Respiratory control of hypoglossal motoneurones in the rat

In this study of adult and neonatal rats, we used cross-correlation analysi s to detect synchronous neuronal events in hypoglossal and phrenic nerves t o infer synaptic connections. We found evidence for the common excitation o f medial and lateral hypoglossal motoneurones in 12 anaesthetized adult rat s but not in 6 in vitro brainstem-spinal cord preparations. We did not find evidence for the common activation of phrenic and hypoglossal motoneurones in 23 adult and 10 neonatal rat preparations. We confirmed this negative r esult by demonstrating that 26 medullary inspiratory neurones activating ph renic motoneurones did not activate hypoglossal motoneurones in 23 adult de cerebrate rats (except in one case). We also found that 15 Botzinger expira tory neurones inhibiting phrenic motoneurones did not inhibit hypoglossal m otoneurones. We conclude that: (1) motoneurones of the medial and lateral h ypoglossal nerve branches receive inspiratory drive from a common premotor population in adult rats, but in neonatal rats adjacent nerve rootlets do n ot; (2) in both adult and neonatal rats phrenic premotor neurones do not mo nosynaptically excite hypoglossal motoneurones; (3) Botzinger expiratory ne urones that inhibit phrenic motoneurones do not inhibit hypoglossal motoneu rones. We therefore suggest that the respiratory control of hypoglossal mot oneurones is separate from that of phrenic motoneurones.