This chapter describes the experiments that are designed to clarify the question of the relative ischemic sensitivities among regions, extending the work to measure the somatosensory evoked potentials (SEPs) and cerebral blood flow (CBF) in thalamus and brainstem. The chapter presents the experimental data from primates on the relationships between features of early (<25 ms latency) components of the SEP generated in the brainstem, thalamus and cerebral cortex, and the local CBF in these regions. The quantitative evidence indicating that the electrophysiological responses in the brainstem and thalamic stages of the somatosensory afferent pathway are relatively resistant to ischemia in comparison to those originating in cortex. A similar gradient of SEP sensitivity along the afferent pathway is present in terms of a reduction in systemic blood pressure; the lemniscal SEP amplitude is only reduced significantly below 20 torr, the response in thalamic ventral postero-lateral (VPL) nucleus below 30 torr, but the cortical response threshold is in the range 30–40 torr. The associated changes in latency of these SEP components in ischemia are described, and discussed in relation to increases in central conduction time observed in clinical or intraoperative practice.
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