Dynamics of heart rate response to sympathetic nerve stimulation

Electrical stimulation of the right cardiac sympathetic nerve was used to achieve a step increase of norepinephrine concentration at the sinus node. The heart rate (HR) response to sympathetic stimulation was characterized by a first-order process with a time delay. For moderate to high intensities of stimulation the mean delay and time constant were 0.7 and 2.1 s, respectively, and for low intensities of stimulation they were 0.4 and 1.1 s, respectively. From the analysis of the HR response to different patterns of nerve stimulation, in vivo neurotransmitter kinetics were estimated. The time constant of norepinephrine dissipation averaged approximately 9 s. These results combined with computer simulations revealed two facets of sympathetic neural control of HR: 1) negligible role of the sympathetic system in beat-to-beat regulation of HR under stationary conditions and 2) ability of HR to react relatively quickly (within a few seconds) to sharp increases in sympathetic nerve traffic.