The terminations and secondary projections of myelinated and non-myelinated fibres of the aortic nerve in the cat.

In anaesthetized cats the projection of myelinated and non-myelinated afferent fibres of the aortic nerve to the brain stem was studied by recording antidromic potentials evoked in the nerve by electrical stimulation of the medulla oblongata. Probable sites of termination of both myelinated and non-myelinated afferent fibres were found to be in the ipsilateral medial lateral subnucleus of the nucleus tractus solitarius, the ipsilateral area postrema, the commissural nucleus and the contralateral nucleus tractus solitarius about the level of the obex. No evidence for a projection of primary afferent fibres in the aortic nerve to the medial reticular formation was found. In a second series of experiments the aortic nerve was electrically stimulated and extracellular recordings were made from neurones in the medulla activated by afferent fibres in this nerve. Neurones activated by myelinated afferent fibres were located in the nucleus tractus solitarius, dorsal vagal motor nucleus, commissural nucleus, area postrema and lateral reticular formation including nucleus ambiguus. Neurones activated by non-myelinated afferent fibres were in or near the medial nucleus tractus solitarius, the area postrema, dorsal motor vagal nucleus and nucleus intercalatus. Few neurones appeared to be activated by both myelinated and non-myelinated afferent fibres and they were located in the medial nucleus tractus solitarius and nucleus ambiguus.