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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

of ions is allowed to occur, which is what happens during an action potential
(Fig. 2.11).
An ion channel is a pore in the membrane through which ions can pass
+
+
2+
under certain circumstances (e.g. there are Na , K , and Ca channels). There
are channels that only open under certain circumstances, such as at a
particular membrane voltage (known as voltage-gated ion channels).
Depolarization in a local area of a neuron changes the voltage in that area,
and if it is of sufficient strength, may cause voltage-sensitive ion channels to
open, allowing ionic diffusion. Thus, adjacent areas become sequentially
depolarized, allowing propagation of the signal. This signal can be propagated
along an axon extremely rapidly. An action potential is an “all-or-none” event,
in that if the depolarizing stimulus is sufficient to reach a threshold value,
the action potential will be initiated and will travel without decrement to the
end of the axon.
After depolarization, the membrane rapidly becomes repolarized by the
+
opening of voltage-dependent K channels that are also opened by
depolarization, but only after a slight delay (approximately 1 millisecond).
+
Na channels also, do not stay open, but are inactivated after a certain period
of time. These factors enable rapid transmission and termination of messages.

Neurotransmitter release
Action potentials allow a message to be propagated along an axon within
one neuron. However, for communication to be complete, this message must
be transmitted between neurons. This is accomplished at the synapses of
the terminal buttons, through the release of neurotransmitter.
Neurotransmitters are chemical substances that are released from one neuron
and that interact with receptors on another neuron to affect a change in that
neuron. They will be discussed in further detail below.
The terminal buttons contain small structures known as vesicles, which
are packages of neurotransmitter that have been transported to the cell body.
When an action potential arrives at the terminal button, voltage-sensitive
2+
2+
Ca channels open, allowing Ca to flow into the terminal button and activate
a number of processes that cause the release of neurotransmitter into the
synaptic cleft. Once in the cleft, neurotransmitters diffuse across and bind to
postsynaptic receptors.
The chemical message needs a means of termination, and this occurs by
several mechanisms. One is by enzymatic degradation of the neurotransmitter
in the cleft, and another is by active reuptake of the neurotransmitter by the
presynaptic membrane. One of cocaine’s primary mechanisms of action is
to block the reuptake of neurotransmitters, thereby increasing their
concentration in the synaptic cleft, and increasing their effects. Amphetamine
acts by reversing the uptake mechanism, so that neurotransmitter is released
into the synaptic cleft independently of action potentials. These mechanisms
will be discussed in more detail in Chapter 4.

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