UNIT ACTIVITY IN RETICULAR FORMATION AND NEARBY STRUCTURES

Abstract

A single-unit analysis of the "reticular activating system" confirmed and extended some of the conclusions by Magoun and his coworkers. Single units were also recorded at other sites. Electrical or physiological stimulation was used. Some reticular units are fired by an afferent volley from any paw and by an auditory stimulus, but the latency, number of spikes and interspike intervals differ with the afferent source. Other units have a more restricted receptive field. A reticular unit may be fired by cutaneous and visceral afferent volleys or by somatic afferent volleys and corticofugal discharges. Visual and somatic afferent stimuli may fire a superior collicular unit. Increasing the strength of peripheral or cortical stimuli above the threshold for unit firing usually reduces the average latency for firing and increases the number of spikes. Most units were fired by the peripheral A alpha-beta groups. A few units fired consistently only after the addition of the A delta group. Both fast and slow afferent fibers may fire the same unit. The patterns of firing evoked include: single-discharge, high-frequency bursts, low-frequency sustained discharge and a characteristic sequence of early discharge-silent period-delayed discharge. The "delayed discharge" was analyzed with condition-ing-testing stimulation. The discharge latency differed among units, but usually lay between 12 and 80 msec. following peripheral stimulation and between 2 and 15 msec. following cortical stimulation. Rarely, the sensorimotor cortex activated the mid-brain reticular formation with a synaptic delay of less than 1 msec. Occlusion, inhibition and facilitation in reticular units were shown by manipulation of stimulus strengths and test intervals, and by choice of afferent sources. Repeated high-frequency bursts of spontaneous activity and " recruitment" following peripheral stimulation at 2-5/sec. are more commonly observed in the medial and lateroposterior thalamic nuclei than at the pontine-midbrain level where spontaneous activity is more often sustained (tonic). How the reticular net is activated is deduced from the discharge latencies of reticular units and from the difference in behavior of neighboring reticular units.