PACEMAKER POTENTIAL AND C-FIBRE'S IMPULSES EVOKED BY STIMULI OF THE VAGUS NERVE

Abstract

The relationship between pacemaker potential and C-fibre's potential evoked by stimuli of the vagus nerve was studied intracellularly in the frog's sinus venosus. In all experiments the heart of Rana catesbeiana was excised together with the vagosympathetic nerve trunk. The ventricle and a large part of the auricles were cut away and the sinus venosus was pinned on wax in a bath with dorsal wall underneath. Vagal cardiac branches entering along the venae cavae superiores were dissected 5 mm to the point at which the division of pulmonary and cardiac branch could be seen (50μ or less in diameter). The extracellular recording electrodes were applied there. On the other hand the pacemaker potential was recorded intracellularly from the sinus venosus through a glass microelectrode.The following results were obtained.1) There were three major groups of nerve fibres as judged by conduction velocity in the cardiac nerve branch.2) The fast groups vary in conduction rate from 20 m per sec. to approximately 8 m per sec., may be regarded as B fibre. 3) The slow groups vary in conduction rate from 2 m per sec. to approximately 0.3 m per sec., regarded as C-fibre.4) The other groups had an intermediate velocity between. them, regarded as B2 fibre.5) The fast groups had a lower threshold and the slow and intermediate groups had a higher threshold.6) The post tetanic hyperpolarization (PTH) in the C-fibre showed the maximum value in stimulus duration between 30 and 75msec. It agreed very closely with that of hyperpolarization of pacemaker.7) Both hyperpolarization of C fibre and pacemaker cell were inhibited by K and ouabain.From these results it could be presumed that spontaneous activity in pacemaker cell was similar to activation of electrogenic sodium pump of C fibre in the PTH.