The point where a skeletal muscle fibre comes into contact with a motor neuron carrying nerve impulses from the central nervous system. The impulses travel from the neuron to the muscle fibre by means of a neurotransmitter, in a similar way to the transmission of impulses across a synapse between two neurons. The neurotransmitter—acetylcholine (ACh) in vertebrates—is released from vesicles at the terminal knob of the motor neuron into a small gap (the cleft), where it diffuses to the end plate of the muscle fibre. Here the ACh binds to receptors in the muscle cell membrane, causing the ligand-gated ion channels controlled by the receptors to open. This allows a net influx of sodium ions (Na+) to enter the muscle cell, and some depolarization of the membrane occurs—an excitatory postsynaptic potential. If sufficient ACh is released by nerve endings (see summation), the depolarization spreads and activates voltage-gated sodium channels in the end plate and elsewhere in the muscle cell membrane, which triggers an action potential. This is propagated throughout the muscle fibre, causing it to contract. The receptor-bound ACh is subsequently broken down by the enzyme acetylcholinesterase into its components (acetate and choline), which re-enter the presynaptic nerve cell to be reused. During development or regeneration, the formation of a neuromuscular junction depends on signal molecules, such as agrin, which recruit components required for the synaptic connections. A single axon can have numerous terminal branches, each of which innervates a single muscle fibre.