478 Chapter 12. Nerve impulses[[Student version, January 17, 2003]]
Figure 12.21:(Drawing based on structural data.) Cross-section of a chemical synapse (see also Figure 2.9 on page
38). The end of an axon is shown at the top, with two synaptic vesicles full of neurotransmitter molecules inside and
one in the process of fusing with the axon’s plasma membrane, dumping its contents into the synaptic cleft. The
receiving (or postsynaptic) dendrite (or dendritic spine) is shown at bottom. Neurotransmitters diffusing across the
cleft bind to receptor proteins embedded in the dendrite’s membrane. In some cases these receptors are ligand-gated
ion channels. [From Goodsell, 1993.]
muscle cell, ultimately activating the myosin molecules generating muscle contraction (Figure 10.1
on page 355).
Thus the neuromuscular connection involves two distinct steps: presynaptic release of acetyl-
choline, followed by its postsynaptic activity. One particularly clear way to separate these steps
experimentally involves the alkaloid curare, which paralyzes skeletal muscle. Stimulating a motor
neuron in the presence of curare leads to normal action potentials and the release of normal amounts
of acteylcholine, but no muscle contraction. It turns out that curare competes with acetylcholine
for binding to the postsynaptic ligand-gated ion channels, inhibiting their normal action in a way
analogous to competitive inhibition in enzymes (Problem 10.4). Other neurotoxins, for example
the one in cobra venom, work similarly.
Tostop muscle contraction after the neuron stops supplying action potentials, an enzyme
calledacetylcholinesteraseis always present in the synaptic cleft, breaking down neurotransmitter
molecules shortly after their release into their components, acetate and choline. Meanwhile, the
neuron is constantly replenishing its supply of synaptic vesicles. It does this by actively transport-
ing choline back in, using it to synthesize acetylcholine, and by actively recovering the lipid bilayer
that fused with the neuron’s outer membrane and repackaging the acetylcholine into new vesicles.