Effect of anodal blockade of myelinated fibers on vagal C-fiber afferents.

The effects of monopolar cathodal, monopolar anodal, and bipolar anodal polarizing currents on vagal A- and C-fiber activity were studied in anesthetized dogs. Monopolar cathodal polarization consistently produced excitation of spontaneous and evoked A- and C-fibers. Monopolar anodal and bipolar anodal polarizations differentially blocked A-fibers as a function of fiber diameter. The specific purpose of this study was to compare the effects of monopolar anodal and bipolar anodal currents on C-fiber excitability. Small fiber preparations were dissected from the vagal trunk, cut centrally, and placed on recording electrodes. A constant-current stimulus pulse was applied to the nerve at various distances from the blocking electrodes. The stimulus current strength was increased until an isolated C-fiber spike was observed. This value was defined as 100% of threshold. The stimulus current was then reduced to zero, the blocking current was increased slowly to 100 microA, and the procedure repeated. Threshold data obtained in this manner for each set of stimulation electrodes was plotted as a function of distance from the blocking electrode(s) for both modes of anodal blockade. No significant change in C-fiber excitability was observed with bipolar anodal blockade, whereas excitability was significantly (P less than or equal to 0.05) decreased using the monopolar technique. Thus, monopolar anodal block may reduce the possibility of asynchronous C-fiber discharge, which has been associated with a bipolar block of A-fibers.