Neural pathways that influence diaphragm activity and project to the lumbar spinal cord in cats

Abstract

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.