Respiratory function of the larynx in developing sheep and the influence of sleep state

The activities of laryngeal abductor and adductor muscles were recorded in conjunction with other respiratory muscles in unanaesthetized chronically monitored foetal, newborn and adult sheep. In foetuses, rhythmical breathing movements occurred in the presence of rapid eye movements and a low voltage electrocorticogram (REM state) and were accompanied by phasic laryngeal abduction; adductor muscles and intercostals were largely inactive. During the foetal non-REM state laryngeal adductor muscles were tonically active at a low level, but the glottis remained open. Deep inspiratory efforts which occurred at low frequency in this behavioural state were not accompanied by abductor activity and resembled inspiratory efforts associated with regurgitation in ruminants. After birth, expiratory adductive activity of the larynx was also maximal during non-REM sleep and negligible during REM sleep. Active laryngeal adduction was accompanied by delayed expiratory airflow, prolonged TE and reduced respiratory frequency. Adduction of the larynx is dependent on behavioural state and appears to be the principal means of regulating TE in sheep, particularly during early post-natal development.