Oxidative capacity and capillary density of diaphragm motor units

Abstract

Motor units in the cat diaphragm (DIA) were isolated in situ by microdissection and stimulation of C5 ventral root filaments. Motor units were classified based on their isometric contractile force responses and fatigue indexes (FI). The muscle fibers belonging to individual units (i.e., the muscle unit) were identified using the glycogen-depletion method. Fibers were classified as type I or II based on histochemical staining for myofibrillar adenosine triphosphatase (ATPase) after alkaline preincubation. The rate of succinate dehydrogenase (SDH) activity of each fiber was determined using a microphotometric procedure. The location of capillaries was determined from muscle cross sections stained for ATPase after acid (pH = 4.2) preincubation. The capillarity of muscle unit fibers was determined by counting the number of capillaries surrounding fibers and by calculating the number of capillaries per fiber area. A significant correlation was found between the fatigue resistance of DIA units and the mean SDH activity of muscle unit fibers. A significant correlation was also observed between DIA unit fatigue resistance and both indexes of muscle unit fiber capillarity. The mean SDH activity and mean capillary density of muscle unit fibers were also correlated. We conclude that DIA motor unit fatigue resistance depends, at least in part, on the oxidative capacity and capillary density of muscle unit fibers.

Impact of unilateral denervation on transdiaphragmatic pressure

“…As in skeletal muscles, motor units in the DIAm comprise motor neurons and the muscle fibers they innervate (Fournier and Sieck, 1988a; Sieck, 1994; Sieck and Fournier, 1989). The properties of both phrenic motor neurons and muscle fibers are matched (Enad et al, 1989; Sieck, 1994) and critically determine the efficacy of the central nervous system in accomplishing specific motor tasks. During neural activation, motor units are recruited in an orderly fashion, based on the intrinsic electrophysiological properties of motoneurons such that, for a given synaptic input, smaller more excitable motor neurons innervating more fatigue resistant muscle fibers (type I and IIa fibers comprising type S and type FR motor units) are recruited before larger motor neurons innervating more fatigable muscle fibers (type IIx and IIb fibers comprising type FInt and type FF motor units) (Butler et al, 1999; Henneman et al, 1965; Mantilla and Sieck, 2011; Sieck and Fournier, 1989).…”

Functional impact of sarcopenia in respiratory muscles

“…Each motor unit type is composed of muscle fibers that are homogeneous with respect to their metabolic properties and contractile protein composition, specifically the expression of myosin heavy chain (MyHC) isoforms (Enad et al, 1989; Gransee et al, 2012; Greising et al, 2012; Sieck, 1994; Sieck et al, 1989a, 1996). In fact, this relationship forms the basis of muscle fiber type classification (Brooke and Kaiser, 1970; Edstrom and Kugelberg, 1968; Schiaffino et al, 1989; Sieck et al, 1985).…”

“…This model of DIAm motor unit recruitment also depends on estimates of the proportion of motor unit types and the forces they contribute. The proportion of different motor unit types can be estimated based on the proportion of different muscle fiber types and measurements of the innervation ratio of each motor unit type (based on glycogen depletion) (Enad et al, 1989; Fournier and Sieck, 1988b). The force generated by each motor unit type depends on the total cross-sectional area (CSA) of the muscle fibers comprising each unit, which can be estimated from fiber type proportion, average fiber CSA and the innervation ratio.…”

Cross-bridge kinetics in respiratory muscles

“…For example, the succinate dehydrogenase (SDH) activity of type I and IIa fibres is significantly higher than that of type IIx and IIb fibres. The higher oxidative capacities of type I and IIa fibres certainly contribute to the greater fatigue resistance of these fibres as compared to type IIx and IIb fibres [23,53]. However, fatigue resistance may also be related to fibre type differences in the ATP consumption rate of different MyHC isoforms.…”