Conceptual Model of Ventilatory Muscle Recruitment and Diaphragmatic Fatigue

Abstract

In the diaphragm as with other respiratory muscles, motor units (a motoneuron and the muscle fibers it innervates; Fig. 1) are the final effector pathway by which the central nervous system controls the level of ventilation. The forces generated by the diaphragm are controlled by changing the number of activated motor units (recruitment coding) and/or by modifying the discharge frequency of recruited units (frequency coding). As in other skeletal 113-1 muscles, the motor units that comprise the diaphragm vary considerably in their contractile and fatigue properties1. The overall strength of the diaphragm and its susceptibility to fatigue depend upon the motor units recruited by the central nervous system. For example, under conditions requiring prolonged low level force production by the diaphragm (e.g., quiet breathing), the nervous system might recruit only those units that are fatigue resistant. In contrast, during other conditions where the diaphragm generates increased levels of force (e.g., sneezing, gagging), the recruitment of more fatigable motor units would also be required. Thus, it is important to consider what types of diaphragm motor units are available for recruitment, both in the generation of a respiratory rhythm and in the production of nonventilatory forces.