Surface and intramedullary potentials evoked by stimulation of the glossopharyngeal nerve in frogs

The bulbar potentials evoked by afferent volleys in the bullfrog glossopharyngeal nerve and in its 2 distal branches were studied. Following supramaximal electric stimulation of the peripheral nerve, the potential consisting of 2 triphasic deflections (S1 and S2) of presynaptic origin and 4 postsynaptic negative waves, N1, N2, N3 and N4, having the peak latency of 5, 8, 30 and 80 ms, respectively, was obtained along the nucleus fasciculus solitarius. By lowering the stimulus intensity to the threshold for exciting mechanosensitive fibers, only S1 followed by N1, N3 and N4 was produced, whereas, at higher intensities, S2 which accompanied by N2 became apparent. N1 and N2 waves were distributed over the bulbar dorsal surface with the maximal amplitude at 1-2 mm rostral to the obex and 0.5-1 mm lateral from the midline, the negativity being found maximal at the depth 0.5-1 mm from the surface. The surface-recorded N2 potential evoked by stimulation of the medial branch distributed caudal to that produced by stimulation of the lateral branch. Of intramedullary-recorded 4 negative waves, only N1 caused by volleys in the lateral branch distributed deeper layer than the one evoked by those in the other branch.