Closed loop model of vagally-mediated baroreflex control of heart rate

Vagal control of the heart is the most rapidly acting mechanism in the baroreceptor reflex. We present here a computational model of the baroreceptor vagal control of heart rate which incorporates physiologically plausible representations of both the peripheral cardiovascular system, including ventricular-vascular coupling, and central neuronal mechanisms, including NTS neurons and cardiac vagal motoneurons which incorporate a variety of ionic currents. The cardiovascular and neural systems are coupled on the afferent and efferent sides by the baroreceptors and by the vagal control of the SA node respectively. The complete system shows a number of interesting responses to various experimental perturbations. The open loop relationship between carotid sinus pressure and heart rate is realistic. At high heart rates the cardiac output and arterial pressure saturate, becoming independent of rate. Electrical stimulation of the vagus nerve can entrain the heart over limited frequency ranges, due to the phase-dependent effects of the vagal impulses on the SA node. When the reflex loop is closed (arterial pressure=carotid sinus pressure) the system is essentially stable but shows small apparently chaotic beat to beat variability in rate and pressure which disappear if the reflex is interrupted.