Human foot outperforms the hand in mechanical pain discrimination

Abstract

Tactile discrimination has been extensively studied, but mechanical pain discrimination remains poorly characterised. Here, we measured the capacity for mechanical pain discrimination using a two-alternative forced choice paradigm, with force-calibrated indentation stimuli (Semmes-Weinstein monofilaments) applied to the hand and foot dorsa of healthy human volunteers. In order to characterise the relationship between peripheral nociceptor activity and pain perception, we recorded single-unit activity from myelinated (A) and unmyelinated (C) mechanosensitive nociceptors in the skin using microneurography. At the perceptual level, we found that the foot was better at discriminating noxious forces than the hand, which stands in contrast to that for innocuous force discrimination, where the hand performed better than the foot. This observation of superior mechanical pain discrimination on the foot compared to the hand could not be explained by the responsiveness of individual nociceptors. We found no significant difference in the discrimination performance of either the myelinated or unmyelinated class of nociceptors between skin regions. This suggests the possibility that other factors such as skin biophysics, receptor density or central mechanisms may underlie these regional differences.

Significance Statement Standard clinical practice for diagnosing neuropathies and pain disorders often involves assessing thresholds for pain or light touch. The ability to discriminate between different stimulus intensities is a separate but equally important sensory function, however this is not typically assessed in the clinic, and so studying this may provide insights into pain signalling mechanisms. Here, we investigated the ability of healthy individuals to discriminate between different forces of painful indentation. We found that the foot was better at this than the hand. This difference could not be explained by the firing activity of peripheral nociceptors (pain-signalling neurons) between the two regions, suggesting that mechanisms other than nociceptor sensitivity are involved.