Myofibrils


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Myofibrils

 

contractile filaments in the protoplasm of striated muscle fibers of the skeletal musculature, myocardium, and muscles with double diagonal striation. They range in diameter from 0.5 to several microns.

In cross section myofibrils are round, angular, or oval. Most of them are made up of very fine protein filaments called myofilaments, or protofibrils. There are two types of myofilaments: thick myofilaments, which consist primarily of myosin and are about 1,500 nanometers (nm) long and 10–15 nm in diameter; and thin myofilaments, which consist primarily of actin and are 1,000–1,100 nm long and 5–8 nm in diameter. Other proteins found in myofilaments are tropomyosin B, which occurs in thin myofilaments of all kinds; tropomyosin A, or paramyosin, which is present in the thick myofilaments of muscles with double diagonal striation; α-actinine and β-actinine; and troponin.

Thin myofilaments adhere to the Z band, a complex intertwining of protein filaments. The portion of a myofibril that lies between two Z bands is called the sarcomere. Thick myofilaments make up the anisotropic band (the A band), the solid, birefringent portion of a myofibril. Thin myofilaments are partly wedged in between the thick myofilaments (the zone of overlap). The sections of sarcomeres that are located on either side of the Z membranes and that contain only thin myofilaments are referred to as the isotropic, or I band. The central zone of the A band, which lacks thin myofilaments, is called the H zone. An M band consisting of short (40-nm) M-threads running along the longitudinal axis of the myofibril can usually be seen in the center of the H zone. The length of the M-threads equals the width of the M band. On both sides of the M band are the H subzones, narrow (approximately 130 nm) bands lighter than the rest of the H zone. Distributed evenly over the entire length of the thick myofilaments are projections (“bridges”) that are evidently the ends of myasin molecules that branch off the myofilaments. The H subzones appear to be lighter because the middle of the thick myofilaments lacks the myosin bridges.

The hypothetical structure of myofibrils is open to criticism. For example, when the myofibrils are stretched a great deal, the thin myofilaments should emerge completely from the A band, and the sarcomere should fragment. However, this does not happen, possibly because of the existence of a third type of myofilament—“ultrathin filaments,” which connect the Z bands.

REFERENCES

Loewy, A., and P. Siekevitz. Struktura i funktsii kletki. Moscow, 1971. (Translated from English.)
Hill, A. Mekhanika myshechnogo sokrashcheniia. Moscow, 1972. (Translated from English.)
References in periodicals archive ?
However, in areas that lack hydrostatic resistance and at the level of individual muscle cells, the myofibrils must act against the connective fibers of the mesoglea.
Myofibrils showed multifocal areas of degeneration and necrosis, with or without mineralization, with non-suppurative inflammation mainly by macrophages, as well as fibrosis as a repair response in chronic cases (COOPER & VALENTINE, 2016).
In the ultrastructure analysis, the long-term mild diabetic group presented well-organized myofibrils and myelin figures associated with degenerated organelles (Figure 4b).
This compartment is composed of red muscle fibers, slow twitch and oxidative metabolism, have small diameter (between 25 and 45 [micro]m), excellent blood supply, high concentration of myoglobin, many mitochondria in the subsarcolemmal region and between the myofibrils, and many droplets lipids (Altringham & Johnston, 1981, Johnston, 1981, Sanger & Stoiber, 2001).
Besides, it must be pointed out that abalone and all the meat species have a good water holding capacity, due to their protein feature of a myofibril origin [52].
After 12 weeks of terminal differentiation, mature sarcomeres were observed to be assembled into myofibrils. Morphological hallmarks, including I- band of actin filaments and A-band with distinct M line across myosin filaments, were clearly visible.
(a-c) Representative electron micrographs for mitochondria, myofibrils, and sarcoplasmic reticulum, respectively.
Li, "Inhibition of frozen storage-induced oxidation and structural changes in myofibril of common carp (Cyprinus carpio) surimi by cryoprotectant and hydrolysed whey protein addition," International Journal of Food Science & Technology, vol.
Severe global myocardial hypokinesia and increase in CPK due to destruction in myofibrils are the most important clinical presentation of sunitinib-induced rhabdomyolysis.
Trinick, "On the mechanism of water holding in meat: the swelling and shrinking of myofibrils," Meat Science, vol.
The fetal type shows a haphazard mixture of round to spindled mesenchymal cells showing myofibrils. The cells show a gradient of maturation toward the periphery (figure 1), with an associated myxoid mucopolysaccharide matrix.