Central distribution of the cervical vagus nerve in Old and New World primates

The central distribution of the cervical vagus nerve was examined in Old and New World primates using anterograde transganglionic and retrograde horseradish peroxide (HRP) histochemistry. Crystals of HRP were applied to the cut central end of the cervical vagus nerve in two Old World (one bonnet, one cynomolgus) and two New World (squirrel) monkeys. Bright- and darkfield examination of coronal sections from the pons, medulla, and upper cervical spinal cord revealed two major concentrations of retrogradely labeled cells in the ipsilateral dorsal motor nucleus (DMX) and nucleus ambiguous (NA). DMX was heavily labeled, containing about 5 times as many labeled cells as NA. The anterograde distribution of reaction product did not extend as far in the rostrocaudal plane as did the retrograde distribution. Labeled afferent fibers entered the medulla at the level of the caudal dorsal cochlear nucleus, joined the solitary tract, and descended to the obex. Ipsilateral terminal label first appeared at the level where the nucleus of the solitary tract (NST) abuts the IVth ventricle. The terminal field grew in extent and density, until at the level of the area postrema (AP), the distribution extended throughout the medial NST, ventrolateral NST, and AP. Contralateral terminal label was sparse and restricted to the medial NST. In the commissural division of the solitary nucleus, sparse reaction product was present bilaterally, with the denser concentration ipsilateral to the treated nerve. Examination of peripheral ganglia revealed labeled somata in the nodose, jugular, and superior cervical ganglia.