Respiratory effects of pontine, medullary and spinal cord midline sections in the rabbit

Experiments were performed on halothane anaesthetized, paralyzed rabbits with vagi intact or cut. In vagotomized rabbits separation of both halves of the medulla by a midline section resulted in a 'desynchronization' of both phrenic and both efferent vagal inspiratory volleys. Activities of all these nerves fall and respiratory frequency decreased. In animals with intact vagi the inspiratory volleys generated by each half of the brainstem were locked to the respirator (and consequently to each other). Elimination of the phasic feedback by stopping the respirator led to 'desynchronization'. Extension of the incision through the pons and midbrain increased phrenic amplitude almost to control values. During 'desynchronized' firing the coincident volleys exhibited markedly bigger amplitude than those which appeared in the opposite phase, i.e. during the silent period of the other phrenic. Hemisection at C1 level or a cervical spinal cord midline myelotomy eliminated the amplitude differences between coinciding and non-coinciding volleys. Amplitude of efferent vagal activity was always constant and synchronous with the ipsilateral phrenic. Our findings indicate the existence of bulbo-spinal excitatory and inhibitory pathways which affect phrenic motoneurones from the opposite side. We conclude that integration at pontine and cervical spinal cord level may significantly influence the respiratory output.