The Central Nervous System
Other important pontine nuclei are part of the reticular for-
mation and some help the medulla oblongata maintain the nor-
mal rhythm of breathing.
, 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
, 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
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
are ﬁber tracts that connect the medulla
to the cerebellum dorsally. Situated lateral to the pyramids,
Facial nerve (VII)
(c) Dorsal view
Dorsal root of
first cervical nerve
Trochlear nerve (IV)
Superior cerebellar peduncle
Inferior cerebellar peduncle
Vestibulocochlear nerve (VIII)
Glossopharyngeal nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Middle cerebellar peduncle
Dorsal median sulcus
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
a system of small nuclei scattered through the core of the brain
stem (see pp. 450–451).
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
As its name suggests (
bridge), the pons is chieﬂy
composed of conduction tracts. Tey are oriented in two dif-
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
connect the pons bilaterally with the two sides of the cerebel-
lum dorsally (Figure 12.13). Tese ﬁbers issue from numer-
, which relay “conversations” between the
motor cortex and cerebellum.
Several cranial nerve pairs issue from pontine nuclei. Tey
(Figures 12.13a, b and 12.14b). We discuss the
cranial nerves and their functions in Chapter 13.