Coma and Brain Death

The term coma describes a severe decrease in mental function due to structural, physiological, or metabolic impairment of the brain. A person in a coma is characterized by a sustained loss of the capacity for arousal even in response to vigorous stimulation. There is no outward behavioral expression of any mental function, the eyes are closed, and sleep-wake cycles disappear. Coma can result from extensive damage to the cerebral cortex; damage to the brainstem arousal mechanisms; interruptions of the connections between the brainstem and cortical areas; metabolic dysfunctions; brain infections; or an overdose of certain drugs, such as sedatives, sleeping pills, and, in some cases, narcotics.

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

TABLE 13-1 Sleep-Wakefulness Stages

Stage

Behavior

EEG (see Figures 13-3 and 13-4)

Alert wakefulness

Awake, alert with eyes open.

Beta rhythm (faster than 13 Hz).

Relaxed wakefulness

Awake, relaxed with eyes closed.

Mainly alpha rhythm (8-13 Hz) over the parietal and occipital lobes. Changes to beta rhythm in response to internal or external stimuli.

Relaxed drowsiness

Fatigued, tired, or bored; eyelids may narrow and close; head may start to droop; momentary lapses of attention and alertness. Sleepy but not asleep.

Decrease in alpha-wave amplitude and frequency.

NREM (slow-wave) sleep

Stage 1

Stage 2

Stages 3 and 4

Light sleep; easily aroused by moderate stimuli or even by neck muscle jerks triggered by muscle stretch receptors as head nods; continuous lack of awareness.

Further lack of sensitivity to activation and arousal.

Deep sleep; in stage 4, activation and arousal occur only with vigorous stimulation.

Alpha waves reduced in frequency, amplitude, and percentage of time present; gaps in alpha rhythm filled with theta (4-8 Hz) and delta (slower than 4 Hz) activity.

Alpha waves replaced by random waves of greater amplitude.

Much theta and delta activity, predominant delta in stage 4.

REM (paradoxical) sleep

Deepest sleep; greatest relaxation and difficulty of arousal; begins 50-90 min after sleep onset, episodes are repeated every 60-90 min, each episode lasting about 10 min; dreaming occurs, rapid eye movements behind closed eyelids; marked increase in brain O2 consumption.

EEG resembles that of alert awake state.

Suprachiasmatic nucleus

Suprachiasmatic nucleus

Suprachiasmatic Nucleus

Thalamus

Posterior hypothalamus

Brainstem nuclei of the reticular activating system

Thalamus

Posterior hypothalamus

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