Fibers

Skeletal muscle fibers consist of long cylindrical fibers with multiple ovoid nuclei located peripherally beneath the sarcolemma (plasma membrane) and with striations composed of alternating dark and light bands.

• The dark bands are called A bands because they are anisotropic (birefringent) in polarized light. In the center of the A band a paler region, the H band, is seen in relaxed muscle.

■ The light bands are called I bands (isotropic), and a dark transverse line, the Z line, bisects each 1 band

These bands and the Z lines are well demonstrated in electron micrographs of skeletal muscle (Figure 1-3-2).

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Myofibrils

Skeletal muscle fibers contain 1- to 2-mm myofibrils that lie in the sarcoplasm (cytoplasm) parallel to the long axis of the muscle fiber (Figure 1-3-2). Myofibrils are composed of a series of sarcomeres that consist of interdigitating polarized thin filaments and bipolar thick filaments (Figure 1-3-3). The sarcomeres are the basic units of contraction of striated muscle.

Figure 1-3-2. EM of Skeletal Muscle

Sarcomere Structure

The banding pattern seen in striated muscle is caused by the arrangement of thin and thick myofilaments (Figure 1-3-3).

• Thick filaments occupy the central portions of the sarcomere.

■ Thin filaments attach at one end to the Z line and run parallel to, and between, the thick filaments.

• I bands are composed of thin filaments only.

■ A bands are composed mosdy of thick filaments and the thin filaments between them.

■ H bands are composed of thick filaments only.

Thin filaments

Thin filaments are composed of the proteins actin, tropomyosin, and troponin.

• Actin is a long fibrous structure (F-actin) composed of two strands of spherical or globular G-actin monomers twisted in a double helix. The filament is polar and contains myosin-binding sites on the G-actLn monomers.

• Tropomyosin is a polar molecule containing two polypeptide chains in the form of an a-helix. The tropomyosin molecules lie head-to-tail to form filaments that lie in the grooves of the actin helix.

■ There are two triads in each sarcomere, which are present at the junction between the A and I bands.

• These units serve to couple excitation of muscle cells to their contraction (excitation-contraction coupling).

Sarcomere

I band A band

I band A band

Terminal cisterna Transverse tubules

Figure i-3-4. Striated Muscle Fiber Showing Sarcoplasmic Reticulum andT-Tubule System

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