title: Electrophysiological and psychophysical correlates of spatial summation to noxious heat: the possible role of A-delta fibers.
abstract: Although spatial summation of pain (SSP) is central to the processing of pain intensity and quality, its mechanism is not fully understood. We previously found greater heat SSP in hairy than in glabrous skin, suggesting that perhaps A-mechano-heat II (AMH-II) nociceptors are the dominant subserving system. In order to further explore the role of A-delta fibers in heat-induced SSP, we analyzed the electrophysiological correlates of SSP under conditions that minimize the influence of skin thicknesses. Among 17 subjects, fast rate of rise (70 °C/sec) heat stimuli that induced a pre-fixed, similar, SSP magnitude for hairy and glabrous skin were repeatedly administered using large and small probes, during which time the contact heat-evoked potentials (CHEPs) and pain ratings were recorded. Both N2 and P2 amplitudes were larger in hairy than in glabrous skin, but a differential effect of SSP was found on the CHEPs. Despite similar psychophysical SSP in hairy and glabrous skin, the electrophysiological SSP reflected in N2 but not P2 amplitude was larger in hairy skin. Nevertheless, regardless of skin type, SSP was manifested by an increase in P2 amplitudes. Considering the uniform psychophysical SSP for the two skin types, the fast stimulation rate and lower activity of AMH-II in glabrous skin, a greater electrophysiological SSP in hairy than in glabrous skin may suggest that SSP is mainly subserved by AMH nociceptors. The overall SSP effect, manifested in greater P2 amplitude, may reflect specific brain responses aimed to prepare the individual to an increased potential tissue damage.
authors.affiliation: Laboratory of Clinical Neurophysiology, Department of Neurology, Rambam Health Care Campus, Faculty of Medicine, Technion and Institute of Technology, Haifa, Israel.
authors.forename: Y
authors.initials: Y
authors.lastname: Granovsky
authors.affiliation: Department of Physical Therapy, Sackler Faculty of Medicine, Tel-Aviv University, Ramat Aviv, 69978, Tel Aviv, Israel.
authors.forename: N
authors.initials: N
authors.lastname: Raz
authors.affiliation: Department of Physical Therapy, Sackler Faculty of Medicine, Tel-Aviv University, Ramat Aviv, 69978, Tel Aviv, Israel. rutidef@post.tau.ac.il.|Sagol School of Neuroscience, Tel-Aviv University, Ramat Aviv, Tel Aviv, Israel. rutidef@post.tau.ac.il.
authors.forename: R
authors.identifier: 0000-0002-2719-4007
authors.initials: R
authors.lastname: Defrin
mesh_terms: D000328:Adult; D000344:Afferent Pathways / Q000502:physiology; D005071:Evoked Potentials / Q000502:physiology; D005260:Female; D064368:Healthy Volunteers; D006358:Hot Temperature; D006801:Humans; D008297:Male; D009412:Nerve Fibers / Q000502:physiology*; D009619:Nociceptors / Q000502:physiology*; D017288:Pain Threshold / Q000502:physiology* / Q000523:psychology; D010812:Physical Stimulation; D011601:Psychophysics; D011930:Reaction Time / Q000502:physiology; D012867:Skin / Q000294:innervation; D064232:Visual Analog Scale; D055815:Young Adult
keywords: A-mechano-heat nociceptors; Contact heat-evoked potentials (CHEPS); Skin type; Spatial summation