Axonal branching of canine sympathetic postganglionic cardiopulmonary neurons. A retrograde fluorescent labeling...

In 11 dogs fluorescent retrograde tracers were injected into physiologically identified left-sided sympathetic cardiopulmonary nerves. When two different ipsilateral cardiopulmonary nerves were injected, labeled cells from each injected nerve had overlapping distributions in the middle cervical and stellate ganglia. Most retrogradely labeled neurons were located in the middle cervical ganglion and cranial pole of the stellate ganglion. Following the injection of two different tracers into two different nerves, some neurons in the middle cervical ganglion were retrogradely labeled with two tracers. Double-labeled neurons were rarely found in the stellate ganglion. There were areas within the ganglia in which labeled neurons projected predominantly to one cardiopulmonary nerve. In the thoracic autonomic nervous system Fast Blue was transported most effectively. Bisbenzimide was not transported as well as Fast Blue and Nuclear Yellow was very poorly transported in cardiopulmonary nerves. The results demonstrate that some efferent postganglionic sympathetic neurons project axons into at least two different cardiopulmonary nerves and that an anatomical substrate for axo-axonal reflexes exists in the thoracic sympathetic nervous system.