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250 nm

Actin molecule

Actin molecule

25 nm

Myosin molecule

Figure 2. Fibrillar and molecular structure of the contractile elements of muscle. The skeletal muscle fiber in longitudinal section (a), as visualized by the light microscope; (b) and (c), by the electron microscope; and (d), as reconstructed from X-ray crystallo-graphic evidence. Source: Réf. 1, used with permission.

25 nm

Myosin molecule

Figure 2. Fibrillar and molecular structure of the contractile elements of muscle. The skeletal muscle fiber in longitudinal section (a), as visualized by the light microscope; (b) and (c), by the electron microscope; and (d), as reconstructed from X-ray crystallo-graphic evidence. Source: Réf. 1, used with permission.

tochondria, and the normal complement of cell organelles (Fig. 2). In particular, the cell is dominated by a unique arrangement of filamentous proteins, sarcoplasmic reticulum (termed endoplasmic reticulum in nonmuscle cells), and glycogen particles. In an electron micrograph, longitudinal sections show that the filaments consist of both thick (17 nm wide) and thin (8 nm wide) filaments corresponding mainly to the proteins myosin and actin, respectively. The thin filaments are attached to densely staining structures termed Z-lines, whereas the myosin filaments lie between the actin filaments and are joined to each other at the middle portion by an M-line. These structures, which together form a repeating unit from Z-line to Z-line termed a sarcomere, can be seen together with a further explanation in Figure 2. The part of the sarcomere containing myosin filaments is termed the A-band, and the part containing actin filaments is termed the I-band. Other filament structures, including filaments found in the gap of overstretched muscle (3,4), which have been now termed titin filaments, and proteins such as desmin and nebulin, are found with certain isolation, extraction, and staining procedures (see "Muscle to Meat" as follows). In transverse sections, the filaments appear as various size dots, and the Z-lines appear as smudges. When tissue is prepared from muscle at various stages of contraction, longitudinal sections show various degrees of interdigitation and overlap of thick and thin filaments. Transverse sections made at various levels of the sarcomere show corresponding patterns of dots associated with the overlap of the thick and thin filaments. Muscle and meat biochemistry have been reviewed (5).

Lying at the edge of the actin and myosin filament overlap, there is a system of tubules, termed T-tubules, derived from invaginations of the sarcolemma. The other tubular system, that is, the sarcoplasmic reticulum, lies within the cytoplasm of the muscle cell. The sarcoplasmic reticulum stores calcium ions. It is the controlled release of the calcium from the sarcoplasmic reticulum, raising the cytoplasmic calcium levels from 10"8 to 10" 4 M, that causes a contraction (a shortening) in living muscle. The calcium ions are then retaken up into the sarcoplasmic reticulum and the muscle cell relaxes. The release of calcium from the sarcoplasmic reticulum is triggered by a wave of depolarisation via the T-tubules. Contraction occurs by the actin and myosin filaments sliding with respect to each other by attachment, release, and reattachment of portions of the myosin molecules (namely, myosin heads, also termed crossbridges), with specific regions of the actin filaments. This contractile activity, initiated via release of calcium from the sarcoplasmic reticulum, is activated and sustained by adenosine triphosphate (ATP). In the presence of calcium and absence of ATP (<1 //mol/g [6]), the muscles are in rigor, usually as a consequence of death. Eventually, ATP is completely transformed to inosine monophosphate, one of the flavor compounds of meat.

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