HUMAN BIOLOGY

256 Chapter 13

What is consciousness?

• Consciousness is a spectrum of mental activity that includes
  state of arousal and sleep.


• Signals from the reticular activitating system (RAS) of the
  reticular formation in the brain stem arouse and activate the
  cerebral cortex.

taKe-hoMe Message

reticular formation A net-
work of brain stem neurons
that processes sensory
information and sends
related signals to other
parts of the CNS.

Consciousness

scalp during an EEG (electroencephalogram). The patterns

show up as tracings like those in Figure 13.21B.

Most of the time you spend sleeping is “slow-wave

sleep.” During this stage, your heart rate, breathing, and

muscle tone change very little and you can be easily roused.

Approximately every 90 minutes, however, a sleeper nor-

mally enters a period of REM (rapid-eye-movement) sleep,

in which the eyelids flicker and the eyeballs move rapidly

back and forth. Sleepers dream during REM sleep, and it is

much harder to wake up during this time. Most research

subjects awakened from REM sleep report they were expe-

riencing vivid dreams.

You may know from personal experience that a sleep-

deprived person tends to feel cranky and have diffi-

culty concentrating. Sleep is important for the brain, but

researchers don’t know exactly why. Although neural

activity changes during sleep, the brain clearly is not rest-

ing. Sleep may be a time when the brain does tasks such

as consolidating memories and firming up connections

involved in learning.

n    Consciousness is a spectrum of brain states such as

alertness and stages of sleep.

The spectrum of consciousness ranges from being wide

awake and fully alert to drowsiness, sleep, and coma.

All states of consciousness depend on the reticular

formation, a two-part network of interconnected neurons

that runs through the brain stem (Figure 13.21A). It receives

and processes incoming sensory information, then sends

signals to other parts of the CNS. One part of the formation,

called the RAS (for reticular activat-

ing system) sends signals upward

to the thalamus that stimulate it

to arouse and activate the cerebral

cortex. Depending on how much

the cortex is stimulated, it responds

in ways that determine the level of

consciousness, including sleeping and waking. The central

role of the RAS in this brain function is why brain stem

damage often results in coma.

The second part of the reticular formation receives

signals from the hypothalamus and relays them to spinal

cord neurons. These signals govern skeletal muscle activ-

ity that helps maintain balance, posture, and muscle tone.

The reticular system also filters incoming signals, helping

the brain distinguish between important and unimportant

ones. It is this filtering that enables you to sleep through

many sounds but to waken to specific ones, such as a cat

meowing to be let out or a baby crying.

The patterns for full alertness and other states of con-

sciousness can be detected by electrodes placed on the

Figure 13.21 Several states of consciousness occur in the brain. a Location of the reticular formation in the brain stem.

B EEG patterns for various stages of consciousness. (© Cengage Learning)

Cerebellum

Midbrain

Pons

Medulla

Reticular

formation^012345

Time (sec)

Awake

(beta waves)

Eyes closed, relaxed

(alpha waves)

Dozing

(theta waves)

Deep sleep

(delta waves)

a B

13.9

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