Nerve Supply to Muscle Fibers
Muscle fibers are innervated by motor neurons.
Motor neurons are nerve cells that transmit information from the nervous system (brain and spinal cord) to voluntary muscles. They are efferent neurons.
The group of muscle fibers innervated by the same motor neuron is called a motor unit.
The distal part of the motor neuron branches into many enlargements, the axon terminals, which release a neurotransmitter called acetylcholine at their ends. This is received by the cell membrane of the muscle fiber at the neuromuscular junction (NMJ).
Once acetylcholine binds with the cell membrane, that causes a biochemical chain reaction within the muscle cell which in the end results with the muscle fiber contraction.
You can find interactive tutorials and diagrams on the innervation of muscle fibers in the articles below.