Physiologic Anatomy of the Neuromuscular Junction — The Motor End Plate. Figure
7-1A and B shows the euromuscular junction from a large, myelinated nerve fiber to a skeletal muscle
fiber. The nerve fiber forms a complex of branching nerve terminals that invaginate into the surface of
the muscle fiber but lie outside the muscle fiber plasma membrane. The entire structure is called the
motor end plate. It is covered by one or more Schwann cells that insulate it from the surrounding
fluids. Figure 7-1C shows an electron micrographic sketch of the junction between a single axon
terminal and the muscle fiber membrane. The invaginated membrane is called the synaptic gutter or
synaptic trough, and the space between the terminal and the fiber membrane is called the synaptic space or synaptic cleft. This space is 20 to 30 nanometers wide. At the bottom of the gutter are
numerous smaller folds of the muscle membrane called subneural clefts, which greatly increase the
surface area at which the synaptic transmitter can act. In the axon terminal are many mitochondria that
supply adenosine triphosphate (ATP), the energy source that is used for synthesis of an excitatory
transmitter, acetylcholine. The acetylcholine in turn excites the muscle fiber membrane. Acetylcholine
is synthesized in the cytoplasm of the terminal, but it is absorbed rapidly into many small synaptic vesicles, about 300,000 of which are normally in the terminals of a single end plate. In the synaptic
space are large quantities of the enzyme acetylcholinesterase, which destroys acetylcholine a few
milliseconds after it has been released from the synaptic vesicles.
