The distribution and functional properties of nociceptive neurons with masseteric input in the cat thalamus

Abstract

I investigated the distribution of nociceptive neurons responding to masseteric nerve stimulation in the nucleus ventralis posteromedialis proper (VPM) and intralaminar (IL) nuclei of the thalamus. The activity of single thalamic neurons in cats anesthetized with urethane and chloralose was assessed extracellularly. Thirty neurons with masseteric inputs were observed around the periphery (shell region) of the posterior half of the VPM, and most of these were excited by mechanical stimulation in the contralateral trigeminal integument. Of these, 20 neurons displayed a graded response to touch, pressure, and noxious mechanical stimulation to the center of the receptive field, while the surrounding area was responsive only to noxious mechanical stimulation. They were therefore considered to be wide dynamic range (WDR) neurons, and were located more anteriorly in the VPM. The other 10 neurons responded only to noxious mechanical stimulation. Twenty-six neurons were found in the nuclei centralis lateralis and parafascicularis of the IL nuclei, and most of these were excited by noxious stimulation of the spinal nerve territory. Some of the VPM and IL neurons responded to injection of 3% NaCl solution into the masseter. These findings suggest that masseteric inputs, such as masseteric pain information, are mainly conveyed to the cortex via WDR neurons and IL nociceptive neurons.