Transmission electron micrograph (TEM) of a neuromuscular junction. This is a junction between a nerve cell, or neuron (light grey band from left centre to top right), and a muscle (lower right and top left), where a nerve passes a signal to the muscle telling it to contract. Where the nerve axon (fibre) meets the muscle it branches 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. This binds to receptors on the muscle cell, causing the muscle to contract. Magnification: x2,000 when printed at 10 centimetres wide.