Origin and course of crossed medullary pathways to spinal sympathetic neurons in the cat

In 12 chloralosed vagotomized cats medullary structures projecting to contralateral spinal sympathetic neurons were identified by activating fibre terminals in the intermediolateral nucleus (ILN) at functionally and histologically identified cardioacceleratory sites at the T2 level and exploring the contralateral medulla for antidromically evoked field potentials.Evoked field potentials were recorded at 20 sites in 130 penetrations in the left medulla and at 14 sites in 70 penetrations in the right medulla; responses were recorded mainly from sites in the nucleus gigantocellularis, the nucleus paramedium reticularis and the nucleus lateralis reticularis.In an additional 20 cats the pathways of crossed fibres from these nuclei to the ILN were identified by observing the effects on the evoked field potentials of surgical and electrolytic lesions in the medulla and spinal cord. Evoked potentials from the left nucleus gigantocellularis were abolished by a midline section in the caudal medulla, or by right medullary hemisection (5 mm caudal to the obex), or by transection of the right ventral funiculus (C6–C7). Potentials from the left nucleus paramedium reticularis were abolished by right medullary hemisection (5 mm caudal to the obex) or by transection of the right ventral funiculus (C5–C7). Potentials from the left nucleus lateralis reticularis were abolished by left spinal hemisection 2–3 mm rostral to the level of the stimulating electrode or by transection of either the left dorsolateral or the left ventral funiculus (C5–C7).It is concluded that: a) spinal sympathetic neurons receive inputs from discrete nuclei in the contralateral medulla; b) autonomic fibres from the nucleus gigantocellularis and the nucleus paramedium reticularis to the contralateral ILN cross the midline in the caudal medulla and descend in the contralateral ventral funiculus; c) fibres from the nucleus lateralis reticularis descend in the ipsilateral dorsolateral and ventral funiculi and cross the midline in the spinal cord close to their level of termination.