Cerebellum
The cerebellum is the second largest structure of the brain. It is located in the metencephalon and occupies the inferior
and posterior aspect of the cranial cavity. The cerebellum is sepa
rated from the overlying cerebrum by a transverse fissure . A portion of the meninges called the tentorium cerebelli
extends into the transverse fissure. The cerebellum consists of two hemispheres and a central constricted area called
the vermis. The falx cerebelli is the portion of the meninges that partially extends between the hemispheres.
Like the cerebrum, the cerebellum has a thin, outer layer of gray matter, called the cerebellar cortex, and a thick,
deeper layer of white matter. The cerebellum is convoluted into a series of slender, parallel gyri. The tracts of white
matter within the cerebellum have a distinctive branching pattern called the arbor vitae, which can be seen in a sagittal
view
Three paired bundles of nerve fibers, called cerebellar peduncles, support the cerebellum and provide it with tracts
for communicating with the rest of the brain . The cerebellar peduncles are as follows:
1. Superior cerebellar peduncles connect the cerebellum with the midbrain. The fibers within these peduncles
originate primarily from specialized dentate nuclei within the cerebellum and pass through the red nucleus to the
thalamus and then to the motor areas of the cerebral cortex. Impulses through the fibers of these peduncles provide
feedback to the cerebrum.
2. Middle cerebellar peduncles convey impulses of voluntary movement from the cerebrum through the pons and to
the cerebellum.
3. Inferior cerebellar peduncles connect the cerebellum with the medulla oblongata and the spinal cord. They
contain both incoming vestibular and proprioceptive fibers and outgoing motor fibers.
The principal function of the cerebellum is coordinating
skeletal muscle contractions by recruiting precise motor units within the muscles. Impulses for voluntary muscular
movement originate in the cerebral cortex and are coordinated by the cerebellum. The cerebellum constantly initiates
impulses to selective motor units for maintaining posture and muscle tone. The cerebellum also adjusts to incoming
impulses from proprioceptors within muscles, tendons,joints, and special sense organs. A proprioceptor is a sensory
nerve ending that is sensitive to changes in the tension of a muscle or tendon.
Pons
The pons can be observed as a rounded bulge on the underside of the brain, between the midbrain and the medulla
oblongata . The pons consists of white fiber tracts that course in two principal directions. The surface fibers extend trans-
versely to connect with the cerebellum through the middle cerebellar peduncles. The deeper longitudinal fibers are part
of the motor and sensory tracts that connect the medulla with the tracts of the midbrain.
Scattered throughout the pons are several nuclei associated with specific cranial nerves. The cranial nerves that have
nuclei within the pons include the trigeminal (V), which transmits impulses for chewing and sensory sensations from the
head; the abducens (VI), which controls certain movements of the eyeball; the facial (VII), which transmits impulses for
facial movements and sensory sensations from the taste buds; and the vestibular branches of the vestibulocochlear (VIII),
which maintain equilibrium.
Other nuclei of the pons function with nuclei of the medulla oblongata to regulate the rate and depth of breathing.
The two respiratory centers of the pons are called the apneustic and the pneumotaxic areas
Trauma or diseases of the cerebellum, such as a stroke or cerebral palsy, frequently cause an impairment of skeletal muscle
function. Movements become jerky and uncoordinated in a condition known as ataxia. There is also a loss of equilibrium, resulting in
a disturbance of gait. Alcohol intoxication causes similar uncoordinated body movements.
Myelencephalon
The medulla oblongata, contained within the myelencephalon, connects to the spinal cord and contains nuclei for the
cranial nerves and vital autonomic functions.
Medulla Oblongata
The medulla oblongata, or simply medulla, is a bulbous structure about 3 cm (1 in.) long that is continuous with the
pons anteriorly and the spinal cord posteriorly at the level of the foramen magnum. Externally, the medulla resembles
the spinal cord, except for the two triangular, elevated structures, called pyramids, on the ventral side and an oval
enlargement, called the olive, on each lateral surface. The fourth ventricle, the space within
the medulla, is continuous posteriorly with the central canal of the spinal cord and anteriorly with the cerebral aqueduct.
The medulla is composed of vital nuclei and white matter that form all of the descending and ascending tracts
communicating between the spinal cord and various parts of the brain.
A sagittal section of the medulla oblongata and pons showing the cranial nerve nuclei of gray matter.
Most of the fibers within these tracts cross over to the opposite side through the pyramidal region of the medulla,
permitting one side of the brain to receive information from and send information to the opposite side of the body
The gray matter of the medulla consists of several important nuclei for the cranial nerves, sensory relay, and for
