ABSTRACT
In skeletal muscles, including the diaphragm, the final element of neural control is the motor unit, each composed of a motoneuron and the muscle fibers it innervates (i.e., the muscle unit; Liddell and Sherrington, 1925; Sherrington, 1992; Fig. 1). These motor units can vary considerably in their physiological, histochemical, and biochemical properties (Burke, 1981; Burke et al. , 1973; Enad et al., 1989; Fournier and Sieck, 1988b; Hamm etal., 1988; Martinet al. , 1988; Nemeth et al., 1981, 1986; Sieck and Fournier, 1989; Sieck et al., 1989a). It is this heterogeneity of motor unit properties that affords the nervous system a basis for control of muscle force generation during different motor behaviors. At the same time, it is the cumulative contractile and fatigue properties of the motor unit pool that determines the constraints under which the diaphragm muscle responds to the various mechanical demands placed on it during different ventilatory and nonventilatory behaviors.
