Low pH enhances response of thin muscle afferents to mechanical stimuli.

We examined whether acidic buffer sensitizes thin muscular afferents to mechanical stimulus using single-fiber recording. The EDL muscle with the common peroneal nerve attached was excised from 17-male SD rats under pentobarbital anesthesia. After identifying single thin afferent fibers, mechanical stimulus was applied to their receptive fields using a servo-controlled mechanical stimulator before and after pH 6.2-solution was applied to the receptive field. The effect of pH 7.4-solution was similarly examined as a control. Application of pH 7.4-solution did not induce any change in the mechanical threshold or increase in the number of discharges evoked during stimulation over the number of spontaneous discharges (response magnitude). However, pH 6.2-solution significantly lowered the mechanical threshold and increased the response magnitude, suggesting that the mechanical sensitivity of thin muscle afferents was augmented by exposure to acidic pH, which is seen during exercise. This change could be involved in respiratory control during exercise.