The nucleus retroambiguus as possible site for inspiratory rhythm generation caudal to obex

We investigated whether spinalized animals can produce inspiratory rhythm. We recorded spinal inspiratory phrenic (PNA) and cranial inspiratory hypoglossal (HNA) nerve activity in the perfused brainstem preparation of rat. Complete transverse transections were performed at 1.5 (pyramidal decussation) or 2mm (first cervical spinal segment) caudal to obex. Excitatory drive was enhanced by either extracellular potassium, hypercapnia or by stimulating arterial chemoreceptors. Caudal transections immediately eliminated descending network drive for PNA, while the cranial inspiratory HNA remained unaffected. After transection, PNA bursting remained sporadic even during enhanced excitatory drive. This implies, cervical spinal circuits lack intrinsic rhythmogenic capacity. Rostral transections also abolished PNA immediately. However, HNA also progressively lost its amplitude and rhythm. Chemoreceptor activation only triggered tonic, non-rhythmic HNA. Thus the integrity of ponto-medullary circuitry was maintained. Our results suggest that an area overlapping the caudal nucleus retroambiguus provides critical ascending input to the ponto-medullary respiratory network for inspiratory rhythm generation.