Identification of afferents contributing to sympathetically evoked activity in wide-dynamic-range neurons.

The purpose of this study was to determine which types of mechanoreceptor afferents contribute to sympathetically evoked activity in wide-dynamic-range (WDR) neurons--the spinal neurons thought to mediate sympathetically maintained pain. The experimental approach was to record and compare activity evoked in single WDR neurons, hair afferents, and slowly adapting type I (SAI) afferents in anesthetized cats. During electrical stimulation of the sympathetic trunk, WDR neurons responded with either an early transient burst of activity, sustained activity, or both. The early transient response was observed only in neurons with piloerection in the receptive field; this response had a similar time course to sympathetically evoked activity in hair afferents with piloerection in the receptive field. The sustained response that occurred in some WDR neurons was independent of piloerection and was similar in time course to the response evoked in SAI afferents by sympathetic stimulation. We conclude that hair and SAI afferents contribute to different components of sympathetically evoked activity in WDR neurons and that both types of afferents are likely to be involved in sympathetically maintained pain in humans.