408
UNIT THREE
Medulla Oblongata
The
medulla oblongata
(me
˘-dul
ah ob
long-ga
tah) is an
enlarged continuation of the spinal cord, extending from the
level of the foramen magnum to the pons (see F g. 11.20).
Its dorsal surface fl attens to form the fl oor of the fourth ven-
tricle, and its ventral surface is marked by the corticospinal
tracts, most of whose F
bers cross over at this level. On each
side of the medulla oblongata is an oval swelling called the
olive,
from which a large bundle of nerve F bers arises and
passes to the cerebellum.
The ascending and descending nerve F
bers connecting
the brain and spinal cord must pass through the medulla
oblongata because of its location. As in the spinal cord, the
white matter of the medulla surrounds a central mass of gray
matter. Here, however, the gray matter breaks up into nuclei
separated by nerve F bers. Some of these nuclei relay ascend-
ing impulses to the other side of the brainstem and then on
to higher brain centers. The
nucleus gracilis
and the
nucleus
cuneatus,
for example, receive sensory impulses from F bers
of the fasciculus gracilis and the fasciculus cuneatus and
pass them on to the thalamus or the cerebellum.
Other nuclei in the medulla oblongata control vital vis-
ceral activities. These nuclei include the following:
1.
Cardiac center.
Peripheral nerves transmit impulses
originating in the cardiac center to the heart, where they
increase or decrease heart rate.
Pyramidal tract
Pons
Optic nerve
Optic chiasma
Thalamus
Third
ventricle
Fourth
ventricle
Spinal cord
Pituitary gland
Pineal gland
Cerebellar
peduncles
Medulla
oblongata
Optic tract
Mammillary body
(a)
(b)
Olive
Cerebral
peduncles
Superior
colliculus
Inferior
colliculus
Corpora quadrigemina
Reticular
formation
Spinal cord
Cerebral
aqueduct
Corpora
quadrigemina
Corpus
callosum
Thalamus
Hypothalamus
Diencephalon
Medulla
oblongata
Pons
Midbrain
FIGURE 11.21
The reticular formation (shown in gold) extends
from the superior portion of the spinal cord into the diencephalon.
FIGURE 11.20
±Brainstem.±(
a
) Ventral view of the brainstem. (
b
) Dorsal view of the brainstem with the cerebellum removed, exposing the fourth
ventricle.
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