Static and dynamic responses of spinothalamic tract neurons to mechanical stimuli.

The activity of primate spinothalamic tract neurons was investigated. Units were identified by antidromic activation from the diencephalon. Most had a spontaneous discharge. The fastest spontaneous discharge rates were from neurons activated by receptors in muscles or joints. However, such activity could be decreased by repositioning the hindlimb. Spontaneous activity was also altered by changes in anesthetic level. Time-interval histograms demonstrated the patterns of spontaneous discharge. Many units could be excited by noxious stimulation. Graded step displacements of the skin often evoked a slowly adapting discharge which was a function of pressure (or displacements above a threshold of some hundreds of microns). Responses continued to increase as pressures became noxious. Rapidly adapting responses of hair-activated and low-threshold spinothalamic tract neurons were tested with ramp stimuli and found to signal acceleration (or a higher derivative of position), velocity, or a combination of these. Responses to stimulus acceleration were prominent when hairs were displaced, while responses to stimulus velocity were seen with displacement of the skin. Possible receptor types which might account for the observations are discussed.