Human Physiology, 14th edition (2016)

230 Chapter 8

trigeminal (V), abducens (VI), facial (VII), and vestibu-

locochlear (VIII). Other nuclei of the pons cooperate with

nuclei in the medulla oblongata to regulate breathing. The

two respiratory control centers in the pons are known as

the apneustic and the pneumotaxic centers. Damage to the

ventral pons can produce a rare condition called locked-in

syndrome, characterized by paralysis of almost all voluntary

muscles so that communication by the aware, awake person

is possible only by eye blinks.

The cerebellum, containing about 50 billion neurons, is

the second largest structure of the brain. Like the cerebrum, it

contains outer gray and inner white matter. Fibers from the cer-

ebellum pass through the red nucleus to the thalamus, and then

to the motor areas of the cerebral cortex. Other fiber tracts con-

nect the cerebellum with the pons, medulla oblongata, and spi-

nal cord. The cerebellum receives input from proprioceptors

(joint, tendon, and muscle receptors) and, working together

with the basal nuclei and motor areas of the cerebral cortex,

participates in the coordination of movement.

The cerebellum is needed for motor learning and for coor-

dinating the movement of different joints during a movement.

It is also required for the proper timing and force required for

limb movements. The cerebellum, for example, is needed to

touch your nose with your finger, bring a fork of food to your

mouth, or find keys by touch in your pocket or purse.

Interestingly, these functions must operate through spe-

cific cerebellar neurons known as Purkinje cells, which pro-

vide the only output from the cerebellum to other brain regions.

Further, Purkinje cells produce only inhibitory effects on the

motor areas of the cerebral cortex. Acting through this inhibi-

tion, the cerebellum aids in the coordination of complex motor

skills and participates in motor learning.

Current research suggests that the cerebellum may have

varied and subtle functions beyond motor coordination. Dif-

ferent investigations have implicated the cerebellum in the

acquisition of sensory data, memory, emotion, and other

higher functions. The cerebellum may also have roles in

schizophrenia and autism. How these possible cerebellar

functions are achieved and how they relate to the control of

the glutamate-releasing axons that project from the prefrontal

cortex to the nucleus accumbens and other structures of the

limbic system to exert control over the drug-seeking behavior.

Hindbrain

The rhombencephalon, or hindbrain, is composed of two

regions: the metencephalon and the myelencephalon. These

regions will be discussed separately.

Metencephalon

The metencephalon is composed of the pons and the cerebel-

lum. The pons can be seen as a rounded bulge on the under-

side of the brain, between the midbrain and the medulla

oblongata ( fig.  8.22 ). Surface fibers in the pons connect to

the cerebellum, and deeper fibers are part of motor and sen-

sory tracts that pass from the medulla oblongata, through

the pons, and on to the midbrain. Within the pons are sev-

eral nuclei associated with specific cranial nerves—the

Figure 8.22 Respiratory control centers in the brain
stem. These are nuclei within the pons and medulla oblongata
that control the motor nerves required for breathing. The location
of the reticular formation is also shown.

Medulla oblongata

Reticular formation

Apneustic area

Pneumotaxic area

Midbrain

Brain stem
respiratory
centers

Pons

Rhythmicity area

CLINICAL APPLICATION

Cerebellum disorders are usually characterized by ataxia —

the loss of coordination, resulting in disorders of gait, bal-

ance, eye movements, and swallowing. The movements may

resemble those of someone intoxicated with alcohol, and

indeed alcohol has been shown to affect the cerebellum. A

person with cerebellum damage may display intention tremor

when attempting to touch an object, where the limb misses

the object and then moves in the opposite direction and back

again, resulting in oscillation of the limb. Cerebellum disor-

ders may have a variety of causes, including trauma, stroke,

transient ischemic attack (TIA), and viral infections. If caused

by a virus, the symptoms will usually resolve spontaneously.