Regulation and Integration of the Body
channels open and Ca
Depolarization of the membrane by the ac-
tion potential opens not only Na
channels but voltage-
channels as well. During the brief time the Ca
channels are open, Ca
ﬂoods down its electrochemical
gradient from the extracellular ﬂuid into the terminal.
entry causes synaptic vesicles to release neurotrans-
mitter by exocytosis.
Te surge of Ca
into the axon ter-
minal acts as an intracellular messenger. A Ca
) binds Ca
and interacts with
the SNARE proteins that control membrane fusion (see
Figure 3.14). As a result, synaptic vesicles fuse with the axon
membrane and empty their contents by exocytosis into
the synaptic cle±. Ca
is then quickly removed from the
terminal—either taken up into the mitochondria or ejected
from the neuron by an active Ca
In short, transmission of nerve impulses along an axon and
across electrical synapses is a purely electrical event. However,
chemical synapses convert the electrical signals to chemical sig-
nals (neurotransmitters) that travel across the synapse to the
postsynaptic cells, where they are converted back into electrical
Information Transfer Across Chemical Synapses
In Chapter 9 we introduced a specialized chemical synapse
called a neuromuscular junction (p. 286). Te chain of events
that occurs at the neuromuscular junction is simply one exam-
ple of the general process that we will discuss next and show in
Focus on a Chemical Synapse
Action potential arrives at axon terminal.
sion at a chemical synapse begins with the arrival of an ac-
tion potential at the presynaptic axon terminal.
Cell body (soma)
Axodendritic, axosomatic, and axoaxonal synapses.
electron micrograph of incoming ﬁbers at axosomatic synapses (5300