Discharge of abdominal muscle α and γ motoneurons during expiratory loading in cats

Abstract

Expiratory responses were elicited in abdominal α and γ motoneurons of anesthetized cats by continuous positive airway pressure, tracheal occlusion, lung inflation, or step-changes in expiratory airflow resistance. Abdominal α motoneurons were silent during quiet breathing but became rhythmically active whenever expiration was opposed. In addition, the responses of abdominal α motoneurons to an increase in expiratory airflow resistance included an increase in discharge frequency, an earlier firing time of individual neurons, recruitment of successively larger motor fibers, and an increased duration of discharge. Abdominal γ motoneurons discharged continuously during quiet breathing and an increased frequency of discharge during the expiratory phase of the respiratory cycle was evident in approximately one-third of the fibers. This respiratory modulation was enhanced, or initiated if absent, by imposing a load during expiration. Bilateral cervical vagotomy abolished both the respiratory modulation of abdominal γ motoneurons and expiratory activity in abdominal α motoneurons indicating the importance of supraspinal structures. Coactivation of abdominal α and γ motoneurons during active expiration also suggests that a segmental reflex involving abdominal muscle spindles may be capable of providing automatic compensation for changes in expiratory load. In conclusion, both vagal and dorsal root information appear to contribute to the proprioceptive control of abdominal muscle expiratory activity.