The effects of electrical stimulation of myelinated and non-myelinated vagal fibres on heart rate in the rabbit.

The bradycardia evoked by electrical stimulation of the peripheral end of the rabbit vagus nerve is mediated by both myelinated and non-myelinated fibres. Stimulation of myelinated fibres at 5 Hz for 20 s resulted in a fall in heart rate from a control value of 204 +/- 1.3 to 182 +/- 1.2 beats min-1 (means + S.E. of means). On stimulation of both myelinated and non-myelinated fibres, heart rate fell to 169 +/- 2.1 beats min-1. Heart rate returned rapidly to control value following stimulation of myelinated fibres but only slowly after stimulation of both myelinated and non-myelinated fibres. Thus recruitment of non-myelinated fibres increased both the magnitude and duration of the bradycardia. Atropine abolished all effects of vagal stimulation on heart rate. Hexamethonium abolished the effect of myelinated fibres on heart rate; however, recruitment of non-myelinated fibres still produced a bradycardia. It is suggested that the prolonged effect following stimulation of non-myelinated fibres may reflect the persistent action of a non-cholinergic excitatory transmitter at the ganglion.