Thermoreceptive innervation of human glabrous and hairy skin: a contact heat evoked potential analysis

The human palm has a lower heat detection threshold and a higher heat pain threshold than hairy skin. Neurophysiological studies of monkeys suggest that glabrous skin has fewer low threshold heat nociceptors (AMH type 2) than hairy skin. Accordingly, we used a temperature-controlled contact heat evoked potential (CHEP) stimulator to excite selectively heat receptors with C fibers or Aδ-innervated AMH type 2 receptors in humans. On the dorsal hand, 51 °C stimulation produced painful pinprick sensations and 41 °C stimuli evoked warmth. On the glabrous thenar, 41 °C stimulation produced mild warmth and 51 °C evoked strong but painless heat sensations. We used CHEP responses to estimate the conduction velocities (CV) of peripheral fibers mediating these sensations. On hairy skin, 41 °C stimuli evoked an ultra-late potential (mean, SD; N wave latency: 455 (118) ms) mediated by C fibers (CV by regression analysis: 1.28 m/s, N=15) whereas 51 °C stimuli evoked a late potential (N latency: 267(33) ms) mediated by Aδ afferents (CV by within-subject analysis: 12.9 m/s, N=6). In contrast, thenar responses to 41 and 51 °C were mediated by C fibers (average N wave latencies 485(100) and 433(73) ms, respectively; CVs 0.95-1.35 m/s by regression analysis, N=15; average CV=1.7 (0.41) m/s calculated from distal glabrous and proximal hairy skin stimulation, N=6). The exploratory range of the human and monkey palm is enhanced by the abundance of low threshold, C-innervated heat receptors and the paucity of low threshold AMH type 2 heat nociceptors.