CARDIOTOPIC ORGANIZATION OF THE NUCLEUS AMBIGUUS - AN ANATOMICAL AND PHYSIOLOGICAL ANALYSIS OF NEURONS REGULATING ATRIOVENTRICULAR-CONDUCTION

Previous data indicate that there are anatomically segregated and phys iologically independent parasympathetic postganglionic vagal motoneuro ns on the surface of the heart which are capable of selective control of sinoatrial rate, atrioventricular conduction and atrial contractili ty. We have injected a retrograde tracer into the cardiac ganglion whi ch selectively regulates atrioventricular conduction (the AV ganglion) . Medullary tissues were processed for the histochemical detection of retrogradely labeled neurons by light and electron microscopic methods . Negative dromotropic retrogradely labeled cells were found in a long column in the ventrolateral nucleus ambiguus (NA-VL), which enlarged somewhat at the level of the area postrema, but reached its largest si ze rostral to the area postrema in an area termed the rostral ventrola teral nucleus ambiguus (rNA-VL). Three times as many cells were observ ed in the left rNA-VL as compared to the right (P < 0.025). Retrograde ly labeled cells were also consistantly observed in the dorsal motor n ucleus of the vagus (DMV). The DMV contained one third as many cells a s the NA-VL. The right DMV contained twice as many cells as the left ( P < 0.05). These data are consistent with physiological evidence that suggests that the left vagus nerve is dominant in the regulation of AV conduction, but that the right vagus nerve is also influential. While recording the electrocardiogram in paced and non-paced hearts, L-glut amate (GLU) was microinjected into the rNA-VL. Microinjections of GLU caused a 76% decrease in the rate of atrioventricular (AV) conduction (P < 0.05) and occasional second degree heart block, without changing heart rate. The effects of GLU were abolished by ipsilateral cervical vagotomy. These physiological data therefore support the anatomical in ference that CNS neurons that are retrogradely labeled from the AV gan glion selectively exhibit negative dromotropic properties. Retrogradel y labeled negative dromotropic neurons displayed a round nucleus with ample cytoplasm, abundant rough endoplasmic reticulum and the presence of distinctive somatic and dendritic spines. These neurons received s ynapses from afferent terminals containing small pleomorphic vesicles and large dense core vesicles. These terminals made both asymmetric an d symmetric contacts with negative dromotropic dendrites and perikarya , respectively. In conclusion, the data presented indicate that there is a cardiotopic organization of ultrastructurally distinctive negativ e dromotropic neurons in the NA-VL. This central organization of paras ympathetic preganglionic vagal motoneurons mirrors the functional orga nization of cardioinhibitory postganglionic neurons of the peripheral vagus nerve. These data are further discussed in comparison to a recen t report on the light microscopic distribution and ultrastructural cha racteristics of negative chronotropic neurons in the NA-VL(42). The da ta support the hypothesis that anatomically separated and functionally selective parasympathetic preganglionic vagal motoneurons in the NA m ay independently control AV conduction and cardiac rate.