176
Chapter 6
input fi bers convey information about specifi c events in the
environment, whereas others control levels of cortical excit-
ability, determine states of arousal, and direct attention to
specifi c stimuli.
The subcortical nuclei are heterogeneous groups of
gray matter that lie deep within the cerebral hemispheres.
Predominant among them are the
basal nuclei
(also known
as
basal ganglia
), which play an important role in control-
ling movement and posture and in more complex aspects of
behavior.
The diencephalon, which is divided in two by the nar-
row third cerebral ventricle, is the second component of the
forebrain. It contains two major parts: the thalamus and the
hypothalamus. The
thalamus
is a collection of several large
nuclei that serve as synaptic relay stations and important inte-
grating centers for most inputs to the cortex. It also plays a key
role in general arousal and focused attention.
The
hypothalamus
lies below the thalamus and is on
the undersurface of the brain. Although it is a tiny region
that accounts for less than 1 percent of the brain’s weight,
it contains different cell groups and pathways that form the
master command center for neural and endocrine coordina-
tion. Indeed, the hypothalamus is the single most important
control area for homeostatic regulation of the internal envi-
ronment. Behaviors having to do with preservation of the
individual (for example, eating and drinking) and preservation
of the species (reproduction) are among the many functions of
the hypothalamus. The hypothalamus lies directly above and
modulates the function of the
pituitary gland,
an important
endocrine structure, which is attached to the hypothalamus
by a stalk (Chapter 11).
Thus far we have described discrete anatomical areas
of the forebrain. Some of these forebrain areas, consisting of
both gray and white matter, are also classifi
ed together in a
functional system called the
limbic system.
This intercon-
nected group of brain structures includes portions of frontal-
lobe cortex, temporal lobe, thalamus, and hypothalamus, as
well as the fi ber pathways that connect them (
Figure 6–40
).
Besides being connected with each other, the parts of the
limbic system connect with many other parts of the central
nervous system. Structures within the limbic system are asso-
ciated with learning, emotional experience and behavior, and
a wide variety of visceral and endocrine functions. In fact, the
hypothalamus coordinates much of the output of the limbic
system into behavioral and endocrine responses.
Cerebellum
The cerebellum consists of an outer layer of cells, the cerebel-
lar cortex (don’t confuse this with the cerebral cortex), and
several deeper cell clusters. Although the cerebellum does not
initiate voluntary movements, it is an important center for
coordinating movements and for controlling posture and bal-
ance. To carry out these functions, the cerebellum receives
information from the muscles and joints, skin, eyes and ears,
Frontal lobe
Olfactory
bulbs
Spinal cord
Thalamus
Hypothalamus
Hippocampus
Septal
nuclei
Figure 6–40
Structures of the limbic system (violet) and their anatomic relation to the hypothalamus (purple) are shown in this partially transparent view of
the brain.
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