Motor End Plate
The axons of the motor neurons (A1) arborize in the muscles so that each muscle fiber (AB2) is reached by an axonal branch (A3). 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 fin-gers, it may supply 50 – 60 muscle fibers in other muscles. The anterior horn cell and its axon (α-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 my-elin sheaths before terminating and form tangled ramifications. In the terminal re-gion, the surface of the muscle fiber forms a flat eminence (hence the term end plate) (A4).
The area of axonal arborization (A5) con-tains a number of cell nuclei. The nuclei lying on top of the axonal ramifications belong to Schwann cells that envelop the axon terminals (teloglia) (B6). The nuclei lying beneath the ramifications (B7) are muscle fiber nuclei in the region of the end plate. At the junction between axoplasm and sarcoplasm, the axon terminals are sur-rounded by a palisade layer (B8) which con-sists of infoldings of the sarcolemma, as shown by electron microscopy.
The axons terminate with boutonlike swell-ings (B9) that dip into the surface of the end plate. These grooves are lined by the mem-brane of the sarcoplasm (sarcolemma) and a basement membrane. The heavily folded sarcolemma of the grooves (subneural clefts) (C10) greatly enlarges the surface area of the muscle fiber.
The motor end plate is a specialized syn-apse. Its presynaptic membrane is the axo-lemma (C11), and its postsynaptic mem-brane is the folded sarcolemma (C12). The substance transmitting nerve impulses to the muscle fiber is acetylcholine. It is con-tained in clear synaptic vesicles (BC13). Upon stimulation of the axon, the neu-rotransmitter is released into the synaptic cleft, resulting in receptor-mediated (ni-cotinic acetylcholine receptors) depolariza-tion of the membrane of the muscle fiber.