Treede R D, Meyer R A, Campbell J N
Department of Neurosurgery, School of Medicine, Johns Hopkins University, Baltimore, Maryland 21205, USA.
J Neurophysiol. 1998 Sep;80(3):1082-93. doi: 10.1152/jn.1998.80.3.1082.
To compare the heat responses of mechanically sensitive and mechanically insensitive A-fiber nociceptors, an electrical search technique was used to locate the receptive fields of 156 A-fibers that innervated the hairy skin in the anesthetized monkey (77 A beta-fibers, 79 A delta-fibers). Two-thirds of these afferents were either low-threshold mechanoreceptors (n = 91) or low-threshold cold receptors (n = 11). Nine A beta-fibers and 41 A delta-fibers were cutaneous nociceptors, and four A delta-fibers innervated subcutaneous tissue. The majority of cutaneous A-fiber nociceptors were heat sensitive (43/50 = 86%). Heat-insensitive cutaneous A-fiber nociceptors consisted of one cold nociceptor, three silent nociceptors, and three high-threshold mechanoreceptors. Two types of response were observed to an intense heat stimulus (53 degrees C, 30 s). Type I (n = 26) was characterized by a long latency (mean: 5 s) and a late peak discharge (16 s). Type II (n = 17) was characterized by a short latency (0.2 s) and an early peak discharge (0.5 s). Type I fibers exhibited faster conduction velocities (25 vs. 14 m/s) and higher heat thresholds (> 53 vs. 47 degrees C, 1-s duration) than type II fibers. The possibility that the type I heat response was a result of sensitization was tested in three fibers by determining the heat threshold to 30-s duration stimuli (42-46 degrees C). For this long stimulus duration heat thresholds were reproducible across multiple runs, and the threshold to the 1-s duration stimulus was not altered by these tests. Thus fibers with a type I heat response were not high-threshold mechanoreceptors that developed a heat response through sensitization. Fibers with a type II heat response had significantly higher mechanical thresholds (median: 15 bar) than fibers with a type I heat response (5 bar). This finding accounts for the observation that type II heat responses were infrequently observed in earlier studies wherein the 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 heat stimuli, and adaptation to sustained heat stimuli similar to that seen in C-fiber nociceptors. First pain sensation to heat is served by type II A-fiber nociceptors that are mechanically insensitive. Type I A-fiber nociceptors likely signal pain to long-duration heat stimuli and may signal first pain sensation to mechanical stimuli.
为比较机械敏感和机械不敏感的A类纤维伤害感受器的热反应，采用电搜索技术定位了156条支配麻醉猴有毛皮肤的A类纤维的感受野（77条Aβ纤维，79条Aδ纤维）。这些传入纤维中三分之二是低阈值机械感受器（n = 91）或低阈值冷感受器（n = 11）。9条Aβ纤维和41条Aδ纤维是皮肤伤害感受器，4条Aδ纤维支配皮下组织。大多数皮肤A类纤维伤害感受器对热敏感（43/50 = 86%）。对热不敏感的皮肤A类纤维伤害感受器包括1条冷伤害感受器、3条静息伤害感受器和3条高阈值机械感受器。对强烈热刺激（53℃，30秒）观察到两种反应类型。I型（n = 26）的特征是潜伏期长（平均：5秒）和放电峰值出现晚（16秒）。II型（n = 17）的特征是潜伏期短（0.2秒）和放电峰值出现早（0.5秒）。I型纤维比II型纤维表现出更快的传导速度（25对14米/秒）和更高的热阈值（>53对47℃，持续1秒）。通过确定对30秒持续时间刺激（42 - 46℃）的热阈值，在三条纤维中测试了I型热反应是敏化结果的可能性。对于这种长刺激持续时间，热阈值在多次测试中是可重复的，并且这些测试未改变对1秒持续时间刺激的阈值。因此，具有I型热反应的纤维不是通过敏化产生热反应的高阈值机械感受器。具有II型热反应的纤维比具有I型热反应的纤维具有显著更高的机械阈值（中位数：15巴对5巴）。这一发现解释了在早期研究中较少观察到II型热反应的现象，在早期研究中搜索技术依赖于机械反应性。具有II型反应的纤维对热刺激表现出分级反应，对重复热刺激有明显疲劳，并且对持续热刺激的适应与C类纤维伤害感受器类似。对热的第一痛觉由机械不敏感的II型A类纤维伤害感受器介导。I型A类纤维伤害感受器可能向长时间热刺激传递痛觉信号，并且可能向机械刺激传递第一痛觉信号。
