Muscle fibers diameter

Figure 2. Erb s illustration of the pathology of muscle from patients with Duchenne muscular dystrophy. Note the variation in muscle fiber diameter, fiber-splitting, deposition of fat and infiltration of connective tissue. Drawing from several biopsies produced during final decade of 19th century. 

Muscle biopsy is usually undertaken to confirm the provisional clinical diagnosis. Because the skin lesions normally precede those in muscle, biopsies of muscle taken early may show little abnormality. Inflammatory foci may be scanty or absent and muscle fiber diameters may be normal. However typical biopsies show discrete foci of inflammatory cells, with a predominance of B-lymphocytes (see Figure 18). These cells are situated in perimysial connective tissue rather than in the en-domysium and are often also perivascular in location. Muscle fiber necrosis occurs in JDM but muscle fibers do not appear to be the primary target of the disordered immune process. Rather, it is the micro vasculature of the muscle which appears to degenerate first and muscle necrosis is preceded by capillary necrosis, detectable at the ultrastructural level. 

Samples should be evaluated for muscle fiber diameters (and the uniformity of fiber sizes), centrally located myonuclei (a sign of a regenerated muscle fiber), fibrosis, fatty infiltration, and atrophied muscle fibers. 

Muscle mass affects the toxin s response. More toxin is needed locally to produce a desired effect in areas of increased muscle mass. Histologic examination of orbicularis oculi musculature after treatment with botulinum toxin shows no evidence of alteration of muscle fiber diameter, disruption of internal muscle architecture, or pathologic changes in the motor end plates. 

Age Muscle fiber diameter (um) End-plate area (fitrP ) End-plate perimeter 9[Pg.318]

Fig. 11. EDL muscle fiber diameter 7 days after peroneal nerve crush Saline treatment (black column) BIM 22015...

EDL muscle fiber diameter following nreve crush at 2 days of age and peptide treatment... 

Muscle fibers also atrophy following neonatal denervation. Treatment with ACTH 4-10 but not with a-MSH, exacerbates the muscle atrophy in 7 day old neonates. By 15 days, however, both melanocortins effect an increase in muscle fiber diameter, overcoming the denervation atrophy seen in the saline-treated controls. A comparison of muscle fiber diameters from 15 day old lesioned neonates treated with melanocortins shows them to be comparable in size to normal saline-treated pups of the same age (Table 7). 

Nerve regrowth in neonatal rats appears to be even more sensitive to melanocortin administration than regrowth in the adult rat. Endplate morphology, muscle fiber diameter, muscle metabolism, motor unit size and electrophysiological parameters all respond positively to these peptides by 15 days of age, although there are variations in response during the first week of postnatal life. 

The structure of heart myocytes is different from that of skeletal muscle fibers. Heart myocytes are approximately 50 to 100 p,m long and 10 to 20 p,m in diameter. The t-tubules found in heart tissue have a fivefold larger diameter than those of skeletal muscle. The number of t-tubules found in cardiac muscle differs from species to species. Terminal cisternae of mammalian cardiac muscle can associate with other cellular elements to form dyads as well as triads. The association of terminal cisternae with the sarcolemma membrane in a dyad structure is called a peripheral coupling. The terminal cisternae may also form dyad structures with t-tubules that are called internal couplings (Figure 17.31). As with skeletal muscle, foot structures form the connection between the terminal cisternae and t-tubule membranes. 

In severe neonatal nemaline myopathy virtually every muscle fiber shows multiple rods and all muscle fiber types are affected. However in juvenile cases, two different patterns of fiber type involvement are seen. In one there is a clear size difference between type 1 fibers, which are abnormally small (hypotrophic or atrophic) and which contain numerous nemaline rods, and type 2 fibers, which are either of normal diameter or hypertrophic and contain few, if any, nemaline rods. Other patients show a gross predominance of type 1 muscle fibers, again with rods virtually confined to this fiber type. These findings may be explicable in terms of the involvement of isoforms of a-actinin specific to slow and fast muscle fiber types. 

Describe the factors that influence the strength of skeletal muscle contraction including multiple motor unit summation, asynchronous motor unit summation, frequency of nerve stimulation, length-tension relationship, and diameter of the muscle fiber... 

Finally, slow-twitch muscle fibers have a small diameter. This facilitates the diffusion of oxygen through the fiber to the mitochondria where it is utilized. Taken together, each of these characteristics enhances the ability of these fibers to utilize oxygen. Therefore, in slow-twitch oxidative muscle... 

The diameter of the muscle fiber is influenced by two major factors ... 

Actin filaments are the thinnest of the cytoskeletal filaments, and therefore also called microfilaments. Polymerized actin monomers form long, thin fibers of about 8 nm in diameter. Along with the above-mentioned function of the cytoskeleton, actin interacts with myosin ( thick ) filaments in skeletal muscle fibers to provide the force of muscular contraction. Actin/Myosin interactions also help produce cytoplasmic streaming in most cells. 

Skeletal muscles consist of bundles of long muscle fibers, which are single cells of diameter 10-100 pm formed by the fusion of many embryonic cells. The lengths are typically 2-3 cm in mammals but may sometimes be as great as 50 cm. Each fiber contains up to 100-200 nuclei. Typical cell organelles are present but are often given special names. Thus, the plasma membrane (plasmalemma) of muscle fibers is called the sarcolemma. The cytoplasm is sarcoplasm, and mitochondria may be called sarcosomes. The major characteristic of muscle is the presence of the contractile myofibrils, organized bundles of proteins 1-2 pm in diameter and not separated by membranes from the cytoplasm. Since they occupy most of the cytoplasm, a substantial number of myofibrils are present in each muscle fiber. 

A closer look at striated muscle fibers shows that they themselves are assemblies of fine, hairlike structures known as myofibrils (Fig. 1A, B). Myofibrils may be about 2 to 5 jxm in diameter, with cell organelles such as mitochondria and membranous systems called T-tubules and the sarcoplasmic reticulum (SR) sandwiched between them (Fig. 2B). 

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