Chapter 12
The Central Nervous System
441
12
ing “inner room,” which well describes this deep, well-hidden
brain region that makes up 80% of the diencephalon.
Te thalamus is
the
relay station for information com-
ing into the cerebral cortex. Within the thalamus are a large
number of nuclei, most named according to their location
(Figure 12.11a)
. Each nucleus has a functional specialty, and
each projects fibers to and receives fibers from a specific re-
gion of the cerebral cortex.
Afferent impulses from all senses and all parts of the body
converge on the thalamus and synapse with at least one of its
nuclei. For example, the ventral posterolateral nuclei receive
impulses from the general somatic sensory receptors (touch,
pressure, pain, etc.), and the
lateral
and
medial geniculate bodies
(jě-nik
9
u-lāt; “knee shaped”) are important visual and auditory
relay centers, respectively.
Within the thalamus, information is sorted out and “edited.”
Impulses having to do with similar functions are relayed as a
group via the internal capsule to the appropriate area of the sen-
sory cortex as well as to specific cortical association areas. As
the afferent impulses reach the thalamus, we have a crude rec-
ognition of the sensation as pleasant or unpleasant. However,
specific stimulus localization and discrimination occur in the
cerebral cortex.
In addition to sensory inputs, virtually
all
other inputs as-
cending to the cerebral cortex funnel through thalamic nuclei.
Tese include:
Inputs that help regulate emotion and visceral function from
the hypothalamus (via the anterior nuclei)
Instructions that help direct the activity of the motor cortices
from the cerebellum and basal nuclei (via the ventral lateral
and ventral anterior nuclei, respectively)
Inputs for memory or sensory integration that are projected
to specific association cortices (via pulvinar, lateral dorsal,
and lateral posterior nuclei)
In summary, the thalamus plays a key role in mediating sen-
sation, motor activities, cortical arousal, learning, and memory.
It is truly the gateway to the cerebral cortex.
Hypothalamus
Named for its position below (
hypo
) the thalamus, the
hypo-
thalamus
caps the brain stem and forms the inferolateral walls
of the third ventricle (Figure 12.10). Merging into the midbrain
inferiorly, the hypothalamus extends from the optic chiasma
(crossover point of the optic nerves) to the posterior margin of
the mammillary bodies.
Te
mammillary bodies
(mam
9
mil-er-e; “little breast”),
paired pealike nuclei that bulge anteriorly from the hypothala-
mus, are relay stations in the olfactory pathways. Between the
optic chiasma and mammillary bodies is the
infundibulum
(in
0
fun-dib
9
u-lum), a stalk of hypothalamic tissue that connects
the
pituitary gland
to the base of the hypothalamus. Like the
thalamus, the hypothalamus contains many functionally im-
portant nuclei (Figure 12.11b).
Despite its small size, the hypothalamus is the main visceral
control center of the body and is vitally important to overall
the putamen (“pod”) and globus pallidus (“pale globe”) together
form a lens-shaped mass, sometimes called the
lentiform nucleus
,
these two nuclei are functionally separate.
Te basal nuclei are functionally associated with the
subtha-
lamic nuclei
(located in the lateral “floor” of the diencephalon)
and the
substantia nigra
of the midbrain (see Figure 12.14a).
Te basal nuclei receive input from the entire cerebral cortex,
as well as from other subcortical nuclei and each other. Via re-
lays through the thalamus, the output nucleus of the basal nuclei
(globus pallidus) and the substantia nigra project to the premo-
tor and prefrontal cortices and so influence muscle movements
directed by the primary motor cortex. Te basal nuclei have no
direct access to motor pathways.
Te precise role of the basal nuclei has been elusive because of
their inaccessible location and because their motor functions over-
lap with those of the cerebellum. In addition to their motor func-
tions, the basal nuclei play a role in cognition and emotion. In all
of these cases, the basal nuclei seem to filter out incorrect or inap-
propriate responses, passing only the best response on to the cortex.
For example, in motor activity, the basal nuclei are particu-
larly important in starting, stopping, and monitoring the inten-
sity of movements executed by the cortex, especially those that
are relatively slow or stereotyped, such as arm-swinging during
walking. Additionally, they inhibit antagonistic or unnecessary
movements. Disorders of the basal nuclei result in either too
much movement (as in Huntington’s disease) or too little move-
ment (Parkinson’s disease); see p. 464.
Check Your Understanding
4.
What anatomical landmark of the cerebral cortex separates
primary motor areas from somatosensory areas?
5.
Mike, who is left-handed, decided to wear his favorite T-shirt
to his anatomy class. On his T-shirt were the words “Only
left-handed people are in their right minds.” What does this
statement mean?
6.
Which type of fiber allows the two cerebral hemispheres to
“talk to each other”?
7.
Name the components of the basal nuclei.
For answers, see Appendix H.
Diencephalon
Describe the location of the diencephalon, and name its
subdivisions and functions.
Forming the central core of the forebrain and surrounded by
the cerebral hemispheres, the
diencephalon
consists largely of
three paired structures—the thalamus, hypothalamus, and epi-
thalamus. Tese gray matter areas collectively enclose the third
ventricle
(Figure 12.10)
.
Thalamus
Te
thalamus
consists of bilateral egg-shaped nuclei, which form
the superolateral walls of the third ventricle (Figures 12.8 and
12.10). In most people, an
interthalamic adhesion
(
intermedi-
ate mass
) connects the nuclei.
Talamus
is a Greek word mean-
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