Only one motor neuron innervates each individual muscle fiber, as the University of Washington explains. However, each motor neuron is connected to multiple muscle fibers within a bundle. This is termed a single motor unit.
A single motor neuron can innervate many muscle fibers to form a motor unit, notes the University of Texas. Motor neurons release acetylcholine at their synapses with the muscle fibers in a cleft termed the neuromuscular junction. This release of neurotransmitter is unusual in the nervous system, as it is very large and enough to depolarize all the muscle fibers in the motor unit above their threshold, according to the University of Washington.