Alterations in somatic afferent transmission through the thalamus by central mechanisms and barbiturates.

A study was undertaken in the cat of the influence of central regulatory mechanisms determining EEG states associated with wakefulness and sleep upon somatic afferent transmission through the thalamus. Findings were compared with the effects of barbiturate compounds to explore the basis of the anesthetic state. Reduction of latenicy, abbreviation of form of response and elimination of facilitatory periods from the recovery cycle characterized thalamic transmission during EEG arousal. Obverse features were associated with EEG synchrony, whether spontaneous or induced by reticular lesions. The findings oppose the concept of the thalamus as a stable relay in the somatic afferent path and the implications of variable input-output relationships for theories of perception are discussed. The effects of barbiturates in light doses resembled changes with EEG synchrony and are attributed to functional block of ascending influences of the brain stem reticular formation, whose susceptibility has earlier been emphasized. In larger doses, increase in latency, reduction of amplitude and prolongation of recovery time of thalamic responses were of such magnitude as to suggest that in greater concentration, barbiturates exert a depressant influence directly upon thalamic relay nuclei.