Cardiac and non-cardiac preganglionic neurones of the thoracic vagus nerve: an HRP study in the cat.

Using the HRP method, we studied cardiac and non-cardiac preganglionic neurones of the thoracic vagus nerve in the cat. The inferior cardiac branches of the right thoracic vagus nerve were differentiated from the neighboring non-cardiac branches by positive cardiac slowing following electrical stimulation. A small amount of crystalline HRP was placed at the central cut end of the cardiac or non-cardiac branches. After a survival period of 2 days, the medulla oblongata, the stellate ganglion (SG), the superior cervical (SCG) and the nodose ganglion (NG) were serially sectioned and processed for histochemical demonstration of HRP. In the medulla oblongata, HRP-labeled cells were mainly located in 2 regions: (1) within and around the ambiguous complex (site A); (2) within the dorsal vagus nucleus (site X). After applying HRP to the cardiac branches, 968 HRP-labeled cells were observed in 7 animals bilaterally with the ipsilateral dominance (94%); 81% of them were found in site A. After applying HRP to the non-cardiac branches, 447 HRP-labeled cells were seen in 2 animals bilaterally with the ipsilateral dominance (92%); 79% of them were found in site X. Many HRP-labeled cells were found in the SG and NG, indicating that the cardiac branches are composed of sensory afferent and sympathetic efferent fibres as well as cardiac vagus efferent fibres.