neurotransmitter. Together, the gap and the "connecting" membranes of the neurons
are called the synapse (Figure 12-8). The gap is called the synaptic cleft.
Figure 12-8. A synapse.
a. Many synaptic vesicles (bundles of neurotransmitters) are found in the
terminal bulb (bouton) of the first neuron. Each vesicle contains a quantum, a specific
amount, of neurotransmitter or a substance used to make the neurotransmitter.
b. When the impulse reaches the bouton, these vesicles are stimulated to
release their neurotransmitter. The neurotransmitter then passes out of the bouton,
through the presynaptic membrane, into the synaptic cleft. On the other side of the
synaptic cleft is the postsynaptic membrane. This is the receptor site of the second
c. The neurotransmitter is located only in the terminal bulb of the first neuron.
For this reason, impulses travel in only one direction through the synapse, from the first
to the second neuron. Since this process consumes much energy, there are many
well-developed mitochondria in the bouton, or terminal bulb.
12-19. THE NEUROMUSCULAR JUNCTION
While the synapse is the "connection" between two neurons, the neuromuscular
junction (Figure 12-9) is the "connection" between a motor neuron and a striated muscle