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2. BRAIN MECHANISMS: NEUROBIOLOGY AND NEUROANATOMY

Above the hindbrain is the mesencephalon or midbrain (Fig. 2.1), which
contains two areas that are very important in substance dependence. The
ventral tegmental area (VTA) is rich in dopamine cell bodies, and projects to
the limbic system and forebrain regions. The VTA is involved in signalling the
importance of stimuli that are critical to survival such as those associated
with feeding and reproduction. However, many psychoactive drugs also have
powerful effects on this brain area, which contributes to the development of
dependence by signalling to the brain that psychoactive substances are very
important from a motivational perspective. The dopaminergic projection
from the VTA to the nucleus accumbens (discussed below) is known as the
mesolimbic dopamine system, and is the neurotransmitter system that is
most strongly implicated in the dependence-producing potential of
psychoactive drugs (Wise, 1998). This key concept will be discussed in more
detail in Chapters 3 and 4. Another important midbrain structure is the
substantia nigra, which also has dopaminergic projections to the forebrain,
but these pathways are involved in coordinating and executing movements
of the body. Degeneration of neurons in the substantia nigra leads to the
characteristic symptoms of Parkinson disease.
Finally, there is the prosencephalon or forebrain, which is composed of
the diencephalon and the telencephalon (cerebral hemispheres) (Fig. 2.1).
Important areas of the diencephalon (Fig. 2.2) are the thalamus, the
hypothalamus, and the posterior lobe of the pituitary gland. The
hypothalamus is critical for regulating hormonal signals and basic bodily

Fig. 2.2 Diencephalon
The figure shows the location of the two lobes of the thalamus, joined by the
massa intermedia. Beneath the thalamus lies the hypothalamus and posterior
pituitary gland, which regulate autonomic, endocrine and visceral functions.
Cerebral hemispheres

Right thalamus
Massa intermedia
Left thalamus
Hypothalamus
Cerebellum
Pons

Medulla

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

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