diencephalon(upper end, where the brainstem meets the cerebrum). Twelve pairs of
nerves, collectively referred to as the cranial nerves, originate in the brainstem and sub-
serve sensory and motor function in the head and neck. One of these nerves, the vagus
nerve(so called because it wanders throughout the thorax and abdomen), is important
to the autonomic nervous system (section 4.3.1). Within the medulla are a number of
neurons actively involved in the biosynthesis of epinephrine (section 4.3.2). Lying
partly in the pons and partly in the mesencephalon is the locus ceruleus(ornucleus
pigmentosus), which is rich in norepinephrine-containing neurons and thus plays a role
in the adrenergic neurotransmitter systems (see section 4.3.1). The dorsal (back)
portion of the pons and mesencephalon is referred to as the tegmentum and contains a
variety of nerve fibre tracts. The mesencephalon contains the substantia nigra, a region
of the brain that is intimately involved with the dopamine neurotransmitter and is thus
involved in the medicinal chemistry of Parkinson’s disease (section 4.4.4). The dien-
cephalon is divided into the following regions:thalamus,hypothalamus,subthalamus,
andepithalamus. The thalamus acts as a relay station that transmits, correlates, and
integrates all ascending sensory information from the body on its way to the cerebrum.
The hypothalamus has a regulatory influence over the autonomic nervous system and is
the point at which the nervous system (using neurotransmitters as messengers) and the
endocrine system (using hormones as messengers) interface. The hypothalamus also
produces responses to emotional changes and to needs signaled by hunger and thirst;
NEUROTRANSMITTERS AND THEIR RECEPTORS 199Figure 4.4 General structure of the brain: the central nervous system consists of the spinal cord and
the brain. The brain consists of the brain stem (medulla oblongata, cerebellum, pons, mesencephalon,
diencephalon) and the cerebrum (cerebral hemispheres, subcortical white matter, basal ganglia).