Bulbo-spinal respiratory effects originating from the splanchnic afferents

Phrenic nerve responses to non-hypertensive stimulation of the visceral afferents originating from the pancreatico-duodenal area and running along the greater splanchnic nerve were studied on cats anaesthetized with sodium thiopentone, paralysed with gallamine and vagotomized. In these intact animals, a biphasic response, short activation followed by a long inhibition, was recorded in the phrenic nerve. In animals then spinalized between C1 and C2 this splanchnico-phrenic reflex persisted. The latency of the first phase was unchanged, but its amplitude increased. In intact animals, the unitary activity of 16 inspiratory neurons and 10 expiratory neurons was recorded in the part of the medulla oblongata corresponding to the ventral respiratory group. Simultaneously the overall activity of the contralateral phrenic nerve was recorded. The stimulation of splanchnic afferents caused a long inhibition of the bulbar inspiratory neurons and an activation of the expiratory neurons. The latency of the bulbar inhibition was consistently less than the latency of the phrenic inhibition. The fact that this reflex effect persisted in spinal animals with an activation phase of the same latency proves that the bulbar centres and the phrenic motoneurons are independently connected to the splanchnic afferents. The possible functional significance of the reflex is discussed.