Schematic diagram of neuromuscular motor function

The motor unit (neuromuscular apparatus). Voluntary muscle control begins with a nerve impulse from the brain, which is relayed by an upper motor neuron to muscle by a lower motor neuron. A) Detail of the motor end plate, where nerve meets muscle. A nerve impulse causes discharge of acetylcholine into the synaptic space, where it attaches to receptors on muscle cells and causes muscle fiber contraction. B) Detail of skeletal muscle. In this microscopic depiction, type 1 (slow twitch) fibers are pale and type 2 (fast twitch) are dark.