a. The Synapse. A synapse (figure 11-2) is a "connection" between two
Figure 11-2. A synapse.
(1) First neuron. An axon terminates in tiny branches. At the end of each
branch is found a terminal bulb. Synaptic vesicles (bundles of neurotransmitter) are
located within each terminal bulb. That portion of the terminal bulb which faces the
synaptic cleft is thickened and is called the presynaptic membrane. This is the mem-
brane through which neurotransmitters pass to enter the synaptic cleft.
(2) Synaptic cleft. The synaptic cleft is the space between the terminal bulb
of the first neuron and the dendrite or cell body of the second neuron.
(3) Second neuron. The terminal bulb of the first neuron lies near a site on
a dendrite or the cell body of the second neuron. The membrane at this site on the
second neuron is known as the postsynaptic membrane. Within the second neuron is a
chemical that inactivates the used neurotransmitter.
b. The Neuromuscular Junction. A neuromuscular junction (figure 11-3) is a
"connection" between the terminal of a motor neuron and a muscle fiber. The
neuromuscular junction has an organization identical to a synapse. However, the bulb
is larger. The postsynaptic membrane is also larger and has foldings to increase its