Structure–activity relationships in rodent diaphragm muscle fibers vs. neuromuscular junctions
Highlights
► Diaphragm muscle in mice has a greater activation history than rats. ► Susceptibility to neuromuscular transmission failure is less in mice than rats. ► Fatigue-resistant fibers contribute to a greater fraction of muscle mass in mice. ► Rats, but not mice, show fiber type differences in neuromuscular junction morphology. ► Functional differences across species are not reflected in neuromuscular morphology.
Introduction
Within the diaphragm muscle (DIAm), the properties of a phrenic motor neuron and the muscle fibers it innervates are matched to comprise a motor unit (Enad et al., 1989, Fournier and Sieck, 1988, Sieck, 1988, Sieck et al., 1989a, Sieck et al., 1996). Diaphragm motor units are selectively recruited to accomplish different ventilatory and non-ventilatory behaviors such that fatigue-resistant motor units are recruited during sustained ventilatory behaviors while more fatigable units are recruited during the shorter-duration, higher-force non-ventilatory behaviors involved in airway clearance (e.g., coughing, sneezing) (Fournier and Sieck, 1988, Mantilla et al., 2010, Mantilla and Sieck, 2011, Sieck, 1991, Sieck and Fournier, 1989). Forces generated by the DIAm to accomplish quiet breathing (i.e., eupnea) represent ∼27% of maximal force (transdiaphragmatic pressure generated by bilateral supramaximal phrenic nerve stimulation) in rats (Mantilla et al., 2010), ∼17% in cats (Fournier and Sieck, 1988, Sieck and Fournier, 1989) and ∼10% in humans (Sieck, 1991). This scaling across species is consistent with the scaling of other ventilatory parameters such as tidal volume (VT), total lung capacity and ventilation (Lindstedt and Schaeffer, 2002, Stahl, 1967). Respiratory rate and inspiratory duty cycle also increase as animal size decreases. This scaling of ventilatory requirements generally matches the distribution of DIAm motor unit (muscle fiber) types across these species (Fournier and Sieck, 1988, Mantilla et al., 2010, Mantilla and Sieck, 2011, Sieck, 1991, Sieck, 1995, Sieck and Fournier, 1989). Accordingly, the proportion of type I and IIa fibers (comprising fatigue-resistant motor units) across species is also scaled to match the fractional motor unit recruitment that is necessary to accomplish sustained ventilatory behaviors (Mantilla and Sieck, 2011).
In the rat DIAm, neuromuscular junction (NMJ) morphology varies across motor unit (muscle fiber) types (Mantilla et al., 2004a, Prakash et al., 1996b, Rowley et al., 2007, Sieck and Prakash, 1997). For example, NMJs at type I and IIa fibers are smaller and less complex compared to NMJs at type IIx and/or IIb fibers. This suggests that NMJ morphology is matched to the functional demands of different motor unit types. Indeed, susceptibility to neuromuscular transmission failure varies across motor unit (muscle fiber) types (Ermilov et al., 2007, Johnson and Sieck, 1993, Kugelberg and Lindegren, 1979). In the rat DIAm, the safety factor for neuromuscular transmission is higher at type IIx and/or IIb fibers compared to type I or IIa fibers (Ermilov et al., 2007, Wood and Slater, 1997, Wood and Slater, 2001). However, with repetitive stimulation, quantal release at type IIx and/or IIb fibers declines far more rapidly than at type I or IIa fibers (Reid et al., 1999, Rowley et al., 2007) providing a basis for the demonstrated greater susceptibility for neuromuscular transmission failure at these fibers (Johnson and Sieck, 1993, Sieck and Prakash, 1995).
Given the scaling of ventilatory parameters that has been demonstrated across species, we hypothesized that because of the mouse DIAm is more active (prolonged inspiratory duty cycle and greater weight-adjusted ventilation) than the rat, the mouse DIAm will have: (1) greater relative proportion of fatigue-resistant motor units (type I or IIa fibers) and (2) reduced susceptibility to neuromuscular transmission failure. We also hypothesized that NMJ morphology in the mouse DIAm will be matched to the functional demands of different motor unit types.
Section snippets
Methods
Adult male Sprague-Dawley rats (body weight ∼ 300 g) and adult male C57BL/6J mice (body weight ∼ 25 g) were used in these experiments. All procedures were approved by the Institutional Animal Care and Use Committee.
Ventilatory parameters
In awake, unrestrained adult male Sprague-Dawley rats and C57BL/6J mice, whole body plethysmography was used to determine , VT, and duration of inspiration and expiratory phases as well as total respiratory cycle duration.
Discussion
The results of this study indicate that compared to the rat, the mouse had increased VT, (both normalized for body weight) and a prolonged inspiratory duty cycle during eupnea. These results suggest that the mouse DIAm has a greater activation history compared to the rat during eupneic ventilation. Consistent with these differences in ventilatory patterns, the mouse DIAm has a greater relative contribution of fatigue-resistant (type I or IIa) muscle fibers to total DIAm mass compared to the
Acknowledgments
Supported by NIH grants HL096750, AR051173, and a Career Development Award from Mayo Clinic.
References (67)
- et al.
Myosin heavy chain isoforms in postnatal muscle development of mice
Biol. Cell
(2003) - et al.
Topographic comparison of neuromuscular junctions in mouse slow and fast twitch muscles
Neuroscience
(1984) - et al.
Neuromuscular transmission failure during postnatal development
Neurosci. Lett.
(1991) - et al.
Neural control of phrenic motoneuron discharge
Respir. Physiol. Neurobiol.
(2011) - et al.
Synaptic vesicle pools at diaphragm neuromuscular junctions vary with motoneuron soma, not axon terminal, inactivity
Neuroscience
(2007) - et al.
Chronic assessment of diaphragm muscle EMG activity across motor behaviors
Respir. Physiol. Neurobiol.
(2011) - et al.
Diaphragm motor unit recruitment in rats
Respir. Physiol. Neurobiol.
(2010) - et al.
Phrenic motor unit recruitment during ventilatory and non-ventilatory behaviors
Respir. Physiol. Neurobiol.
(2011) - et al.
Measurements of motoneuron somal volumes using laser confocal microscopy: comparisons with shape-based stereological estimations
Neuroimage
(1993) - et al.
Application of the Cavalieri principle in volume estimation using laser confocal microscopy
Neuroimage
(1994)