Responses of neurons in primate ventral posterior lateral nucleus to noxious stimuli

Recordings were made from the caudal part of the ventral posterior lateral (VPLc) nucleus of the thalamus in anesthetized macaque monkeys. In addition to many neurons that responded only to weak mechanical stimuli, scattered neurons were found that responded to both innocuous and noxious stimulation or just to noxious stimulation of the skin. A total of 73 such neurons were examined in 26 animals. Noxious stimuli included strong mechanical stimuli (pressure, pinch, and squeezing with forceps) and graded noxious heat (from 25°C adapting temperature to 43, 45, 47, and 50°C). The responses of the VPLc neurons increased progressively with greater intensities of noxious stimulation. The stimulus-responses function when noxious heat stimuli were used was a power function with an exponent greater than one. Repetition of the noxious heat stimuli revealed sensitization of the responses of the thalamic neurons to such stimuli. The threshold for a response to noxious heat was lowered, and the responses to supra-threshold noxious heat stimuli were enhanced. The responses of VPLc neurons to noxious heat stimuli adapted after reaching a peak discharge frequency. The rate of adaptation was slower for a stimulus of 50°C than for one of 47°C. For the six neurons tested, responses to noxious heat were dependent on pathways ascending in the ventral part of the lateral funiculus contralateral to the receptive field (ipsilateral to the thalamic neuron). In two cases, the input to the thalamic neurons from axons of the dorsal column was also conveyed by way of a crossed pathway in the opposite ventral quadrant. In another case, access to the thalamic neuron by way of ascending dorsal column fibers was demonstrated. The thalamic neurons had restricted contralateral receptive fields that were somatotopically organized. Neurons with receptive fields on the hindlimb were in the lateral part of the VPLc nucleus, whereas neurons with receptive fields on the fore-limb were in medial VPLc. Ninety percent of the VPLc neurons tested that responded to noxious stimuli could be activated antidromically by stimulation of the surface of SI sensory cortex. It was possible to confirm that many of these cells project to the SI sensory cortex by using microstimulation. Successful micro-stimulation points were either within the SI cortex or in the white matter just beneath the cortex. We conclude that some neurons in the VPLc nucleus are capable of signaling nociceptive stimuli. The nociceptive information appears to reach these cells through the ventral part of the lateral funiculus on the side contralateral to the receptive field, presumably by way of the spinothalamic tract. The VPLc cells are somatotopically organized, and they are thalamocortical neurons that project to the SI sensory cortex. It is speculated that the VPLc nucleus and SI cortex play a role in nociception.