Abstract
Smaller diaphragmatic motor unit potentials (MUPs) compared to MUPs of limb muscles lead to the hypothesis that diaphragmatic muscle fibers, being the generators of MUPs, might be also smaller. We compared autopsy samples of costal diaphragm and vastus lateralis of healthy men with respect to fibers' size and expression of slow myosin heavy chain isoform (MyHC-1) and fast 2A isoform (MyHC-2A). Diaphragmatic fibers were smaller than fibers in vastus lateralis with regard to the mean minimal fiber diameter of slow-twitch (46.8 versus 72.2 μm, p < 0.001), fast-twitch (45.1 versus 62.4 μm, p < 0.001), and hybrid fibers (47.3 versus 65.0 μm, p < 0.01) as well as to the mean fiber cross-sectional areas of slow-twitch (2376.0 versus 5455.9 μm2, p < 0.001), fast-twitch (2258.7 versus 4189.7 μm2, p < 0.001), and hybrid fibers (2404.4 versus 4776.3 μm2, p < 0.01). The numerical proportion of slow-twitch fibers was higher (50.2 versus 36.3%, p < 0.01) in costal diaphragm and the numerical proportion of fast-twitch fibers (47.2 versus 58.7%, p < 0.01) was lower. The numerical proportion of hybrid fibers did not differ. Muscle fibers of costal diaphragm have specific characteristics which support increased resistance of diaphragm to fatigue.
Highlights
Diaphragm, a principal inspiratory muscle of humans, is a highly specialized skeletal muscle unique in its ability to contract continuously and rhythmically
Muscle fibers were classified according to the expression of myosin heavy chain isoforms (MyHC) by indirect immunoperoxidase method as described previously [7]; briefly, slow fibers were demonstrated by BA-D5 antibody immunoreactive with β/slow MyHC-I in rats [8] and humans [9]; fast fibers were demonstrated by A4.74 antibody immunoreactive with MyHC-IIA and MyHC-IIX in humans [7]
The mean fiber diameters and the mean fiber cross-sectional areas were similar among different fiber types of costal diaphragm
Summary
A principal inspiratory muscle of humans, is a highly specialized skeletal muscle unique in its ability to contract continuously and rhythmically. Muscle fiber size is one of the variables which contribute to the amplitude of MUPs [2], suggesting that muscle fibers of costal diaphragm, being the generators of MUPs, might be smaller than those of limb muscles. Studies about muscle fiber diameters in human costal diaphragm are sparse and contradictory quoting similar [3], smaller [4, 5], or bigger [6] size of diaphragmatic fibers with respect to limb muscle fibers. The aim of the present study was to establish normative morphometric data of muscle fibers in human costal diaphragm with respect to the size of slow-twitch and fast-twitch muscle fibers and their numerical proportions
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