Human Physiology, 14th edition (2016)

The Central Nervous System 231

the normal cycles of sleep and wakefulness that result, depend

upon the activation and inhibition of neural pathways that go

from the pons through the midbrain reticular formation, an

interconnected group of neurons (from the Latin rete 5 net).

This constitutes an ascending arousal system known as the

reticular activating system (RAS).

The RAS includes groups of cholinergic neurons (neurons

that release ACh) in the brain stem that project to the thalamus;

these neurons enhance the transmission of sensory informa-

tion from the thalamus to the cerebral cortex. Other groups of

RAS neurons located in the hypothalamus and basal forebrain

release monoamine neurotransmitters (dopamine, norepineph-

rine, histamine, and serotonin) and project to various locations

in the cerebral cortex ( fig. 8.23 ).

These arousal neural pathways of the RAS are inhibited

by another group of neurons located in the ventrolateral pre-

optic nucleus (VLPO) of the hypothalamus, which release the

inhibitory neurotransmitter GABA. The activity of VLPO and

other GABA-releasing neurons is increased with the depth of

sleep, and these neurons are believed to both cause and stabi-

lize sleep. The inhibitory neurons of the VLPO and the arousal

neurons that release monoamine neurotransmitters are believed

to mutually inhibit each other, creating a switch that controls

falling asleep and waking up.

There are other neurons of the RAS, located in the

lateral hypothalamic area ( LHA ), that release polypeptides as

motor coordination are presently incompletely understood
and controversial.

Myelencephalon

The myelencephalon is composed of only one structure, the
medulla oblongata, often simply called the medulla. About
3 cm (1 in.) long, the medulla is continuous with the pons
superiorly and the spinal cord inferiorly. All of the descend-
ing and ascending fiber tracts that provide communication
between the spinal cord and the brain must pass through the
medulla. Many of these fiber tracts cross to the contralateral
side in elevated triangular structures in the medulla called the
pyramids. Thus, the left side of the brain receives sensory
information from the right side of the body and vice versa.
Similarly, because of the decussation of fibers, the right side
of the brain controls motor activity in the left side of the body
and vice versa.
Many important nuclei are contained within the medulla.
Several nuclei are involved in motor control, giving rise to
axons within cranial nerves VIII, IX, X, XI, and XII. The vagus
nuclei (there is one on each lateral side of the medulla), for
example, give rise to the highly important vagus (X) nerves.
Other nuclei relay sensory information to the thalamus and
then to the cerebral cortex.
The medulla contains groupings of neurons required for
the regulation of breathing and of cardiovascular responses;
hence, they are known as the vital centers. The vasomotor
center controls the autonomic innervation of blood vessels; the
cardiac control center, closely associated with the vasomotor
center, regulates the autonomic nerve control of the heart; and
the respiratory center of the medulla acts together with cen-
ters in the pons to control breathing.

Reticular Activating System

in Sleep and Arousal

In order to fall asleep, we must be able to “tune out” sensory
stimulation that ascends to the cerebral cortex. Conversely, we
awake rather quickly from sleep when the cerebral cortex is
alerted to incoming sensory information. These abilities, and

Clinical Investigation CLUES

Kevin had coordination problems and swayed when he

walked, but denied that he drank alcohol. His arm oscil-

lated when he tried to point.

• Damage to what brain region might account for
  these symptoms?


• What medical term is used to describe these
  symptoms?

Figure 8.23 The reticular activating system

(RAS). The groups of neurons shown in orange project to the

thalamus, where they enhance the arousal of the cerebral cortex

to sensory information relayed from the thalamus. The groups of

neurons shown in red project to various locations in the cerebral

cortex and more directly arouse the cerebral cortex to ascending

sensory information. Activity of the RAS promotes wakefulness,

and inhibition of the RAS promotes sleep.

Cerebellum

Brainstem

Thalamus

Medulla

Pons

Hypothalamus