Ventromedial thalamic neurons convey nociceptive signals from the whole body surface to the dorsolateral neocortex

The somatosensory properties of ventromedial (VM) thalamic neurons were inv estigated in anesthetized rats by examining their responses to calibrated c utaneous stimuli. A population of neurons within the lateral part of the ve ntromedial thalamus (VMl) showed two peaks of activation after percutaneous electrical stimuli, regardless of which part of the body was stimulated. T he early and late peaks were elicited by A delta- and C-fiber activities wi th mean conduction velocities of 12.9 +/- 0.9 and 1 +/- 0.2 m/sec, respecti vely. These responses were strongly depressed or blocked after microinjecti ons within the medullary subnucleus reticularis dorsalis of xylocaine or th e NMDA antagonist MK-801. None of the VMl neurons responded to innocuous cu taneous or proprioceptive stimuli. In contrast, all these neurons responded to noxious mechanical and thermal stimulation of the limbs and showed mono tonic increases in their discharges to increasingly strong noxious cutaneou s stimuli. In addition, some VMl neurons were antidromically activated by s timulation in layer I of the dorsolateral frontal cortex. These findings su ggest that the rat VMl conveys and encodes cutaneous nociceptive inputs fro m any part of the body surface to layer I of the dorsolateral neocortex. Th is reticulo-thalamocortical network may allow any signal of pain to gain ac cess to widespread areas of the neocortex and thus help prime the cortex fo r attentional reactions and/or the coordination of motor responses.