CAN NEURONS IN THE NUCLEUS AMBIGUUS SELECTIVELY REGULATE CARDIAC RATEAND ATRIOVENTRICULAR-CONDUCTION

Previous anatomic data have described the distribution of presumptive negative chronotropic and negative dromotropic neurons in the ventro-l ateral nucleus ambiguus (NA-VL) following injections of retrograde tra cers into physiologically selective parasympathetic intracardiac gangl ia, Negative dromotropic neurons were preferentially distributed in th e rostral NA-VL (rNA-VL). Negative chronotropic neurons were preferent ially distributed in the caudal NA-VL (cNA-VL). Significant numbers of both types of cardio-inhibitory neurons were observed to overlap in a n intermediate level of the NA-VL (iNA-VL). In the present report, we have examined the effects of microinjections of the excitatory amino-a cid glutamate (GLU) into the cNA-VL and iNA-VL on cardiac rate and AV conduction while recording the electrocardiogram in paced and non-pace d cat hearts. The data indicate that: (i) excitation of neurons in the cNA-VL causes a 58 +/- 17% reduction in cardiac rate, without influen cing AV conduction; and (ii) excitation of neurons in the iNA-VL cause s both a reduction in heart rate (68 +/- 12%) and a decrease in the ra te of AV conduction (38 +/- 7%), These physiological results support t he anatomical inference that neurons in the cNA-VL that are retrograde ly labeled from physiologically selective parasympathetic intracardiac ganglia selectively exhibit negative chronotropic properties. Further more, the data indicate that there is a longitudinal cardiotopic organ ization of both negative chronotropic and negative dromotropic neurons in the NA-VL. This CNS organization mirrors the peripheral organizati on of functionally selective cardiac components of the vagus nerve, Fi nally, the data are consistent with the hypothesis that anatomically s eparated and functionally selective parasympathetic preganglionic vaga l motoneurons in the NA independently control cardiac rate and AV cond uction.