Neuromuscular junction, light micrograph. This is a junction between a nerve cell, or neuron, and a muscle, where a nerve passes a signal to the muscle telling it to contract. The nerve axon (fibre) runs from left upper centre to lower centre, where it branches into several terminal boutons (black dots). Each of these meets the muscle at a synapse, a narrow gap. When the nerve signal reaches the bouton it causes the release of the neurotransmitter chemical acetylcholine. This binds to receptors on the muscle cell, causing the muscle to contract.