BRAIN-STEM ORIGIN OF PREGANGLIONIC CARDIAC MOTONEURONS IN THE MUSKRAT

The muskrat, an aquatic rodent with a brisk and reliable diving respon se, shows a remarkable bradycardia after nasal stimulation. However, t he medullary origin of cardiac preganglionic motoneurons is unknown in this species. We injected fat pads near the base of the heart of musk rats with a WGA-HRP solution to label retrogradely preganglionic paras ympathetic neurons that project to the cardiac plexi. Results showed t hat 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 ex ternal formation of the nucleus ambiguus, 5.6% were in the lateral ext reme of the dorsal motor nucleus of the vagus nerve and 5.3% were foun d in the intermediate area in between these two nuclei. Although contr oversy still exists concerning the medullary origin of preganglionic c ardiac motoneurons, our results from muskrats agree with those from mo st other species where preganglionic cardiac motoneurons were located just ventral to the nucleus ambiguus.