The distribution in the cat brain stem of neurones activated by vagal nonmyelinated fibres from the heart and lungs.

In anaesthetized cats, right cardiac vagal branches were electrically stimulated and recordings of evoked 'slow wave' and single neurone activity were made in the brain stem. Short-latency 'slow wave' and multi-neuronal activity evoked by excitation of myelinated vagal afferent fibres were recorded in the medial and lateral subnuclei of the nucleus tractus solitarius, the area postrema, the dorsal vagal motor nucleus, the lateral reticular formation and the nucleus ambiguus. Long-latency responses evoked by vagal non-myelinated fibres were recorded in the medial subnucleus of the nucleus tractus solitarius, the area postrema, dorsal vagal motor nucleus, the parahypoglossal area and the lateral reticular formation dorsal to the nucleus ambiguus. A specific study was made of seventy-two single neurones activated by non-myelinated afferent fibres in the cardiac branch. Thirty-four were shown to be synaptically activated, twenty-one were activated nonsynaptically and seventeen could not be classified. One neurone was also activated by myelinated cardiac afferent fibres, and two by thoracic vagal (including pulmonary) afferent fibres. Neurones were not spontaneously active. Indirect evidence suggests that the majority of the recordings of nonsynaptically activated neurones were likely to be from cell bodies. Neurones were located from the level of the obex to 3.0 mm rostral to it in the medial subnucleus of the nucleus tractus solitarius (45), and in the lateral subnucleus (2), the area postrema and its border with the medial subnucleus of the nucleus tractus solitarius (13), the dorsal vagal motor nucleus (9), the parahypoglossal area (1) and the lateral reticular formation dorsal to nucleus ambiguus (2). Recordings were made from fifteen neurones activated by myelinated fibres in the cardiac vagal branches, and twelve were excited synaptically. The neurones were located in the medial (8) and lateral (3) subnuclei of the nucleus tractus solitarius, the dorsal vagal motor nucleus (1) and the lateral reticular formation (1). Four neurones were also excited by vagal afferent fibres in the thoracic vagal nerve immediately caudal to the caudal cardiac branch.