THE AUGMENTATION OF INTRINSIC LARYNGEAL MUSCLE-ACTIVITY BY AIR-JET STIMULATION OF THE NASAL CAVITY IN DECEREBRATE CATS

The purpose of this study was to examine the functional roles of nasal afferents in modulating the activity of the intrinsic laryngeal muscl es. The electromyographic activities of the intrinsic laryngeal muscle s and major respiratory muscles were recorded in cats during nasal air -jet stimulation. The activities of brainstem respiratory neurons were also recorded to determine which neurons transmit nasal afferent sign als to the intrinsic laryngeal motoneurons. These axonal projections w ere identified by antidromic activation evoked by stimulation to the s pinal cord at C4 level and the laryngeal nerve. The length of the resp iratory cycle was prolonged and the diaphragmatic activity was decreas ed during air-jet stimulation of the nasal cavity. In contrast, the ac tivities of both the intrinsic laryngeal adductor and abductor muscles were increased. Examination of the laryngeal reflexes revealed increa se in the activities of intrinsic laryngeal motoneurons during both re spiratory phases. Most of the respiratory neurons recorded decreased t heir peak firing rate during air-jet stimulation, reflecting decreased diaphragmatic activity; however, the peak firing rate of the bulbospi nal expiratory neurons in the portion of the ventral respiratory group caudal to the obex did not decrease during stimulation. These finding s demonstrate the nasal air-jet stimulation decreases the activities o f major inspiratory muscles in order to avoid inspiration of foreign b odies into the nasal cavity and augments the activities of intrinsic l aryngeal muscles, enabling prompt elicitation of the laryngeal airway reflex. Our findings also suggest that the nasal afferents suppress th e major inspiratory activities by way of brainstem inspiratory neurons , but that the activities of intrinsic laryngeal muscles are controled through undetermined pathway(s) other than the-pathway through respir atory neurons. (C) 1998 Elsevier Science Ireland Ltd. All rights reser ved.