During breathing, the diaphragm and abdominal muscles contract out of phase. However, during other behaviors (including vomiting, postural adjustments, and locomotion) simultaneous contractions are required of the diaphragm and other muscle groups including abdominal muscles. Recent studies in cats using transneuronal tracing techniques showed that in addition to neurons in the respiratory groups, cells in the inferior and lateral vestibular nuclei (VN) and medial pontomedullary reticular formation (MRF) influence diaphragm activity. The goal of the present study was to determine if neurons in these regions have collateralized projections to both diaphragm motoneurons and the lumbar spinal cord. For this purpose, the transneuronal tracer rabies virus was injected into the diaphragm and the monosynaptic retrograde tracer Fluoro-Gold (FG) was injected into the Th13-L1 spinal segments. A large fraction of MRF and VN neurons (median of 72 and 91%, respectively) that were infected by rabies virus were dual-labeled by FG. These data show that many MRF and VN neurons that influence diaphragm activity also have a projection to the lumbar spinal cord, and thus likely are involved in coordinating behaviors that require synchronized contractions of the diaphragm and other muscle groups.
During breathing, the diaphragm and abdominal muscles contract out of phase. However, during other behaviors (including vomiting and postural adjustments) simultaneous increases in the activity of the diaphragm and abdominal muscles are required. Recent studies in cats using transneuronal tracing techniques showed that in addition to neurons in the respiratory groups, cells in the inferior and lateral vestibular nuclei (VN) and medial medullary reticular formation (MRF) influence diaphragm activity. The goal of the present study was to determine if neurons in these regions have collateralized projections to diaphragm motoneurons and the upper lumbar spinal cord, where abdominal motoneurons are located. For this purpose, the transneuronal tracer rabies virus was injected into the diaphragm and the monosynaptic retrograde tracer Fluoro‐Gold (FG) was injected into the L1 spinal segment. A large fraction of the neurons in the MRF and VN (median of 75 and 91%, respectively) that were infected by rabies virus were dual‐labeled by FG. These data show that MRF and VN neurons have connectivity with both phrenic and abdominal motoneurons, and thus can simultaneously regulate the activity of both the diaphragm and abdominal musculature. Future studies should focus on the physiological role of these neurons in producing co‐contractions of inspiratory and expiratory muscles.
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.
We have demonstrated that vestibular stimulation results in distinct changes in blood flow in the upper and lower body, indicating that the sympathetic nervous system has the capacity to elicit regionally‐specific changes in blood flow. Since the RVLM plays a major role in regulating blood flow, the present study tested the hypothesis that distinct populations of RVLM neurons project to the T4 segment (which controls blood flow in the upper body) and the T10 segment (which regulates lower body blood flow). For this purpose, multiple injections of the fluorescent dye Fast Blue were made in cats in the vicinity of the intermediolateral cell column (IML) in T4, whereas multiple injections of the dye Fluoro‐Ruby were made near the IML in T10. RVLM neurons that were single‐labeled for one of the tracers (indicating that they projected to only one segment) were approximately four times more prevalent than those that were double‐labeled (had collateralized projections to both T4 and T10), including catecholaminergic neurons identified through immunohistochemistry. A χ2 test confirmed that double‐labeled cells were less prevalent than single‐labeled RVLM neurons (p<0.001). Our conclusion from these data is that the RVLM has the capacity to independently regulate sympathetic nervous system influences on blood flow in different body regions.
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