Brainstem origin of preganglionic cardiac motoneurons in the muskrat

The muskrat, and aquatic rodent with a brisk and reliable diving response, shows a remarkable bradycardia after nasal stimulation. However, the medullary origin of cardiac preganglionic motoneurons is unknown in this species. We injected fat pads near the base of the heart of muskrats with a WGA-HRP solution to label retrogradely preganglionic parasympathetic neurons that project to the cardiac plexi. Results showed that the preponderance of labeled neurons was in ventrolateral parts of the medulla from 1.5 mm caudal to the obex to 2.0 mm rostral. Eighty-nine percent of the labeled neurons were located bilaterally in the external formation of the nucleus ambiguus, 5.6% were in the lateral extreme of the dorsal motor nucleus of the vagus nerve and 5.3% were found in the intermediate area in between these two nuclei. Although controversy still exists concerning the medullary origin of preganglionic cardiac motoneurons, our results from muskrats agree with those from most other species where preganglionic cardiac motoneurons were located just ventral to the nucleus ambiguus.