Evoked response in the posterior cricoarytenoid to stimulation of the ipsilateral vagus.

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As no complete study has been made of reflex response to stimulation of ipsilateral vagus in regard to extramedullary respiratory control system, the authors have tried to clarify the influence of the stimulation of the vagus on the ipsilateral posterior cricoarytenoid muscle (PCA).<br>Reflex responses in the recurrent laryngeal nerve and motor action potentials of the PCA elicited by stimulation of the ipsilateral vagus were compared with those elicited by stimulation of the internal branch of the superior laryngeal nerve.<br>The latent periods in the case of stimulation of the vagus were much longer and reflex discharges in tonic fibers were not noticeable. In addition, reflex inhibition of spontaneous respiratory activity of the PCA was not re-markable. Decrease in activity of the PCA muscle, which is usually caused by inflation of the lung, was caused by the low frequency of central vagal stimu-lation. On the other hand, high frequency of central vagal stimulation caused increase in activity of this muscle. It is known that forced hyperinflation by introducing excessive amount of air results in increased inspiratory activity (Head's paradoxcal reflex). So it seems most likely that these high frequency of vagal stimulation are equivalent to mechanical inflation by excessive amount of air.<br>Based upon the facts mentioned above, it can be said that reflex response in the PCA elicited by vagal stimulation may not take part in laryngeal reflex closure, but may take part in respiratory control system to regulate the activity of the PCA muscle. The PCA muscle, hence, can be firmly considered to be one of the respiratory muscle from the viewpoint of extramedullary respiratory control system.

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