Insignificant bilateral convergence of preganglionic vagal fibers on postganglionic neurons to the canine heart.

We determined the extent of convergence of preganglionic fibers from the right and left vagus nerves on postganglionic neurons that supply the sinoatrial node in chloralose-anesthetized dogs. We administered hemicholinium-3 and stimulated the right vagus nerve at a high frequency to deplete acetylcholine from the postganglionic parasympathetic neurons supplied by that nerve. We compared the effects of this "depletion regimen" with the responses in two control groups: a stimulation control group, which was subjected to high-frequency right vagus stimulation only, and a drug control group, which received a hemicholinium-3 infusion only. The effects of right vagus stimulation did not differ from those of left vagus stimulation in either of the control groups. In the animals subjected to the depletion regimen, the responses to right vagus stimulation were almost abolished. However, the left vagus nerve retained its ability to prolong cardiac cycle length in these animals. Thus, our experiments indicate that left vagus preganglionic fibers do not converge with right vagus preganglionic fibers on a substantial pool of postganglionic neurons that innervate the canine sinoatrial node.