Pump neurons of the nucleus of the solitary tract project widely to the medulla.

Axonal projections of pump neurons (P-cells) of the nucleus of the solitary tract were studied in Nembutal-anesthetized, paralyzed, and artificially ventilated cats. Extracellular recordings were made from single P-cells which were monosynaptically activated by electrical stimulation of the ipsilateral vagal afferents and distributed in close proximity to the solitary tract. Antidromic stimulation of the medulla showed that all P-cells examined projected their axons to the ipsilateral ventrolateral medulla. Axonal arborizations were found in respiration-related areas and their vicinity between the level of the retrofacial nucleus and the level a few millimeters caudal to the obex. Some of these P-cells sent axon collaterals to the contralateral nucleus of the solitary tract (NTS) as well. This study is the first to show wide spread medullary projections of P-cell axons.