The effects of brainstem transection on respiratory activity in the rabbit.

Mid-sagittal incisions and transverse hemisections of the medulla were performed in rabbits anaesthetized with halothane and artificially ventilated at eupnoeic level. A midsagittal incision extending from the obex to (at least) 4 mm rostral elicited asynchronous (independent) firing in both phrenic nerves. A subsequent transverse hemisection interrupted the activity of the ipsilateral phrenic nerve. An electrical stimulation of the vagal input temporarily restored this activity. A midsagittal incision, which had not completely separate both halves of the medulla, did not affect the synchrony of firing of both phrenic nerves; in these cases also the transverse hemisection did not arrest the phrenic nerve activity. It is concluded that synchronous activity of the inspiratory output depends upon intact connnections crossing in the lower medulla and that proper generation of respiratory pattern depends upon a critical level of excitation created by neurones at rostral pontine and suprapontine levels of the CNS, neighboring structures (presumably including the medial reticular formation) and/or vagal input. A concept of a "critical mass" of active neurones is introduced.