Comparison of respiratory-related trigeminal, hypoglossal and phrenic activities

In decerebrate, paralyzed and vagotomized cats, we recorded activities of hypoglossal and phrenic nerves and of the mylohyoid branch of the trigeminal nerve. At normocapnia, a respiratory-modulated trigeminal discharge could be discerned in most cats. This discharge was characterized by a diminution of activity during neural inspiration and a peak in expiration. In hypercapnia or hypoxia, peak activity increased and its time of occurrence moved to late inspiration. Augmentations of peak trigeminal, hypoglossal and phrenic activities were proportional. Peak trigeminal and hypoglossal activities increased more than phrenic following administrations of protriptyline, strychnine and, in some cats, cyanide or doxapram. Peak trigeminal activity fell more than phrenic after diazepam. Pentobarbital or halothane reduced peak hypoglossal, but not trigeminal, activity more than phrenic. However, after these anesthetics, trigeminal activity became restricted to the inspiratory-expiratory junction. We conclude that trigeminal and hypoglossal activities are more dependent upon processes within the reticular formation than is the bulbospinal-phrenic system. Central and peripheral chemoreceptor influences are distributed equivalently upon trigeminal, hypoglossal and phrenic motoneurons.