Attenuation of the negative chronotropic effect of myelinated fibers by nonmyelinated fibers in the vagus nerve of the rabbit.

In the adult rabbit, both myelinated and nonmyelinated efferent fibers travel from the cervical vagus nerve to the sinoauricular node. The myelinated fibers convey the negative chronotropic effect. The influence of the nonmyelinated fibers was studied. Stimulation (4/sec) of the myelinated fibers, while the compound action potential was monitored continuously, caused a slowing of the heart rate. Subsequent activation of the non-myelinated fibers attenuated this negative chronotropic effect. Stimulation of adrenergic fibers was excluded, based on data from histochemical studies and the effects of blockade of beta- and muscarinic receptors. The lengthening of the RR interval during activation of the myelinated fibers alone was about 3.5 times larger than during activation of all fiber groups. When the conduction of the myelinated fibers was blocked (anodal block), activation of the nonmyelinated fibers did not show any effect on heart rate. It was concluded that the negative chronotropic effect is conducted via small myelinated fibers and that the nonmyelinated fibers modulate the effect of these cardiomotor fibers, without having an effect on heart rate on their own. This modulation is effected either via a presynaptic mechanism on preganglionic fibers or via an interaction with postganglionic neurons. The nonmyelinated fibers constitute a novel peripheral system to modulate heart rate.