408
UNIT 3
Regulation and Integration of the Body
11
2
Voltage-gated Ca
2
1
channels open and Ca
2
1
enters the
axon terminal.
Depolarization of the membrane by the ac-
tion potential opens not only Na
1
channels but voltage-
gated Ca
2
1
channels as well. During the brief time the Ca
2
1
channels are open, Ca
2
1
floods down its electrochemical
gradient from the extracellular fluid into the terminal.
3
Ca
2
1
entry causes synaptic vesicles to release neurotrans-
mitter by exocytosis.
Te surge of Ca
2
1
into the axon ter-
minal acts as an intracellular messenger. A Ca
2
1
-sensing
protein (
synaptotagmin
) binds Ca
2
1
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
2
1
is then quickly removed from the
terminal—either taken up into the mitochondria or ejected
from the neuron by an active Ca
2
1
pump.
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
signals.
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
(Figure 11.17)
:
1
Action potential arrives at axon terminal.
Neurotransmis-
sion at a chemical synapse begins with the arrival of an ac-
tion potential at the presynaptic axon terminal.
Dendrites
Cell body
Axon
Axodendritic
synapses
Axosomatic
synapses
Cell body (soma)
of postsynaptic
neuron
Axon
(b)
Axoaxonal
synapses
Axosomatic
synapses
(a)
Figure 11.16
Synapses.
(a)
Axodendritic, axosomatic, and axoaxonal synapses.
(b)
Scanning
electron micrograph of incoming fibers at axosomatic synapses (5300
3
).
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