Chapter 12
The Central Nervous System
445
12
Other important pontine nuclei are part of the reticular for-
mation and some help the medulla oblongata maintain the nor-
mal rhythm of breathing.
Medulla Oblongata
Te conical
medulla oblongata
(mě-dul
9
ah ob
0
long-gah
9
tah),
or simply
medulla
, is the most inferior part of the brain stem.
It blends imperceptibly into the spinal cord at the level of the
foramen magnum of the skull (Figures 12.10 and 12.12; see
Figure 7.6, p. 206).
Te central canal of the spinal cord continues upward into
the medulla, where it broadens out to form the cavity of the
fourth ventricle. ±ogether, the medulla and the pons form the
ventral wall of the fourth ventricle. [Te dorsal ventricular wall
is formed by a thin capillary-rich membrane called a choroid
plexus which abuts the cerebellum dorsally (Figure 12.10).]
Structures of the Medulla Oblongata
Flanking the midline
on the medulla’s ventral aspect are two longitudinal ridges called
pyramids
, formed by the large pyramidal (corticospinal) tracts
descending from the motor cortex (Figure 12.14c). Just above
the medulla–spinal cord junction, most of these fibers cross
over to the opposite side before continuing into the spinal cord.
Tis crossover point is called the
decussation of the pyramids
(de
0
kus-sa
9
shun; “a crossing”). As a result of this crossover, each
cerebral hemisphere chiefly controls the voluntary movements
of muscles on the opposite side of the body.
Several other structures are visible externally. Te
inferior
cerebellar peduncles
are fiber tracts that connect the medulla
to the cerebellum dorsally. Situated lateral to the pyramids,
Pineal gland
Diencephalon
Floor of
fourth ventricle
Facial nerve (VII)
(c) Dorsal view
Thalamus
Dorsal root of
first cervical nerve
Midbrain
Superior
colliculus
Inferior
colliculus
Trochlear nerve (IV)
Superior cerebellar peduncle
Corpora
quadrigemina
of tectum
Medulla oblongata
Inferior cerebellar peduncle
Vestibulocochlear nerve (VIII)
Glossopharyngeal nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Pons
Middle cerebellar peduncle
Dorsal median sulcus
Choroid plexus
(fourth ventricle)
Figure 12.13
(continued)
Te oval
red nucleus
lies deep to the substantia nigra (Fig-
ure 12.14a). Its reddish hue is due to its rich blood supply and
to the presence of iron pigment in its neurons. Te red nuclei
are relay nuclei in some descending motor pathways that effect
limb flexion, and they are embedded in the
reticular formation
,
a system of small nuclei scattered through the core of the brain
stem (see pp. 450–451).
Pons
Te
pons
is the bulging brain stem region wedged between the
midbrain and the medulla oblongata (see Figures 12.10, 12.12,
and 12.13). Dorsally, the fourth ventricle separates it from the
cerebellum.
As its name suggests (
pons
5
bridge), the pons is chiefly
composed of conduction tracts. Tey are oriented in two dif-
ferent directions:
Te deep projection fibers run longitudinally as part of the
pathway between higher brain centers and the spinal cord.
Te more superficial ventral fibers are oriented transversely
and dorsally. Tey form the
middle cerebellar peduncles
and
connect the pons bilaterally with the two sides of the cerebel-
lum dorsally (Figure 12.13). Tese fibers issue from numer-
ous
pontine nuclei
, which relay “conversations” between the
motor cortex and cerebellum.
Several cranial nerve pairs issue from pontine nuclei. Tey
include the
trigeminal
(tri-jem
9
ĭ-nal),
abducens
(ab-du
9
senz),
and
facial nerves
(Figures 12.13a, b and 12.14b). We discuss the
cranial nerves and their functions in Chapter 13.
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