Fig. 5. Summary of chemical responses of nociceptors to lactic acid of three different pH levels. (  A ) Sample recordings from a single C-fiber innervating 2 mm or less from the incision. After a 5-min baseline, 15 mm lactic acid with pH 6.5, 6.0, and 5.5 was sequentially applied for 5 min before 5-min washout. The interval between each acid application was 15 min. 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, C ) Prevalence of acid-responsive units in (  B ) C-fibers and (  C ) A-fibers. (  D ) Mean discharge rate of each acid-responsive C-fiber during application of 15 mm lactic acid with three different pH levels. Imp = impulse .