Paradoxical Sleep

In searching for the structures involved in paradoxical, or deep, sleep we are in a somewhat better position. When an animal is in that state, we have as clues to guide us not only the electrical activities in the brain but also conclusive and readily observable signs such as the disappearance of tonus in the muscles of the neck. This is the single most reliable mark of paradoxical sleep. Furthermore, it enables us to study animals that have been subjected to drastic operations we cannot use in the study of light sleep because they obliterate the electrical activities that identify the falling-asleep stage.

A cat whose brainstem has been cut through at the level of the pons, so that essentially all the upper part of the brain has been removed, still exhibits the cycle of waking and deep sleep. Such an animal can be kept alive for several months, and with the regularity of a biological clock it oscillates between wakefulness and the state of paradoxical sleep, in which it spends only about 10 percent of the time. This state is signaled, as in normal animals, by the typical slackness of the neck muscles, by the electroencephalographic spikes denoting electrical activity in the pons structures and by lateral movements of the eyeballs.

When, however, we sever the brain stem at a lower level, in the lower part of the pons just ahead of the medulla, the animal no longer falls into paradoxical sleep. The sign that marks this cyc]ical state-periodic loss of muscle tonus-disappears. It seems, therefore, that the onset of paradoxical sleep must be triggered by the action of structures somewhere in the middle portion of the pons. Further experiments have made it possible for us to locate these structures rather precisely. We have found that paradoxical sleep can be abolished by destroying certain nerve cells in a dorsal area of the pons known as the locus coeruleus. Dahlstrom and Fuxe have shown that these cells have a green fluorescence under ultraviolet light and that they contain noradrenalin. Hence it seems that noradrenalin may play a role in producing paradoxical sleep similar to the one serotonin apparently plays in bringing about light sleep.

What mechanism is responsible for the elimination of muscular tonus that accompanies paradoxical sleep ? It seems most likely that the source of this inhibition lies in the spinal cord, and Moruzzi and his colleague Ottavio Pompeiano are making a detailed investigation of this hypothesis.

The objective information about paradoxical sleep developed so far gives us some suggestions about the mechanisms involved in dreaming. The controlling structures apparently are located in the dorsal part of the pons. They give rise to spontaneous excitations that travel mainly to the brain's visual tracts, and it seems possible that this excitation is related to the formation of the images that one "sees" in dreams. Regardless of how strongly the brain is stimulated by these spontaneous impulses (as Edward V. Evarts of the National Institute of Mental Health and others have shown by means of microelectrode recordings of the visual system), during sleep the body's motor system remains inactive because a potent braking mechanism blocks electrical excitation of the motor nerves. This inhibitory mechanism seems to be controlled by the hormone-secreting nerves of the locus coeruleus structure. If this structure is destroyed, the animal may periodically exhibit a spasm of active behavior, which looks very much as if it is generated by the hallucinations of a dream. In such episodes the cat, although it evinces the unmistakable signs of deep sleep and does not respond to external stimuli, will sometimes perform bodily movements of rage, fear or pursuit for a minute or two. The sleeping animal's behavior may even be so fierce as to make the experimenter recoil.

All in all the experimental evidence from mammals obliges us to conclude that sleep has a fundamental duality; deep sleep is distinctly different from light sleep, and the duality is founded on physiological mechanisms and probably on biochemical ones as well. Can we shed further light on the subject by examining animal evolution ?

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