CORTICIFUGAL INFLUENCES ON INTRINSIC BRAINSTEM CONDUCTION IN CAT AND MONKEY

Abstract

Cortical projections into the brainstem reticular formation in cats and monkeys were examined electrophysiologically to determine their influence upon activity induced along an intrinsic brainstem conduction pathway. This provides a method of assessing the nature and extent of corticifugal control of brainstem mechanisms. Effects of cortical conditioning single shocks or itterative stimuli were tested on conduction in a dorsal ascending brainstem system, using single test shocks in the brainstem pathway at known intervals following the cortical conditioning stimulus. Sequences of facilitatory and inhibitory interaction with the brainstem system result from such cortical excitation. Individual cortical loci display different sequences and temporal patterns of interaction. The cortical zones found capable of modifying brainstem conduction are similar in distribution to those known to have major projections to the brainstem, and include the lateral sensorimotor cortex, superior temporal gyrus, posterior orbital cortex, medial frontal cortex, and basal occipitotemporal cortex, including the entorhinal area. This latter area elicited the most profound and lasting influences which were predominantly inhibitory. The observed convergence of corticifugal influences upon the same ascending brainstem system provides an example of transactional as contrasted with interactional systems.