Two types of vagal preganglionic motoneurones projecting to the heart and lungs

Abstract
Cells (84) in the cat''s nucleus ambiguus whose axons projected to the cardiac (74) and pulmonary (10) branches of the right vagus were studied. Their axonal conduction velocities were all in the range of B fibers (2.8-15.5 m/s). Pulmonary branch-projecting neurons were usually spontaneously active (9/10) and fired in phase with inspiration. Their activity showed no pulse modulation. Cardiac branch-projecting neurons (10) had properties indistinguishable from those of pulmonary branch-projecting neurons. Inspiratory-firing cells projecting to either branch are believed to be bronchoconstrictor in function. The remaining 64 cells that projected to the cardiac branch had properties expected of cardioinhibitory neurons. Most (54) were silent until activated by ionophoresis of excitant amino acids. All showed an expiratory discharge when active and of 27 tested, 23 showed a cardiac modulation of their discharge. When the aortic baroreceptors were denervated, the cardiac rhythm was always abolished reversibly by carotid occlusion. Ionophoretic activation of expiratory-firing (presumed cardioinhibitory) cells slowed the heart (15/18 neurons tested). Excited inspiratory-firing cells never had this effect (11 tested). Both types of neurons were found in the nucleus ambiguus, but presumed cardioinhibitory cells were often found more caudal and ventral than presumed bronchoconstrictor neurons.