Essential Cell Biology 5th edition

Ion Channels and Nerve Cell Signaling
417
voltage-gated
Ca 2+ channel
CLOSED
synaptic
cleft
RESTING
NERVE TERMINAL
presynaptic
nerve terminal
neurotransmitter
synaptic
vesicle
neurotransmitter
receptor
voltage-gated
Ca 2+ channel
OPEN
ACTIVATED
NERVE TERMINAL
Ca 2+
arriving action
potential
(electrical
signal)
fused synaptic
vesicle
released
neurotransmitter
(chemical signal)
Figure 12–40 An electrical signal is
converted into a secreted chemical
signal at a nerve terminal. When
an action potential reaches a nerve
terminal, it opens voltage-gated Ca 2+
channels in the plasma membrane,
allowing Ca 2+ to flow into the terminal.
The increased Ca 2+ in the nerve terminal
stimulates the synaptic vesicles to fuse
with the plasma membrane, releasing
their neurotransmitter into the synaptic
cleft—a process called exocytosis
(discussed in Chapter 15).
postsynaptic
cell
Transmitter-gated Ion Channels in the Postsynaptic
Membrane Convert the Chemical Signal Back into
an Electrical Signal
The released neurotransmitter rapidly diffuses across the synaptic cleft
and binds to neurotransmitter receptors concentrated in the plasma membrane
of the postsynaptic target cell. Once released, neurotransmitters
are rapidly removed from ECB5 the E12.39/12.40
synaptic cleft—either by enzymes that
destroy them or by pumps that return them to the nerve terminal or that
transport them into neighboring non-neuronal cells. This rapid removal
of the neurotransmitter limits the duration and spread of the signal and
ensures that when the presynaptic cell falls quiet, the postsynaptic cell
will do the same.
Neurotransmitter receptors can be of various types; some mediate relatively
slow effects in the target cell, whereas others trigger more rapid
responses. Rapid responses—on a time scale of milliseconds—depend on
receptors that are transmitter-gated ion channels (also called ion-channel-coupled
receptors). These constitute a subclass of ligand-gated ion
channels (see Figure 12–27B), and their function is to convert the chemical
signal carried by a neurotransmitter back into an electrical signal. The
channels open transiently in response to the binding of the neurotransmitter,
thus changing the ion permeability of the postsynaptic membrane.
This in turn causes a change in the membrane potential (Figure 12–41).
activated
nerve terminal
INACTIVE
POSTSYNAPTIC CELL
neurotransmitter
in synaptic cleft
inactive neurotransmitter
receptor (transmittergated
ion channel)
postsynaptic
cell
ions
ACTIVATED
POSTSYNAPTIC CELL
neurotransmitteractivated
receptor
change in
membrane
potential
(electrical
signal)
Figure 12–41 A chemical signal is
converted into an electrical signal by
postsynaptic transmitter-gated ion
channels at a synapse. The released
neurotransmitter binds to and opens the
transmitter-gated ion channels in the
plasma membrane of the postsynaptic cell.
The resulting ion flows alter the membrane
potential of the postsynaptic cell, thereby
converting the chemical signal back into an
electrical one (Movie 12.13).