Summary
In mammals, it has long been considered that the bulbo-spinal inspiratory drive provided a direct (monosynaptic) excitation of phrenic motoneurones (Phr Mns). Although such connections have been demonstrated, recent indirect data strongly suggested that the main inspiratory drive is polysynaptic. We tried to directly demonstrate relay respiratory interneurones at the C3–C6 spinal cord level where the Phr Mn pool is located. The experiments were performed on decorticate, unanaesthetized, bilaterally vagotomized and curarized rabbits and the firing pattern of spinal interneurones was compared to the phrenic bursting. Dorsally and dorso-medially to the Phr Mn pool, different classes of inspiratory (54%) and expiratory (46%) interneurones could be identified in the ventral horn. Three classes of inspiratory interneurones were characterized and classified as “I all” (26%), “I late” (43%) and “I tonic” (29%) according to the terminology used by other authors for the bulbospinal inspiratory neurones which drive the spinal respiratory motoneurones. The expiratory interneurones could also be divided into 3 classes: “E all” (48%), “E late” (10%) and “E tonic” (41%). This first direct evidence of inspiratory interneurones at the C3–C6 spinal cord levels can account for the major polysynaptic excitation of the Phr Mns while the presence of numerous expiratory interneurones at this level suggests a polysynaptic bulbo-spinal inhibitory action onto the Phr Mns. These classes of inspiratory and expiratory interneurones did not always coincide with the bulbo-spinal classes of neurones described elsewhere. Unless these discrepancies are due to the different experimental conditions, they may indicate that some of these interneurones are not just relay target cells and they suggest that they might behave as integrative operators between the medullary drive and the Phr Mns.
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Palisses, R., Perségol, L. & Viala, D. Evidence for respiratory interneurones in the C3-C5 cervical spinal cord in the decorticate rabbit. Exp Brain Res 78, 624–632 (1989). https://doi.org/10.1007/BF00230250
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DOI: https://doi.org/10.1007/BF00230250