Perinatal development of laryngeal function.

The resistance of the upper airway is strongly influenced by the action of opposing sets of laryngeal muscles. Expiratory airflow may be retarded by active adduction of the arytenoid cartilages or by a reduction in the activity of abductor muscles. In developing sheep the adductor muscles appear to represent the principal means by which lung recoil is opposed. This mechanism, which is most pronounced during non-rapid-eye-movement sleep, is regulated by afferent traffic from the lungs. In fetal sheep the laryngeal muscles are also influenced by breathing movements and sleep states. The adductor muscles are normally tonically active during non-rapid-eye-movement sleep when rhythmical breathing movements are absent. It is possible that this activity is at least partially responsible for elevated tracheal pressures and depressed flow of tracheal fluid during fetal apnea. This hypothesis has been tested by observing the effects of fetal paralysis and recurrent laryngeal nerve section. These experiments suggest that in the fetus near term the larynx makes a major contribution to upper airway resistance and hence to the maintenance of pulmonary expansion which has been shown to influence lung development.