Recruitment and Frequency Coding of Diaphragm Motor Units During Ventilatory and Non-Ventilatory Behaviors

Abstract

The motor unit, comprised of an alpha motoneuron and the muscle fibers it innervates, is the final common pathway by which the nervous system controls muscle contractions. Since the pioneering studies of Sherrington (13), it has been recognized that the central nervous system controls the force generated by a muscle by changing the number of activated motor units (recruitment coding) or by modifying the discharge frequency of recruited units (frequency coding). In mixed muscles, motor units display a variety of contractile and fatigue properties (2). Thus, to accomplish different motor behaviors, the nervous system has a repertoire of units from which to select, For example, under conditions requiring prolonged force production, the nervous system might select to recruit only those units that are fatigue resistant. Under other conditions requiring shorts bursts of force, unit fatigue resistance might not be an important determinant in unit recruitment. Instead, more fatigable units, which typically generate greater forces might be selectively recruited. The nervous system might also select to increase force by increasing the discharge rate of those units already active.