Fig. 5. Relative resistance of C fibers to block by lidocaine. (A ) Example of unitary activity in one C fiber (conduction velocity = 1.46 m/s). Upper trace is control (10 min after control buffer [PIPES] injection); second to fifth traces were recorded at 10-min intervals after injections of lidocaine, 0.05, 0.075, 0.1, and 0.175%, respectively; and the lowest trace is a record made 40 min after the last lidocaine injection. (B ) Changes in conduction velocity (latency, as percentage of control value) for the same C fiber shown in A. Conduction after lidocaine, 0.25%, failed at (*) and was regained at (#). (C ) The time course of tonic impulse failure after lidocaine, 0.25%, in “fast” (closed triangles) and “slow” (open triangles) C fibers. (D ) The time course of tonic failure of impulses in fast and slow C fibers after lidocaine, 0.5%, injection.