Synchronization of Inspiratory Motor Activity as Compared between Phrenic and Vagus Nerve

Motor synchronization is known to occur in the phrenic nerve of rabbit, cat, and dog.2" It only develops with reinforced inspiratory activity, such as dyspnoea, whereas with quiet, eupnoeic breathing the inspiratory impulses remain asynchronous.8'9 Synchronization does not appear even with strongest reflex tonus of inspiratory muscles and therefore seems to depend on some kind of centrally induced, spontaneous activity.'0"' Its central origin is also demonstrated by the dependence of the frequency of the synchronized efferent volleys on central temperature5 and by direct recording of a similar rhythmic activity from the caudal part of nucleus ambiguus.' The latter findings, however, do not provide any further evidence concerning localization of the source of synchronized activity because they are merely concerned with the outflow at the motoneuronic level of the vagus and particularly the inferior laryngeal nerve. Synchronization of inspiratory neurons seems to occur simultaneously in the various motor nuclei of inspiratory muscles. This was shown for one phrenic nerve and the opposite side of the diaphragm7 and may also be deduced from comparison between one phrenic nerve and the ipsilateral or contralateral vagus or inferior laryngeal nerve.8 Experimental data, however, were obtained from records taken at a relatively low speed and with asymmetrical amplifiers, and may therefore not allow definite conclusions to be drawn concerning identity of frequency and phase relation prevailing between the synchronized activity of different and widely separate pools of inspiratory motoneurons. Since the problem is of great importance with regard to further study of the spontaneous activity of inspiratory centres, the present investigations were undertaken in order to provide more reliable experimental evidence as to the existence of a common pace-making mechanism responsible for synchronized inspiratory innervation in phrenic and vagal outflow.