In the human body, a motor unit consists of a single motor neuron and all the skeletal muscle fibers it innervates. The axonal terminal of the motor neuron interfaces with skeletal muscle at the neuromuscular junction (NMJ).
At the axonal terminal, an electrical signal in the form of an action potential is converted into a chemical signal in the form of a neurotransmitter called acetylcholine (ACh). ACh traverses the gap of the NMJ in order to perpetuate the signal from the motor neuron. After crossing this space in a matter of milliseconds, ACh opens ion channels (nicotinic ACh receptors) present on the outer membrane of skeletal muscle fibers. This triggers an influx of sodium ions into the muscle fiber, depolarizing it. This depolarization leads to internal calcium release, which in turn results in muscle contraction. Physiologists refer to this process as excitation contraction coupling.