12.4. Nerve, muscle, synapse[[Student version, January 17, 2003]] 477
Figure 12.20:(Anatomical drawings.) Two classes of human neurons. These are actual anatomical drawings from
the pioneering work of S. Ram ́on y Cajal in 1909. (a)Apyramidal cell from the rabbit cerebral cortex. The axon
divides near the cell body (or soma, dark blob between “a” and “b”), sending “collateral” branches (labeled “c”) to
connect with nearby cells as well as a main axon (labeled “e”) projecting to distant parts of the brain. The other
branched lines extending from the soma are dendrites (input lines). (Top to bottom, about 0. 8 mm.) (b)APurkinje
cell, showing its extensive dendritic (input) system. The axon is labeled “a.” (Top to bottom, about 0.4mm.) [From
Ram ́on y Cajal, 1995.]
12.4.2 The neuromuscular junction
The best-studied synapse is the junction between a motor neuron and its associated muscle fiber.
As sketched in Figure 12.21, the axon terminals contain large numbers of synaptic vesicles, filled
with the neurotransmitter acetylcholine. In the quiescent state the vesicles are mostly awaiting
release, though a few release spontaneously each second.
As an action potential travels down an axon, it splits into multiple action potentials if the
axon branches, finally arriving at one or more axon terminals. The terminal’s membrane contains
voltaged-gated calcium channels; in response to depolarization, these channels open. The external
concentration of Ca++is in the milllimolar range, but active pumping maintains a much smaller
(micromolar) concentration inside (see Section 11.3.5 on page 432). The resulting influx of calcium
catalyzes the fusion of about 300 synaptic vesicles with the presynaptic membrane in about a
millisecond (Figure 2.9 on page 38). The vesicles’ contents then diffuse across the synaptic cleft
between the neuron and the muscle fiber.
On the other side of the synapse, the muscle cell contains ligand-gated ion channels sensitive
to acetylcholine (Figure 12.19). The release of a single synaptic vesicle generates a measurable,
subthreshold depolarization in the muscle cell. S. Kuffler and coauthors showed that an identical
response could be generated by manually injecting a tiny quantity of acetylcholine (fewer than
10 000 molecules) into the neuromuscular junction. The arrival of an action potential, however,
releases many vesicles at once. The ensuing large depolarization triggers an action potential in the