Secretion of Acetylcholine by the Nerve Terminals When a nerve impulse reaches the
neuromuscular junction, about 125 vesicles of acetylcholine are released from the terminals into the
synaptic space. Some of the details of this mechanism can be seen in Figure 7-2, which shows an
expanded view of a synaptic space with the neural membrane above and the muscle membrane and its
subneural clefts below. On the inside surface of the neural membrane are linear dense bars, shown in
cross section in Figure 7-2. To each side of each dense bar are protein particles that penetrate the
neural membrane; these are voltage-gated calcium channels. When an action potential spreads over the
terminal, these channels open and allow calcium ions to diffuse from the synaptic space to the interior
of the nerve terminal.
