Conduction in Small Fibers Figure 2. Typical results from a sympathetic efferent and a primary afferent. A) Sympathetic efferent fiber exhibiting speeding of conduction to twin-pulse stimulation. Following an electrical twin pulse at the sciatic nerve (60 ms interstimulus interval), the conduction latency of the second action potential (AP) for fiber #2 was 0.4 ms smaller than for the first AP. Thus, the conduction velocity of the second AP increased (i.e., ‘‘speeding’’). Top: Neuronal activity (total sweep length 300 ms) with stimulus artifacts at 60 ms. Bottom: Expanded view of neuronal activity showing five different AP waveforms. Fiber #2 originated from a sympathetic efferent (as identified by collision following electrical stimulation at the sympathetic chain). The other C fibers were not positively identified. B) Sympathetic efferent fiber shows little activity-dependent slowing. Following a train of electrical stimuli (2 Hz, 3 min), the conduction latency for fiber #2 increased by 12.9 ms or 8.7% relative to the latency for the first AP (148.6 ms, conduction velocity = 1.0 m/s). C) The primary afferent fiber exhibited slowing of conduction to twin-pulse stimulation. Following electrical twin pulses at the sciatic nerve (50 ms interstimulus interval) the conduction latency of the second AP was 2.2 ms greater than that of the first. Thus, the conduction velocity of the second AP decreased (i.e., ‘‘slowing’’). Top: Neuronal activity (total sweep length: 400 ms) showing stimulus artifact at 50 ms. Bottom: Expanded view of neuronal activity showing four different AP waveforms. Fibers #1–3 were all primary afferent fibers (as identified by collision following stimulation from the electrode on the dorsal root). D) The primary afferent fiber showed pronounced activity-dependent slowing. Following a train of electrical pulses applied at the sciatic nerve (2 Hz, 3 min), the conduction latency for fiber #2 increased by 60.7 ms or 24.6% relative to the latency for first AP (247.6 ms, conduction velocity = 0.93 m/s). doi:10.1371/journal.pone.0009076.g002 increased continuously throughout the stimulus train (i.e., similar to that shown in Fig. 2); the mean increase in latency at the end of the protocol (29.4 + 1.9%) was substantially greater than for the sympathetic efferent fibers (p,0.001). For 6 afferent fibers (labeled as ‘‘small slowers’’ in Fig. 4A), the time course for the increase in conduction latency was similar to that of the sympathetic efferent fibers (Fig. 4B). The mean increase in latency at the end of the protocol for these afferents (6.7 + 0.5%) was significantly (but not substantially) smaller than for the sympathetic efferents (p,0.05). The ‘‘big slowers’’ and ‘‘small slowers’’ did not differ with regard to their latency difference following twin pulse stimulation (Fig. 3B). However, as can be seen from Fig. 3B, the mean conduction velocity of the small slowers was significantly faster (1.25 + 0.3 m/s) than the big slowers (0.96 + 0.5 m/s, p,0.05). Centripetal Recordings Following Electrical Stimulation at the Cutaneous Terminals In a separate series of experiments, twin-pulse electrical stimulation (ISI = 40–70 ms) was applied to the cutaneous terminals of 46 mechanically-sensitive nociceptive afferent fibers (MSAs) and of 36 mechanically-insensitive afferent/efferent fibers. As shown in Fig. 3C (triangles), all of the MSAs exhibited a slowing of conduction to stimulation with a single twin-pulse. The average twin pulse difference (3.0 + 0.3%) was greater than observed for the afferent fibers in the centrifugal recordings with proximal sciatic nerve stimulation (Fig. 3A, p,0.001). This suggests that the twin-pulse slowing for afferent fibers is more pronounced in the cutaneous arbor than in the proximal parent axon. In contrast, most of the mechanically-insensitive fibers exhibited a speeding to the twin pulse stimulation (circles, Fig. 3C). This PLoS ONE | www.plosone.org 5 February 2010 | Volume 5 | Issue 2 | e9076