Selective innervation of upper and lower thoracic spinal segments by medullary raphe neurons in felines

Abstract

The medullary raphe nuclei mediate a variety of physiological responses, including the regulation of blood pressure. We have demonstrated that vestibular stimulation results in distinct changes in blood flow in the upper and lower body, indicating that the nervous system has the capacity to elicit regionally‐specific changes in blood flow. Prior physiological studies have shown that neurons in the rostral ventrolateral medulla (RVLM) are not responsible for this response patterning. In the present study, we ascertained whether neurons in the medullary raphe nuclei have the capacity to independently regulate sympathetic outflow to different body regions. For this purpose, multiple injections of the fluorescent dye Fast Blue were made into the vicinity of the intermediolateral cell column (IML) in T4, whereas multiple injections of Fluoro‐Ruby were made near the IML in T10. Retrogradely labeled neurons were mapped in the medullary raphe nuclei. Neurons that were single‐labeled for each of the dyes and those that were double‐labeled were noted in approximately equal numbers. These findings show that although some medullary raphe neurons regulate sympathetic outflow from multiple spinal segments, others have the capacity to control activity in specific segments.