Responses of a mechanosensitive muscle afferent fiber from an incised muscle to 15 mm lactic acid with varying pH (pH 6.5–7.0) and unoxygenated synthetic interstitial fluid (SIF; pH 7.6). (A) An example recording of the response of a group IV afferent to lactic acid. The upper panel shows a spike density histogram (bin width = 10 s), and the lower panel shows the digitized oscilloscope tracing. Filled arrowheads mark artifacts generated by the addition of the acid solution; the open arrowheads mark artifacts created during removal of the solution. The receptive field is depicted in the inset. The mean spike density histograms of group IV afferents from the control group (B and D) and incision group (C and E) during 5-m lactic acid exposure at pH 6.5 or 7.0. APs = action potentials; mm = millimolar; mV = millivolts.