The reflex and mechanical response of the inspiratory muscles to an increased airflow resistance

Abstract

Diaphragm and external intercostal motor units were monitored in anesthetized-intact, vagotomized (V x), rhizotomized (R x, T 1-T 12 & C 4-C 7), and V x-R x cats to determine the role of information from the lungs, chest wall, and diaphragm in the reflex response to an increased inspiratory airflow resistance. v t, /.V i, Pes and inspiratory work were simultaneously monitored. The motor responses of the external intercostals to the loading were (1) an increase in the rate of unit activity, (2) earlier firing time, (3) extended firing time, and (4) recruitment of new units. Vagal information contributes to (1) and (4) and is responsible for (3). Thoracic dorsal root information contributes to (1) and (4) and is responsible for (2). All components of the loading response were eliminated after both the vagi and thoracic dorsal roots were sectioned. The motor responses of the diaphragm to loading were (1) and (3); both were eliminated by vagotomy. There was an increase in dPes dt during the loaded inspiration which was sufficient to maintain the inspiratory work rate, even though dV dt was decreased. With all afferent pathways sectioned (loading reflex absent), the inspiratory work rate was still maintained during the loaded inspiration. This suggests that the major portion of the increase in force developed during the loaded inspiration (intact animal) is related to intrinsic properties of the respiratory muscles. The primary function of the “load-compensating” reflex when inspiring against increased airflow resistance seems to be one of providing stability of the rib cage.