LARYNGEAL MOTOR CONTROL IN FROGS - ROLE OF VAGAL AND LARYNGEAL FEEDBACK

Using decerebrate frogs (Rana catesbeiana), we investigated the role o f vagal and laryngeal sensory feedback in controlling motor activation of the larynx. Vagal and laryngeal nerve afferents were activated by electrical stimulation of the intact vagal and laryngeal nerves. Pulmo nary afferents were activated by lung inflation. Reflex responses were recorded by measuring efferent activity in the laryngeal branch of th e vagus (Xl) and changes in glottal aperture. Two glottic closure refl exes were identified, one evoked by lung inflation or electrical stimu lation of the main branch of the vagus (Xm), and the other by electric al stimulation of Xl. Lung inflation evoked a decrementing burst of Xl efferent activity and electrical stimulation of Xm resulted in a brie f burst of Xl action potentials. Electrical stimulation of Xl evoked a triphasic mechanical response, an abrupt glottal constriction followe d by glottal dilatation followed by a long-lasting glottal constrictio n. The first phase was inferred to be a direct (nonreflex) response to the stimulus, whereas the second and third represent reflex responses to the activation of laryngeal afferents. Intracellular recordings of membrane potential of vagal motoneurons of lung and nonlung types rev ealed EPSPs in both types of neurons evoked by stimulation of Xm or Xl , indicating activation of glottal dilator and constrictor motoneurons . In summary, we have identified two novel reflexes producing glottic closure, one stimulated by activation of pulmonary receptors and the o ther by laryngeal receptors. The former may be part of an inspiratory terminating reflex and the latter may represent an airway protective r eflex. (C) 1997 John Wiley & Sons, Inc. J Neurobiol.