Changes in pyramidal tract conduction with experimental brain-stem ischaemia in the monkey

Abstract

The effect of graded ischaemia on intracranial nerve fibre conduction has not previously been studied quantitatively. We measured pyramidal tract (PT) discharges evoked by electrical stimulation of the ipsilateral motor cortex, together with local blood flow (by hydrogen clearance), in the internal capsule and ventral pons of baboons anaesthetised with alpha-chloralose. Changes in conduction time and amplitude of the motor volley were monitored over this PT segment as it was subjected to progressive ischaemia in controlled stages. Conduction time increased significantly, with attenuation of the volley, at average brain-stem tissue flows below 30 ml/100 g/min. Using paired stimuli, we demonstrated relative refractory and supernormal characteristics of PT conduction. In mild ischaemia, the conduction time of the test response decreased, much more than with single stimulation, and in denser ischaemia it increased, relatively more so at smaller inter-stimulus intervals. The data demonstrate the impairment of conduction in a population of CNS axons with reduced local blood flow and indicate that the transmission of relatively rapid sequences of impulses would be the first aspect of conduction to suffer in ischaemia prior to conduction block.