title: The effect of selective electrical stimulation of non-myelinated vagal fibres on heart rate in the rabbit.
abstract: The bradycardia evoked by electrical stimulation of the peripheral cut end of the rabbit vagus nerve is mediated by both myelinated and non-myelinated fibres. The purpose of this study was to assess the effects of non-myelinated fibres on heart rate in the rabbit using selective electrical stimulation techniques. In 8 rabbits selective activation of non-myelinated fibres using reversed polarity triangular shaped pulses (10 Hz, 20 s), resulted in a slowly developing fall in heart rate of 24.1 +/- 1.1 beats/min which outlasted the period of stimulation by 58.4 +/- 4.2 s. In 4 rabbits stimulation of myelinated fibres at 10 Hz for 20 s resulted in a fall in heart rate of 24.5 +/- 2.6 beats/min. On stimulation of both myelinated and non-myelinated fibres heart rate fell by 39.9 +/- 3.2 beats/min. Heart rate returned rapidly to control value following stimulation of myelinated fibres (5.6 +/- 0.5 s) but only slowly after stimulation of both myelinated and non-myelinated fibres (56.7 +/- 4.9 s). Atropine (5 mg/kg, i.v.) abolished all effects of vagal stimulation on heart rate. Hexamethonium (15 mg/kg, i.v.) abolished the effect of myelinated fibres on heart rate but did not affect the fall in heart rate produced by non-myelinated fibres. We suggest that the prolonged effects on stimulation of non-myelinated fibres may reflect the persistent action of a non-cholinergic excitatory transmitter at the cardiac parasympathetic ganglia.
mesh_terms: D000818:Animals; D001285:Atropine / Q000494:pharmacology; D004558:Electric Stimulation; D005071:Evoked Potentials; D006339:Heart Rate / Q000187:drug effects; D018738:Hexamethonium; D006584:Hexamethonium Compounds / Q000494:pharmacology; D009412:Nerve Fibers / Q000502:physiology*; D011817:Rabbits; D014630:Vagus Nerve / Q000502:physiology*
chemical_list: D006584:Hexamethonium Compounds; D018738:Hexamethonium; D001285:Atropine