FIGURE 11.10 • Neuromuscular junction. a. Diagram of a neuromuscular junction. An axon is shown making contact with a muscle cell. Note how the junctional folds of the muscle cell augment the surface area within the synaptic cleft. The external lamina extends throughout the cleft area. The cytoplasm of the Schwann cell is shown covering the axon terminal. (Modified from Kelly DE, Wood RL, Enders AC, eds. Bailey’s Textbook of Microscopic Anatomy. Baltimore: Williams & Wilkins, 1984.) b. Electron micrograph of a neuromuscular junction shows the axon ending within the synaptic cleft of a skeletal muscle fiber. An aggregation of mitochondria (M) and numerous synaptic vesicles (SV ) is visible. The portion of the motor axon ending that is not in apposition to the muscle fiber is covered by Schwann cell cytoplasm (S), but no myelin is present. The muscle fiber shows the junctional folds (JF ) and the subneural clefts (SnC) between them. The external lamina of the muscle fiber is barely evident within the subneural clefts. Other structures present are the aggregated mitochondria of the muscle fiber (M) in the region of the neuromuscular junction, the nucleus (N) of the muscle fiber, and some myofibrils (MF ). 32,000. (Courtesy of Dr. George D. Pappas.) top of the folds. The external lamina extends into the subneural folds (see Fig. 11.10). The synaptic vesicles of the axon terminal release ACh into the cleft, which then binds to nicotinic ACh receptors on the sarcolemma of the striated muscle. The nicotinic ACh receptor in striated muscles is a transmitter-gated Na channel. Binding of ACh opens
ACh Na channels, causing an influx of Na into striated muscle Na receptor cell. This influx results in a localized membrane depolariza tion, which in turn leads to the events described above. An enzyme called acetylcholinesterase (AChE) quickly breaks down the acetylcholine to prevent continued stimulation. For more detail description of ACh function, see Chapter 12.
The muscle fiber cytoplasm that underlies the junctional folds contains nuclei, many mitochondria, rough-surfaced endoplasmic reticulum (rER), free ribosomes, and glycogen. These cytoplasmic organelles are believed to be involved in the synthesis of specific acetylcholine receptors in the membrane of the cleft, as well as acetylcholinesterase.
A neuron along with the specific muscle fibers that it in- nervates is called a motor unit. before depolarization depolarization
A single neuron may innervate several to a hundred or more muscle fibers. Muscles capable of the most delicate movements have the fewest muscle fibers per motor neuron in their motor units. For example, in eye muscles, the innervation ratio is about one neuron to three muscle fibers; in the postural muscles of the back, a single neuron may innervate hundreds of muscle fibers.
The nature of muscle contraction is determined by the number of motor neuron endings as well as by the number of specific types of muscle fibers that are depolarized. Al-voltage sensor though depolarization of a muscle fiber at a single neuromuscular junction is characterized as an “all-or-none” phenomenon, not all nerve terminals discharge at once,
FIGURE 11.11 • Summary of events leading to contraction of which allows a graded response to the contractile stimulus.
skeletal muscle. See the text for a description of the events
Innervation is necessary for muscle cells to maintain their structural integrity.
The motor nerve cell not only instructs the muscle cells to contract but also exerts a trophic influence on the muscle cells.