Vagal and Sympathetic Effects on the Pacemaker Fibers in the Sinus Venosus of the Heart* by Otto

The response of excitable cells to inhibitory nerve impulses has already been studied with intracellular electrodes in several preparations. In the spinal cord of the cat an inhibitory volley sets up a transient polarization of the motoneuron, preventing it from discharging (Brock, Coombs, and Eccles (4)). In the crustacean nerve muscle junction inhibitory impulses have a dual effect. They attenuate the amplitude of the local excitatory potentials and produce a potential change directed towards the resting level, whenever the membrane potential is displaced by applied currents (Fatt and Katz (13)). Mechanisms of inhibition have also been studied recently in isolated nerve cells of crayfish and lobster by Kumer and Eyzaguirre (22). The inhibitory impulses "drive" the cell membrane potential toward an equilibrium level and may produce a polarizing or depolarizing effect, according to the state of the cell membrane. The study, by electrical recording, of the nervous inhibition of the heart goes back to Gaskell (15), who found that the demarcation potential in the tortoise auricle was increased by stimulation of the vagus. More recently Burgen and Terroux (5) measured with intracellular electrodes a polarization in cat auricle on application of acetylcholine, and they present some evidence that this effect may be in part due to an increase in the permeability of the membrane to potassium ions. In the present paper an attempt is made to record the effect of vagal and of sympathetic stimulation on the fibers Of the sinus venosus. This seemed to be of interest since it has been found that the pacemaker fibers in the sinus venosus differ from auricle fibers in showing a slow depolarization during diastole (Trautwein and Zink (25)). I t will be shown that on vagal stimulation the development of this slow depolarization is suppressed and that the membrane potential in the inhibited sinus is driven towards a resting level. Action potentials set up by direct shocks during vaga] inhibition are also affected and block of conduction often occurs.