Chapter 12
The Central Nervous System
443
12
is implicated in
failure to thrive
, a condition characterized by
delay in growth or development that occurs when a child is de-
prived of a warm, nurturing relationship.
Epithalamus
Te most dorsal portion of the diencephalon, the
epithala-
mus
forms the roof of the third ventricle. Extending from its
posterior border and visible externally is the
pineal gland
or
body
(pin
9
e-al; “pine cone shaped”) (see Figures 12.10 and
12.13c). Te pineal gland secretes the hormone
melatonin
(a
sleep-inducing signal and antioxidant; see Chapter 16) and,
along with hypothalamic nuclei, helps regulate the sleep-wake
cycle. Te
posterior commissure
forms the caudal border of the
epithalamus.
Check Your Understanding
8.
Why is the thalamus called the “gateway to the cerebral cortex”?
9.
The hypothalamus oversees a branch of the peripheral
nervous system. Which branch?
For answers, see Appendix H.
Brain Stem
Identify the three major regions of the brain stem, and
note the functions of each area.
From superior to inferior, the brain stem regions are mid-
brain, pons, and medulla oblongata (Figures 12.10, 12.12,
and 12.13). Each roughly an inch long, collectively they ac-
count for only 2.5% of total brain mass. Histologically, the
organization of the brain stem is similar (but not identical)
to that of the spinal cord—deep gray matter surrounded by
white matter fiber tracts. However, the brain stem has nuclei
of gray matter embedded in the white matter, a feature not
found in the spinal cord.
Brain stem centers produce the rigidly programmed, auto-
matic behaviors necessary for survival. Positioned between the
cerebrum and the spinal cord, the brain stem also provides a
pathway for fiber tracts running between higher and lower neu-
ral centers. Additionally, brain stem nuclei are associated with
10 of the 12 pairs of cranial nerves (which we will describe in
Chapter 13), so it is heavily involved with innervating the head.
Midbrain
Te
midbrain
is located between the diencephalon and pons
(
Figure 12.12
and
Figure 12.13
). On its ventral aspect two
bulging
cerebral peduncles
(pĕ-dung
9
klz) form vertical
pillars that seem to hold up the cerebrum, hence their name
meaning “little feet of the cerebrum” (Figure 12.13a, b, and
Figure 12.14a
). Te
crus cerebri
(“leg of the cerebrum”) of
each peduncle contains a large pyramidal (corticospinal)
motor tract descending toward the spinal cord. Te
superior
Frontal lobe
Olfactory bulb
(synapse point of
cranial nerve I)
Optic chiasma
Optic nerve (II)
Optic tract
Mammillary body
Pons
Medulla
oblongata
Cerebellum
Temporal
lobe
Spinal cord
Midbrain
Figure 12.12
Inferior view of the brain,
showing the three parts of the brain stem:
midbrain, pons, and medulla oblongata.
Only
a small portion of the midbrain is visible in this
view. (For a related image, see
A Brief Atlas of the
Human Body
, Figure 49.)
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