State dependence of upper airway respiratory motoneurons: Functions of the cricothyroid and nasolabial muscles of the unanesthetized rat

Abstract

Unanesthetized, free-to-move rats were chronically implanted with recording electrodes in order to study the state dependence of the respiratory center's control of two upper airway respiratory muscles, the cricothyroid and nasolabial muscles, and the diaphragm. Polygraphic and oscilloscopic served to display cortical activity (ECoG) as well as dorsal neck and respiratory muscle activity (EMG). States of vigilance were identified polygraphically. There was: (1) in alertness, expiratory activity of the nasolabial and cricothyroid muscles; (2) in slow wave sleep, either inspiratory and expiratory or only inspiratory activity of the cricothyroid muscle and absence of nasolabial muscle activity; (3) in paradoxical sleep, marked attenuation, irregular nasolabial muscle activity unrelated to respiration; (4) in hypercapnia and hypoxia, a dual inspiratory and expiratory activity of the cricothyroid muscle and expiratory activity of the nasolabial muscle and (5) in pentobarbital anesthesia, inspiratory activity of the cricothyroid muscle and absence of nasolabial muscle activity but inspiratory activity if nasal secretions occurred. Two principal conclusions may be drawn from the findings. Although phrenic motoneurons maintain a basic rhythmically independent of physiologic states, those of cricothyroid and nasolabial motoneurons vary dramatically thereby revealing the non-uniform character of the central nervous system's control of respiratory motoneurons. The composition of inspired air and anesthesia modify central control of upper airway respiratory motoneurons to facilitate airflow.