Bidirectional augmentation of heart rate regulation by autonomic nervous system in rabbits

Although the characteristics of the static interaction between the sympathetic and parasympathetic nervous Systems in regulating heart rate (HR) have been well established, how the dynamic interaction modulates the HR response remains unknown. We therefore investigated dynamic interaction by estimating the transfer function from nerve stimulation to HR using a band-limited Gaussian white-noise technique. The transfer function relating dynamic sympathetic stimulation to HR had characteristics of a second-order low-pass filter. Simultaneous tonic vagal stimulation at 5 and 10 Hz increased gain of the transfer function by 55.0 +/- 40.1 and 80.7 +/- 50.5%, respectively (P < 0.05). The transfer function from dynamic vagal stimulation to HR had characteristics of a first-order low-pass filter. Simultaneous tonic sympathetic stimulation at 5 and 10 Hz increased the gain by 18.2 +/- 17.9 and 24.1 +/- 18.0%, respectively (P < 0.05). Thus interaction augmented dynamic gain bidirectionally, even though it affected mean HR antagonistically. By virtue of this interaction, the autonomic nervous system appears to extend its dynamic range of operation.