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2. BRAIN MECHANISMS: NEUROBIOLOGY AND NEUROANATOMY

dendrites in some areas such as the hippocampus, in response to chronic
use of morphine (Sklair-Tavron et al., 1996; Eisch et al., 2000). Some of the
long-lasting behavioural changes seen in chronic substance use will no doubt
relate to such structural changes. Many of the synaptic changes are thought
to be mediated by processes similar to those discovered for learning and
memory (Hyman & Malenka, 2001).

Conclusion
This chapter has provided an overview of normal brain function, and of the
many distinct processes that interact to produce behaviour. Alterations in
any one of the steps in the process (generation of action potentials, changes
in electrical activity or chemical conductance, neurotransmitter release,
neurotransmitter reuptake, changes in second messenger function, altered
gene expression, altered synaptic connectivity) can alter the function of other
interacting processes, which ultimately can affect behaviour. As will be seen
in the following chapters, psychoactive substances can profoundly alter
neuronal processes, leading to the behaviours characteristic of dependence.
The immediate psychoactive and rewarding effects of substance use can
be explained by understanding the mechanism of action of these substances
at the pharmacological level. Further, the development of tolerance and
withdrawal, and the long-term effects of substance use can also be
understood through knowledge of a drug’s mechanism of action. The effects
of psychoactive substances on more complex processes such as motivation
can also be understood through the knowledge of their effects on the brain.
Their effects on motivational systems in the brain will be discussed further
in Chapter 3. The specific effects of the major psychoactive substances will
be explored in Chapter 4.

BOX 2.1
Neuroimaging techniques
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) uses magnetic fields and radio waves to produce
high-quality two- or three-dimensional images of brain structures without injecting
radioactive tracers. The brain can be imaged with a high degree of detail. Although
MRI gives only static pictures of brain anatomy, functional MRI (fMRI) can provide
functional information by comparing oxygenated and deoxygenated blood, which
provides information on changes in brain activity in specific brain regions in response
to various stimuli such as drugs, sounds, pictures, etc. An fMRI scan can produce
images of brain activity as fast as every second, whereas positron emission
tomography (PET) usually takes 40 seconds or much longer to produce images of
brain activity. Thus, with fMRI, there is greater temporal precision. fMRI has the

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