurotransmitter, such as acetylcholine, in the synaptic cleft upon impulse. The acetylcholine binds to the receptors on the sarcolemma and causes the release of calcium ions from their vesicles. The calcium ions bind to their site on the actin and are significant in the actin-myosin movement that causes contraction of the muscle.