The projection pathway from the tooth pulp to the ipsilateral first somatosensory cortex (SI) in the cat

About 40 per cent of tooth pulp-driven (TPD) neurones received afferent fibres from the ipsilateral tooth pulp. This ipsilateral afferent pathway was investigated in cats anaesthetized with nitrous oxide and halothane. The subcortical temperature of the brain was lowered to about 28 degrees C by perfusion of cold water within a thermode. Cooling the homotopic area contralateral to the recording site caused little change in the firing rate of short-latency TPD neurones upon ipsilateral pulp stimulation (n = 13). Microinjection (1-2 microliters) of 1 per cent lidocaine into the ipsilateral nucleus ventralis posteromedialis (VPM) caused significant diminution in the firing rate of short-latency TPD neurones (n = 11) to ipsilateral stimulation but not of long-latency TPD neurones (n = 8). About 35 per cent (13/35) of the TPD neurones distributed in the medial shell region of the VPM proper responded with short latency to ipsilateral pulp stimulation. These findings suggest that the ipsilateral input to short-latency TPD neurones in the oral area is carried via projection fibres from the ipsilateral VPM but not via commissural ones, and that the ipsilateral input to long-latency neurones is probably relayed in a site other than the ipsilateral VPM.