MYELINATED MECHANICALLY INSENSITIVE AFFERENTS FROM MONKEY HAIRY SKIN - HEAT-RESPONSE PROPERTIES

To compare the heat responses of mechanically sensitive and mechanical ly insensitive A-fiber nociceptors, an electrical search technique was used to locate the receptive fields of 156 A fibers that innervated t he hairy skin in the anesthetized monkey (77 A beta-fibers, 79 A delta -fibers). Two-thirds of these afferents were either low-threshold mech anoreceptors (n = 91) or low-threshold cold receptors (n = 11). Nine A beta-fibers and 41 A delta-fibers were cutaneous nociceptors, and fou r AS-fibers innervated subcutaneous tissue. The majority of cutaneous A-fiber nociceptors were heat sensitive (43/50 = 86%). Heat-insensitiv e cutaneous A-fiber nociceptors consisted of one cold nociceptor, thre e silent nociceptors. and three high-threshold mechanoreceptors. Two t ypes of response were observed to an intense heat stimulus (53 degrees , 30 s). Type I (IZ = 26) was characterized by a long latency (mean: 5 s) and a late peak discharge (16 s). Type II (n = 17) was characteriz ed by a short latency (0.2 a) and an early peal; discharge (0.5 s). Ty pe I fibers exhibited faster conduction velocities (25 vs. 14 m/s) and higher hear thresholds ( >53 vs. 47 degrees C, 1-s duration) than typ e IT fibers. The possibility that the type I heat response was a resul t of sensitization was tested in three fibers by determining the heat threshold to 30-s duration stimuli (42-46 degrees C). For this long st imulus duration heat thresholds were reproducible across multiple runs , and the threshold to the 1-s duration stimulus was not altered by th ese tests. Thus fibers with a type I heat response were not high-thres hold mechanoreceptors that developed a heat response through sensitiza tion Fibers with a type II heat response had significantly higher mech anical thresholds (median: 15 bar) than fibers with a type I heal resp onse (5 bar). This finding accounts For the observation that type TI h eat responses were infrequently observed in earlier studies wherein th e search technique depended on mechanical responsiveness. Fibers with a type II response exhibited a graded response to heat stimuli, marked fatigue to repeated applications of hear stimuli, and adaptation to s ustained heat stimuli similar to that seen in C-fiber nociceptors. Fir st pain sensation to heat is served by type LI A-fiber nociceptors tha t are mechanically insensitive. Type I A-fiber nociceptors likely sign al pain to long-duration hear stimuli and may signal first pain sensat ion to mechanical stimuli.