Neural Substrates of States of Consciousness

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Periods of sleep and wakefulness alternate about once a day; that is, they manifest a circadian rhythm consisting typically of 8 h sleep and 16 h awake. Within the sleep portion of this circadian cycle, NREM sleep and REM sleep alternate, as we have seen.

What physiological processes drive these cyclic changes in states of consciousness? Nuclei in both the brainstem and hypothalamus are involved.

Recall from Chapter 8 that neurons of certain brain-stem nuclei give rise to axons that diverge to affect wide areas of the brain in a highly specific manner, forming a fiber system known as the reticular activating system. This system is, in fact, composed of several separate divisions, distinguished by their anatomical distribution and neurotransmitters. The divisions originate in different nuclei within the brainstem, and some of them send fibers to those areas of the thalamus that influence the EEG. Components of the reticular activating system that release norepinephrine, serotonin, or acetyl-choline—functioning in this instance more as neuro-modulators—are most involved in controlling the various states of consciousness.

One model (Figure 13-6) suggests that the waking state and REM sleep are at opposite ends of a spectrum: During waking, the aminergic neurons (those that release norepinephrine or serotonin) have the major influence, and during REM sleep the cholinergic neurons are dominant. NREM sleep, according to this model, is intermediate between the two extremes.

The aminergic neurons, which are active during the waking state, facilitate both the direction of attention to perceptions of the outer world and the enhanced motor activity that characterize awake behavior. They also inhibit certain of the cholinergic brainstem neurons. As the aminergic neurons stop firing, the cholinergic neurons, released from inhibition, increase their activity.

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

II. Biological Control Systems

13. Consciousness and Behavior

© The McGraw-Hill Companies, 2001

Consciousness and Behavior CHAPTER THIRTEEN

Consciousness and Behavior CHAPTER THIRTEEN

t Norepinephrine and serotonin

i Acetylcholine

Brainstem nuclei that are part of the reticular activating system i Norepinephrine and serotonin

t Acetylcholine c

Waking

NREM sleep

- REM sleep

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