Neuromuscular junction, illustration. This is a junction between a nerve cell, or neuron (upper frame), and a muscle (bottom), where a nerve passes a signal to the muscle telling it to contract. Where the nerve axons (fibres) meet the muscle they branch into several terminal boutons. Each bouton 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 (in purple spheres). This binds to receptors on the muscle cell, causing the muscle to contract.