Abstract Introduction: In humans, cutaneous Aβ afferents are traditionally linked to discriminative touch, while pain is attributed to Aδ and C fibers. However, we previously identified thickly myelinated high-threshold mechanoreceptors (Aβ-HTMRs) that encode noxious skin indentations and evoke painful percepts when selectively activated. These afferents also display finely grained receptive fields and resilience to fatigue during sustained stimulation. Objectives: To characterize the tuning properties of A-HTMRs under controlled mechanical stimulation and compare them with C-HTMRs. Methods: We used a robotic stimulation system capable of delivering precise skin indentations across a wide force range (20–1000 mN). Single-unit axonal recordings (microneurography) were obtained from cutaneous afferents of the radial nerve in awake healthy participants. Both low- and high-threshold mechanoreceptors were recorded. Results: Among 39 recorded mechanoreceptive afferents, HTMRs were distinguished by high mechanical thresholds and lack of response to soft brushing, with conduction velocities in the Aβ- and C-fiber ranges. Both A- and C-HTMRs exhibited force-dependent increases in spike count and firing rate, with A-HTMRs showing significantly stronger responses. Principal component analysis revealed distinct separation between A- and C-HTMRs, driven by A-HTMRs' robust high-force responses. Psychophysical testing indicated painful stimuli were often described as “sharp,” and selective intraneural microstimulation of a single A-HTMR evoked localized “sharp-stinging” pain projected to its receptive field. Conclusion: Robot-controlled stimulation confirmed both A- and C-HTMRs' role in encoding painful mechanical stimuli. The fast conduction, high firing rates, fine receptive fields, and fatigue resilience of A-HTMRs suggest a specialized nociceptive system capable of conveying rich spatial-temporal information, potentially contributing to protective behaviors.
Bouchatta et al. (Fri,) studied this question.