Electrophysiological assessment of the cutaneous arborization of A delta-fiber nociceptors

Little is known about the relationship between the branching structure and function of physiologically identified cutaneous nociceptor terminals. The axonal arborization itself, however, has an impact on the afferent signal t hat is conveyed along the parent axon to the CNS. We therefore developed el ectrophysiological techniques to investigate the branching structure of cut aneous nociceptors. Single-fiber recordings were obtained from physiologica lly identified nociceptors that innervated the hairy skin of the monkey. El ectrodes for transcutaneous stimulation were fixed at two separate location s inside the receptive field. For 32 A delta-fiber nociceptors, distinct st eps in latency of the recorded action potential were observed as the intens ity of the transcutaneous electrical stimulus increased, indicating discret e sites for action potential initiation. The number of discrete latencies a t each stimulation location ranged from I to 9 (3.7 +/- 0.2; mean +/- SE) a nd the mean size of the latency step was 9.9 +/- 1.0 ms (range: 0.4-89.1 ms ). For seven A delta fibers, collision techniques were used to locate the p osition of the branch point where the daughter fibers that innervated the t wo locations within the receptive field join the parent axon. To correct fo r changes in electrical excitability at the peripheral terminals, collision experiments between the two skin locations and between each skin location and a nerve hunk electrode were necessary. Nine branch points were studied in the seven A delta fibers; the mean propagation time from the action pote ntial initiation site to the branch point was 31 +/- 5 ms corresponding to a distance of 54 +/- 10 mm. Almost half of the daughter branches were unmye linated. These results demonstrate that collision techniques can be used to study the functional anatomy of physiologically identified nociceptive aff erent terminals. Furthermore these results indicate that some nociceptive a fferents branch quite proximal to their peripheral receptive held. Occlusio n of action potential activity can occur in these long branches such that t he shorter branches dominate in the response to natural stimuli.