Aortic nerve activation: effect of stimulus train composition on heart rate.

The central end of the sectioned left aortic nerve was stimulated within one heart cycle by a train of electrical pulses, producing a reflex reduction in heart rate in the anesthetized rabbit. Relative importance of the stimulation variables (train duration, pulse frequency, and pulse number) in producing bradycardia was investigated by maintaining one variable constant, while altering the other two over a wide range. At each fixed pulse frequency, maximal bradycardia was obtained by the longest train duration that corresponded with the greatest pulse number. At each fixed train duration, maximal bradycardia was achieved with pulse frequencies between 80--120 Hz and remained maximal up to 160 Hz. Stimulating over wide variations in train duration and pulse frequency at any fixed pulse number produced no significant differences in heart rate reduction at that pulse number. These results indicate the dominant importance of total pulse number per cardiac cycle in determining the magnitude of the heart rate response and reveal an apparent frequency limitation within the reflex.