Assessment of mechanical and thermal thresholds of human C nociceptors during increases in skin sympathetic nerve activity

Objectives: The objective of the present study is to investigate the relationship between C-fiber nociceptor sensitivity and skin sympathetic nerve activity during mental arithmetic. Methods: Single afferent C-fibers were identified simultaneously with spontaneous postganglionic sympathetic discharges and recorded from the peroneal nerve using microneurography in 23 normal subjects. Mechanical and heat thresholds were measured by ‘marking’ the nociceptor with suprathreshold stimuli, causing increased latency after a subsequent threshold stimulus at rest and during mental arithmetic. Skin sympathetic nerve activity was estimated by counting the number of bursts per minute. Results: Thirty-two single C-fibers were identified. Eleven had polymodal receptors (mechanical and heat sensitive), eight were only sensitive to mechanical stimuli, two were only sensitive to heat stimuli, and 11 were insensitive to both. C-fibers were selected when the ratio of skin sympathetic nerve activity during mental arithmetic to that at rest was over 1.00. In 19 selected mechanical sensitive units, average mechanical threshold was 4.86 at rest and 4.84 during mental arithmetic. In 6 selected heat sensitive units, average heat threshold was 45.0°C at rest and 43.4°C during mental arithmetic. However, differences were not statistically significant. Conclusions: Physiological sympathetic stimulation did not affect afferent C-fiber nociceptor sensitivity to mechanical and heat stimuli in healthy subjects