The axons of the motor neurons arborize in the muscles so that each muscle fiber is reached by an axonal branch. The number of muscle fibers supplied by one axon varies considerably. While a single axon may innervate two to three muscle fibers in the muscles of eyes and fingers, it may supply 50 – 60 muscle fibers in other muscles. The anterior horn cell and its axon (alfa-motoneuron) together with the group of muscle fibers it supplies is called a motor unit. When the neuron is stimulated, the muscle fibers contract in unison. The terminal branches of the axon lose their myelin sheaths before terminating and form tangled ramifications. In the terminal region, the surface of the muscle fiber forms a flat eminence called motor end plate.
The area of axonal arborization contains a number of cell nuclei. The nuclei lying on top of the axonal ramifications belong to Schwann cells that envelop the axon terminals (teloglia). The nuclei lying beneath the ramifications are muscle fiber nuclei in the region of the end plate. At the junction between axoplasm and sarcoplasm, the axon terminals are surrounded by a palisade layer which consists of infoldings of the sarcolemma, as shown by electron microscopy. The axons terminate with boutonlike swellings that dip into the surface of the end plate. These grooves are lined by the membrane of the sarcoplasm (sarcolemma) and a basement membrane. The heavily folded sarcolemma of the grooves (subneural clefts) greatly enlarges the surface area of the muscle fiber.
The motor end plate is a specialized synapse. Its presynaptic membrane is the axolemma, and its postsynaptic membrane is the folded sarcolemma. The substance transmitting nerve impulses to the muscle fiber is acetylcholine. It is contained in clear synaptic vesicles. Upon stimulation of the axon, the neurotransmitter is released into the synaptic cleft, resulting in receptor-mediated (nicotinic acetylcholine receptors) depolarization of the membrane of the muscle fiber.