Dynamic interactions between the sympathetic and vagal systems in regulating heart rate

Complex interactions exist between the sympathetic and vagal systems in regulating heart rate. We earlier reported that activation of either one of the sympathetic and vagal nerves augments the dynamic heart rate response to concurrent stimulation of its counterpart. We attributed these interactions to the assumed sigmoidal nonlinearity between autonomic activity and heart rate. To validate this hypothesis, we stimulated the sympathetic and vagal nerves according to Gaussian white noise signals. The stimulation frequencies were chosen so that we could identify the system characteristics over the entire operating range of the heart rate regulation. A neural network analysis revealed nonlinear sigmoidal relationship between the autonomic nervous activity and heart rate. Because of this nonlinearity, we conclude that changes in sympathetic or vagal tone alone can alter the dynamic heart response to stimulation of the other nerve. These results suggest that a high frequency component of the heart rate variability appears to reflect not only vagal activity but also sympathetic activity through the interactions between the sympathetic and vagal systems