The location of cardiac vagal preganglionic motoneurones in the medulla of the cat.

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Abstract
Electrophysiological techniques were used to locate the origin of preganglionic vagal motoneurons supplying the heart of the cat. The right cardiac vagal branches were identified anatomically, and their ability to slow the heart was assessed by electrical stimulation. Control experiments revealed that contamination of cardiac branches by bronchomotor and esophageal efferent fibers was likely to be small. Neurons (57) in the medulla were activated antidromically on stimulating the cardiac branches at up to 5 times the threshold for cardiac slowing. They had axons with conduction velocities between 3-15 m/s, corresponding to B fibers. None of these were located in the region of the dorsal motor nucleus of the vagus, in spite of repeated sampling there, but all were located in the region of the nucleus ambiguus. Histological examination of marked neurons (46 of 57 neurons) revealed that they were associated with its principal column, rostral to the obex. Sampling motoneurons of the dorsal motor nucleus revealed that most sent axons down the thoracic vagus below the cardiac branches. Only 3 of 33 could be activated antidromically by high intensity stimulation of the cardiac branches, but on the basis of their thresholds and conduction velocities, it is argued that they were unlikely to be cardio-inhibitory neurons. Preganglionic cardio-inhibitory neurons apparently arise not in the dorsal motor nucleus, but in the principal column of the nucleus ambiguus.