ORGANIZATION OF THE DIFFUSE THALAMIC PROJECTION SYSTEM

Abstract

IN 1942, Dempsey and Morison (1-3, 9) discovered a thalamic system, low ' pathways.Since then, other proposals concerning the functional significance of this diffuse thalamic projection system have been made, the most dramatic being Jasper's hypothesis that it is the subcortical pacemaker in petit mal epilepsy (4,5,6).Recent demonstration that EEG activation by the brain stem may be mediated, in part at least, by the diffuse thalamic projection system (8, lo), has motivated the present attempt to learn more of its organization.The ' centre median and intralaminar nuclei were found by Dempsey and Morison .(9) to comprise the thalamic components of this system.In the absence of demonstrable connections from them to the cortex, their influence upon cortical electrical activity has since been proposed to be exerted indirectly,, .through the reticular thalamic nuclei (6, 11,12) or through the rhinenceph-. alon (12).In the present study, information concerning the origin of thisdiffuse system has been supplied by localization of thalamic areas which ' evoke recruiting responses upon stimulation, while determination of forec brain regions from which such responses can be recorded has provided data concerning its projections. METHODSCats were employed and the recruiting response, elicited by thalamic stimulation was ' recorded with a Grass model-3 amplifier and inkwriter.The animals were under nem-.butal, Dial or chloraloaane anesthesia or, after preparation under ether and local procaine, were immobilized with beta-erythroidine or by transection of the cord at C1 and maintained with artificial respiration.The thalamus was stimulated with stereotaxically oriented bipolar concentric electrodes.A Goodwin stimulator was employed, the condensor discharges of which had voltages between 1 and 7, a falling phase of 1 rnsec.and a usual frequency of ' 7.5/sec.Grounding both temporal muscles effectively reduced shock artifacts.Regional cortical pickups were between screw electrodes, 1 cm.apart, inserted into the calvarium until their tips rested on the underlying dura.More detailed pickup were obtained with silver ball tips, applied to the exposed cortex with the aid of a Graa~~ multiple electrode holder.The most precise cortical and subcortical records were gained with bipolar '