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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

surface of a dendrite with which a terminal button from another neuron forms
a synapse), allows many different neurons to converge on a single nerve cell,
facilitating the coordination and integration of many complex messages. The
number of dendritic spines can increase or decrease following exposure to
psychoactive substances (Sklair-Tavron et al., 1996; Robinson & Kolb, 1999;
Eisch et al., 2000), thus altering communication between neurons, and most
likely contributing to the behavioural and neurological effects of the
substances. This will also be discussed in more detail at the end of the chapter.

Axon
The axon is a long slender process extending from the cell body, that carries
information from the cell body to the terminal buttons (see Fig. 2.8). Certain
chemicals such as neurotransmitters are transported along the axon, and it
also propagates nerve impulses (see below). The area where the axon leaves
the cell body is known as the axon hillock.

Terminal buttons

The terminal buttons are the bulbous structures found at the end of axons
(see Figs 2.8 and 2.10). At the terminal button, chemical signalling molecules
(which will be discussed more in the section on neurotransmission) are stored
in small packages, or vesicles. When an appropriate signal arrives at the
terminal button, neurotransmitter is released into the synapse or synaptic
cleft, the space between the terminal button and the membrane of the next
cell or dendrite with which it is communicating. The membrane of the
terminal button that is transmitting the message is known as the presynaptic

Fig. 2.10 A terminal button and synapse

Mitochondrion Cisterna releasing a vesicle
filled with neurotransmitter
Synaptic
vesicle

Synaptic cleft post-synaptic Presynaptic
membrane membrane

Source: Reproduced from Pinel, 1990, with permission from the publishers.

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