Descending pressor pathways from the dorsomedial and ventrolateral medulla of cats

Descending pathways of the pressor areas in dorsomedial (DM) and ventrolateral (VLM) medulla were studied physiologically and anatomically. The electrically induced pressor response from either DM or VLM that previously responded to glutamate (Glu) was slightly but not significantly decreased after splitting the medulla at midline. After electrolytic lesion in the ipsilateral lateral tegmental field (LTF) of caudal medulla, 4 mm caudal to the obex, electrically induced pressor responses from DM and VLM previously responded to Glu decreased significantly from 71% to 20% and from 57% to 7%, respectively. The dorsomedial LTF was found to be responsible for the DM responses, while the ventrolateral LTF for the VLM responses. Following electrolytic lesion in the ipsilateral dorsolateral funiculus (DLF) between C4 and C5, the magnitude of DM-induced pressor responses decreased significantly from 74% to 37% and that of the VLM from 75% to 14%. After electrolytic lesion at the ipsilateral intermediolateral cell column (IML) of T2 spinal cord, the pressor responses on DM and VLM stimulations also decreased significantly. Lesioning the remaining IML almost completely abolished the induced pressor responses. Seven to ten days following kainic acid injections into DM or VLM to destroy neurons therein, degenerative fibers (silver impregnation) were found to descend from the medulla to thoracic spinal cord, principally ipsilaterally, but with some fibers crossing to the controlateral medulla. Data from both physiological and anatomical studies suggest that fibers from DM and VLM descend ipsilaterally through different pathways and converge at LTF 4 min caudal to the obex, and then further descend to DLF before reaching the thoracic IML.