Relationship of spontaneous vagal activity to wakefulness and sleep in the cat

Abstract

Permanent bipolar electrodes were implanted in five cats on the cervical vagus nerve, the dura mater overlying frontal and occipital cortices, in the posterior ventromedial thalamus, anterior hypothalamus, ventromedial hypothalamus, lateral geniculate body, dorsal hippocampus, and dorsolateral midbrain reticular formation. Stainless-steel screws in the frontal bone served as eye (EOG) leads. The EMG lead was attached to the skin on the dorsum of the neck. The cats, unanesthetized and unrestrained, were recorded on an 8-channel electroencephalograph from an insulated sound-proof cubicle. The vagus nerve had spontaneous tonic background activity upon which varied phasic activities were superimposed. Long-term recordings demonstrated a parallelism between the spontaneous tonic activity and the normal waking and sleeping pattern of the cat. A low-voltage, fast activity in the waking state, consisting of impulses of approximately equal amplitude and duration, diminished in amplitude and frequency as the animal passed into slow-wave sleep, and finally became atonic, with the disappearance of the tonic background activity, during activated sleep. These observations support the conclusion that a close relationship exists between interoception and ongoing brain activity. The alterations in spontaneous vagal activity provide another factor of distinction between the states of arousal and activated sleep, and may contribute to our understanding of the mechanisms involved in these two states. The findings may provide a possible electrophysiological basis for alterations in visceral activities which have been reported to accompany the different stages of waking and sleep and suggests that a circadian rhythmicity exists in visceral nerve activity.