Maintained and evoked unit activity in the mesencephalic reticular formation of the freely behaving cat

Abstract

Maintained and evoked unit activity in the mesencephalic reticular formation (MRF) was studied in behaving cats. Three behavioral states were distinguished by electrical and behavioral criteria: resting arousal, light sleep, and deep sleep. Most units showed a relatively regular firing pattern. During the sleep-wakefulness cycle MRF units continued to fire steadily with decreasing discharge rates from arousal to light sleep and at increased rates from light to deep sleep over those in resting arousal. Phasic enhancement of unit discharge in deep sleep usually occurred in association with bursts of phasic waves in the visual system. Reticular units decreased discharge rates significantly during attention to visual environmental stimuli in comparison with those in arousal. Reticular formation neurons were confirmed to be the site of input convergence. Responses from stimulation of different cortical sites showed different modes of latency distribution. A tendency was noted in light sleep for MRF responses to occur with longer latency than in arousal and deep sleep. Gradation of unitary responsiveness in the three states was established as light sleep→arousal→deep sleep in order of increasing sensitivity. No clear-cut correlation was noted between the location of units in the MRF and their maintained activity level, but units with high extent of input convergence were found mostly in the ventral MRF. A minority of MRF units showed extremely low-discharge rate throughout the three states. Most, though not all, fired actively with a burst of spikes only when proper stimuli were given to somatic receptive fields.