Fig. 8. Tonic and phasic (use-dependent) effects of lidocaine, 0.25%, on latency and incidence of spikes in C fibers. (A ) Time course of lidocaine action in two C fibers: one “slow” unit (upper traces, triangles; resting conduction velocity = 0.72 m/s) and one “fast” unit (lower traces, circles; conduction velocity = 1.19 m/s). Activity was measured under “tonic” (0.5 Hz, open symbols) and “phasic” (5 Hz, filled symbols) modes of stimulation. The spaces of unconnected symbols indicate the times of spike failure. (B ) Lidocaine induced a use-dependent relief of block at higher-frequency (5-Hz) phasic stimulation in two C fibers (C fiber 3, conduction velocity = 1.19 m/s; C fiber 4, conduction velocity = 1.04 m/s) that were blocked at low-frequency (tonic, 0.5-Hz) stimulation. The topmost trace shows the tonic responses at 6.5 min after lidocaine injection. Both units experienced tonic block of conduction by 14 min after injection (second trace), but block was relieved during one train of phasic stimulation (5 Hz), the activity of C fiber 3 returning earlier than that of C fiber 4. Recovery from tonic block was detected 4 min after phasic stimulation episode (trace not shown).