Fig. 4. Repeated application of pH 6.0 lactic acid to acid-responsive C-fibers 2 mm or less from the incision. (  A ) Sample recording from a single C-fiber. After a 5-min baseline, pH 6.0 lactic acid (15 mm; 32°C) was repeatedly applied (three times) for 5 min at 5-min intervals, followed by 5-min washout. The  upper and lower panels show the digitized oscilloscope tracings and spike density histograms (bin width = 10 s), respectively.  Inset displays the action potential of this unit. CV = conduction velocity. (  B ) Mean discharge rate of each acid-responsive C-fiber 2 mm or less from the incision during repeated application of pH 6.0 lactic acid. The  solid circles represent data from fiber in (  A ). Five-min periods of baseline, acid application, and washout were used to calculate the mean discharge rate. All five units were spontaneously active before acid exposure, and background activity was subtracted from the response data afterwards. Imp = impulse .