The actions of myelinated and non-myelinated vagal fibres on atrial contraction in the rabbit.

The bradycardia and reduction in atrial contraction on electrical stimulation of the peripheral cut end of the vagus nerve in the cat have been attributed solely to the action of myelinated fibres. In the rabbit however, recruitment of non-myelinated fibres produces an additional prolonged bradycardia. The objective of this investigation was to examine the relative contributions of myelinated and non-myelinated fibres to the vagal action on atrial contraction in the rabbit. In 5 rabbits stimulation of myelinated fibres at 10 Hz for 20 s reduced developed atrial tension of the paced heart by 17.1 +/- 2.6%. After stimulation, atrial tension returned rapidly to the control value in 6.5 +/- 0.5 s. Recruitment of non-myelinated fibres resulted in a reduction in developed atrial tension of 29.1 +/- 2.8%. This larger fall in atrial tension outlasted the period of stimulation by 34.1 +/- 4.8 s. Hexamethonium blocked the reduction in atrial contraction due to myelinated fibres but not that due to non-myelinated fibres. Atropine abolished all effects of vagal stimulation on atrial contraction. We conclude that the recruitment of non-myelinated fibres produces an additional prolonged reduction in atrial contraction which is resistant to hexamethonium but blocked by atropine.