Myofibrils


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Related to Myofibrils: sarcoplasmic reticulum, sarcomere, sarcolemma, thick filaments

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 ?
As a comparison with abalone, red meat muscle tissue consists of contractible myofibrils assembled into parallel fibrous bundles interconnected by collagen.
Here the structural hierarchy starts from myofibrils which are composed of repeating units of myosin and actin filaments, called sarcomeres.
The alterations were characterized by the occurrence of intense necrotized muscle fibers (without a nucleus) and degenerating muscle fibres (with a peripheral nucleus), showing loss of cell structure accompanied by myofibril hypercontraction.
The cardiac troponin T-I-C complex (cTnT, cTnI, and TnC) is structurally bound predominantly to the myofibrils, with only a minor fraction of cTnT and cTnI (6%-8%) present in the cytoplasm as soluble intact protein (4,5), although this finding has been disputed (6).
Furthermore, the combination of coactivation, variable external resistance and difficulty in controlling velocity of contractions may result in activated muscle fibres being lengthened and hence possibly ultra-structural damage to the myofibrils.
70% of the water of the flesh is thought to be located in the myofibrils, 20% in the sarcoplasm and 10% in the connective tissue [17].
Typical changes in the adriamycin-induced heart failure are increased free radicals accompanied by a decrease of endogenous antioxidants; thesubsequentincrease in oxidants results in enhanced oxidative stress leading to a slow loss of myofibrils and vacuolization of myocardial cells.
In myofibrils the energy-rich bond is 'returned' to ADP by another, MM CK isoenzyme to form locally a newly synthesized ATP, which ensures the muscle contraction in the myosin ATPase reaction (see Fig.
2003) has added that textural properties are thought to depend on fibrillar diameter, and also other structural parameters including the distance between myofibrils.
This presentation concluded that cardiac myocytes isolated from rats with heart failure do not become desensitized to cardiac myosin activators and that cardiac myofibrils isolated from rats with heart failure respond similarly to a myosin activator in comparison to cardiac myofibrils isolated from experimental controls.
3] Myofibrillar loss is characterized by partial or total loss of myofibrils within the cell, with Z band remnants remaining.
Results from Table 1 demonstrated the marinade containing salt and phosphate increased pH value, which was similar with Offer and Trinick (1983) and Cheng and Sun (2008), who explained that chloride ions formed an ion"cloud"around the meat protein filaments, which resulted an increase of net negative charges and osmotic pressure within the myofibrils, further caused the swelling of filament lattice and the increasing of the pH value.