CENTRAL ORIGIN OF ASYMMETRY IN THE CAROTID SINUS REFLEX *

It is known that the carotid sinus reflex mechanism evokes asymmetrical responses to rising and falling blood pressures. I n the open-loop preparation, a sinusoidal excitation at the isolated carotid sinus results in a fluctuating arterial pressure, which rises more slowly than it falls;’ a square-wave excitation has also been shown to produce an asymmetrical The most conspicuous manifestation of asymmetry is that both a short positive and a negative pulse in the carotid sinus pressure cause a transient decrease in the arterial pressure, indicating a type of nonlinearity that has been called “unidirectional rate ~ens i t iv i ty .”~’~ In the closed-loop preparation, it has been demonstrated that a rise of blood pressure is more effective in slowing the heart than a similar drop of blood pressure is in accelerating it.6 It has been suggested that the known rate-sensitive properties of the baroreceptors may at least partially account for these a symmet r i e~ .~ ’~ Nonlinearity in the neural control of heart rate has also been demon~trated,~ but it appears that the asymmetry in the heart-rate reflex cannot be completely explained by the known nonlinear properties of the heart and/or the baroreceptors.6 The purpose of this paper is to present evidence, obtained by recording the activity of cardiac vagal efferent fibers during transient pressure disturbances, that the central nervous system introduces additional asymmetry to the heart-rate reflex. 2