Vagus Stimulation and Rate Changes in the Turtle Heart

Abstract

Observations previously reported 1 showed that stimulation of either vagus nerve of a turtle might be adjusted to produce some slowing of the rate of the sinus beat although only a part, if any, of the sinus impulses were conducted to the atria and ventricle. Further investigation of vagus stimulation in producing these phenomena are presented here.

In many preparations either the right or left vagus may be stimulated to produce atrial inotropic depression of the atria with no chronotropic effects. Only rarely is it impossible to do this by stimulation of the left vagus. With the right vagus the separation of effects is rarely as pronounced as with the left.

If right vagus stimulation of strength just sufficient to produce definite chronotropic changes be used, continued stimulation usually produces inotropic depression of the sinus and slowing to a rate somewhat more than half the normal rate. Frequently the pacemaker, normally on the right side, then shifts to an abnormal locus, usually to a region in the left side of the sinus symmetrical with the normal pacemaker. This beat may be propagated so that the atria and ventricle respond regularly or with occasional dropped beats. With somewhat stronger stimulation the aberrant pacemaker may continue active but sino-atrial block prevents propagation to the atria and ventricle.

With left vagus stimulation, the interpretation of records because of the aberrant pacemaker is avoided and the chronotropic effect is much reduced. In most preparations, the left vagus contributes but few fibers to the normal pacemaker region. Evidence for this is, for example, the fact that in the majority of preparations, low stimulation rates (ca. 5 per sec.) produce little or no slowing of the heart and no visible inotropic depression of the sinus at any strength of stimulation used.