407
CHAPTER ELEVEN
Nervous System II
system guides behavior that may increase the chance of sur-
vival. In addition, parts of the limbic system interpret sen-
sory impulses from the receptors associated with the sense
of smell (olfactory receptors).
Brainstem
The
brainstem
connects the brain to the spinal cord. It con-
sists of the midbrain, pons, and medulla oblongata. These
structures include many tracts of nerve F
bers and masses of
gray matter called
nuclei
(see
f
gs.
11.15, 11.19, and
11.20
).
Midbrain
The
midbrain
(mesencephalon) is a short section of the
brainstem between the diencephalon and the pons. It con-
tains bundles of myelinated nerve fibers that join lower
parts of the brainstem and spinal cord with higher parts
of the brain. The midbrain includes several masses of gray
matter that serve as refl
ex centers. It also contains the
cere-
bral aqueduct
that connects the third and fourth ventricles
(f
g. 11.21)
.
Two prominent bundles of nerve F
bers on the underside
of the midbrain comprise the
cerebral peduncles.
These F bers
include the corticospinal tracts and are the main motor path-
ways between the cerebrum and lower parts of the nervous
system (see F
g. 11.20). Beneath the cerebral peduncles are
large bundles of sensory F
bers that carry impulses upward
to the thalamus.
Two pairs of rounded knobs on the superior surface
of the midbrain mark the location of four nuclei, known
collectively as
corpora quadrigemina.
The upper masses
(superior colliculi) contain the centers for certain visual
refl
exes, such as those responsible for moving the eyes to
view something as the head turns. The lower ones (inferior
colliculi) contain the auditory refl
ex centers that operate
when it is necessary to move the head to hear sounds more
distinctly (see F
g. 11.20).
Near the center of the midbrain is a mass of gray matter
called the
red nucleus.
This nucleus communicates with the
cerebellum and with centers of the spinal cord, and it pro-
vides refl
exes that maintain posture. It appears red because
it is richly supplied with blood vessels.
Pons
The
pons
appears as a rounded bulge on the underside of
the brainstem where it separates the midbrain from the
medulla oblongata (see fig. 11.20). The dorsal portion of
the pons largely consists of longitudinal nerve F bers, which
relay impulses to and from the medulla oblongata and the
cerebrum. Its ventral portion contains large bundles of trans-
verse nerve F bers, which transmit impulses from the cere-
brum to centers within the cerebellum.
Several nuclei of the pons relay sensory impulses from
peripheral nerves to higher brain centers. Other nuclei func-
tion with centers of the medulla oblongata to maintain the
basic rhythm of breathing.
behind the optic chiasma to which the pituitary gland is
attached; (3) the
posterior pituitary gland,
which hangs
from the fl
oor of the hypothalamus; (4) the
mammillary
(mam
ı˘-lar
e)
bodies,
two rounded structures behind the
infundibulum; and (5) the
pineal gland,
which forms as a
cone-shaped evagination from the roof of the diencephalon
(see chapter 13, p. 511).
The thalamus is a selective gateway for sensory impulses
ascending from other parts of the nervous system to the cere-
bral cortex. It receives all sensory impulses (except those
associated with the sense of smell) and channels them to
appropriate regions of the cortex for interpretation. In addi-
tion, all regions of the cerebral cortex can communicate with
the thalamus by means of descending F
bers.
The thalamus transmits sensory information by synchro-
nizing action potentials. Consider vision. An image on the
retina stimulates the
lateral geniculate nucleus
(LGN) region
of the thalamus, which then sends action potentials to a part
of the visual cortex. Those action potentials are synchro-
nized—fired simultaneously—by the LGN’s neurons only
if the stimuli come from a single object, such as a bar. If
the stimulus is two black dots, the resulting thalamic action
potentials are not synchronized. The synchronicity of action
potentials, therefore, may be a way that the thalamus selects
which stimuli to relay to higher brain structures. Therefore,
the thalamus is not only a messenger but also an editor.
Nerve F bers connect the hypothalamus to the cerebral
cortex, thalamus, and parts of the brainstem so that it can
receive impulses from them and send impulses to them. The
hypothalamus maintains homeostasis by regulating a variety
of visceral activities and by linking the nervous and endo-
crine systems.
The hypothalamus regulates:
1. Heart rate and arterial blood pressure.
2. Body temperature.
3. Water and electrolyte balance.
4. Control of hunger and body weight.
5. Control of movements and glandular secretions of the
stomach and intestines.
6. Production of neurosecretory substances that stimulate
the pituitary gland to release hormones that help
regulate growth, control various glands, and infl
uence
reproductive physiology.
7. Sleep and wakefulness.
Structures in the region of the diencephalon also are
important in controlling emotional responses. Parts of the
cerebral cortex in the medial parts of the frontal and tem-
poral lobes connect with the hypothalamus, thalamus, basal
nuclei, and other deep nuclei. These structures form a com-
plex called the
limbic system.
It controls emotional experi-
ence and expression and can modify the way a person acts,
producing such feelings as fear, anger, pleasure, and sorrow.
The limbic system reacts to potentially life-threatening upsets
in a person’s physical or psychological condition. By causing
pleasant or unpleasant feelings about experiences, the limbic
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