Myofibrils and Myofilaments

The structural and functional subunit of the muscle fiber is the myofibril

Skeletal muscles are composed of fascicles, which in turn are composed of individual muscle fibers. The muscle fiber is filled with longitudinally arrayed subunits called myofibrils (Fig. 10.3). Myofibrils are visible in favorable histologic preparations and are best seen in cross sections of muscle fibers. In these sections they give the fiber a stippled appearance. Myofibrils extend the entire length of the muscle cell.

Myofibrils are composed of bundles of myofilaments

Myofilaments are the individual filamentous polymers of myosin II (thick filaments) and actin and its associated proteins (thin filaments). Myofilaments are the actual contractile elements of striated muscle. The bundles of myofilaments that make up the myofibril are surrounded by a well-developed smooth endoplasmic reticulum (sER), also called the sarcoplasmic reticulum. This reticulum forms a highly organized tubular network around the contractile elements in all striated muscle cells. Mitochondria and glycogen deposits are located between the myofibrils in association with the sER.

Cross-striations are the principal histologic feature of striated muscle

Cross-striations are evident in H&cE-stained preparations of longitudinal sections of muscle fibers. They may also be

250 chapter 10 I Muscle Tissue BOX 10.1

Functional Considerations: Muscle Metabolism and Ischemia

Like all cells, muscle cells depend on the energy source contained in the high-energy phosphate bonds of ATP and phos-phocreatine. The energy stored in these high-energy phosphate bonds comes from the metabolism of fatty acids and glucose. Glucose is the primary metabolic substrate in actively contracting muscle. It is derived from the general circulation as well as from the breakdown of glycogen, which is normally stored in the muscle fiber cytoplasm. As much as 1% of the dry weight of skeletal and cardiac muscle may be glycogen.

In rapidly contracting muscles, such as the leg muscles in running or the extraocular muscles, most of the energy for contraction is supplied by anaerobic glycolysis of stored glycogen. The buildup of intermediary metabolites from this pathway, particularly lactic acid, can produce an oxygen deficit that causes ischemic pain (cramp) in cases of extreme muscular exertion.

Most of the energy used by muscle recovering from contraction or by resting muscle is derived from oxidative phosphorylation. This process closely follows the /3-oxidation of fatty acids in mitochondria that liberates two carbon fragments. The oxygen needed for oxidative phosphorylation and other terminal metabolic reactions is derived from hemoglobin in circulating erythrocytes and from oxygen bound to myoglobin stored in the muscle cells.

seen in unstained preparations of living muscle fibers examined with a phase contrast or polarizing microscope, in which they appear as alternating light and dark bands. These bands are termed the A band and the I band (Fig. 10.3).

In polarizing microscopy, the dark bands are biréfringent; i.e., they alter the polarized light in two planes. Therefore, the dark bands, being doubly refractive, are anisotropic and are given the name A band. The light bands are monorefringent; i.e., they do not alter the plane of polarized light. Therefore, they are isotropic and are given the name I band.

Both the A and I bands are bisected by narrow regions of contrasting density (see Fig. 10.3). The light I band is bisected by a dense line, the Z line, also called the Z disk [Cer. Zwischenscheibe, between disks]. The dark A band is bisected by a less dense, or light, region called the H band [Ger. Hell, light]. Furthermore, bisecting the light H band is a narrow dense line called the M line [Get: Mitte, middle]. The M line is best demonstrated in electron micrographs (Fig 10.4), although in ideal H&cE preparations it can be detected in the light microscope.

As noted above, the cross-banding pattern of striated muscle is due to the arrangement of the two kinds of myofilaments. To understand the mechanism of contraction, this banding pattern must be considered in functional terms.

Myofibril Regions During Contraction

I band H band M line A band overlap

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