Striations

Myosin and actin are not unique to muscle; these proteins occur in all cells, where they function in cellular motility, mitosis, and transport of intracellular materials. In skeletal and cardiac muscle they are especially abundant, however, and are organized in a precise array that accounts for the striations of these two muscle types (fig. 11.4).

Striated muscle has dark A bands alternating with lighter I bands. (A stands for anisotropic and I for isotropic, which refers to the way these bands affect polarized light. To help remember which band is which, think "dArk" and "light.") Each A band consists of thick filaments lying side by side. Part of the A band, where thick and thin filaments overlap, is especially dark. In this region, each thick filament is surrounded by thin filaments. In the middle of the A band, there is a lighter region called the H band,4 into which the thin filaments do not reach.

Each light I band is bisected by a dark narrow Z disc5 (Z line) composed of the protein connectin. The Z disc provides anchorage for the thin filaments and elastic filaments. Each segment of a myofibril from one Z disc to the next is called a sarcomere6 (SAR-co-meer), the functional contractile unit of the muscle fiber. A muscle shortens because its individual sarcomeres shorten and pull the Z discs closer to each other, and the Z discs are connected to the sarcolemma by way of the cytoskeleton. As the Z discs are pulled closer together during contraction, they pull on the sarcolemma to achieve overall shortening of the cell.

The terminology of muscle fiber structure is reviewed in table 11.1; this table may be a useful reference as you study the mechanism of contraction.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. What special terms are given to the plasma membrane, cytoplasm, and smooth ER of a muscle cell?
  2. What is the difference between a myofilament and a myofibril?
  3. List five proteins of the myofilaments and describe their physical arrangement.
  4. Sketch the overlapping pattern of myofilaments to explain how they account for the A bands, I bands, H bands, and Z discs.

5Z = Zwichenscheibe = "between disc"

6sarco = muscle + mere = part, segment

Saladin: Anatomy & I 11. Muscular Tissue I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

412 Part Two Support and Movement

412 Part Two Support and Movement

Saladin Muscle Striations

Elastic filament

Thin filament

Thick filament

Elastic filament

Thick filament

Thick And Thin Myofilaments

Figure 11.4 Muscle Striations and Their Molecular Basis.

(a) Five myofibrils of a single muscle fiber, showing the striations in the relaxed state. (b) The overlapping pattern of thick and thin myofilaments that accounts for the striations seen in figure a. (c) The pattern of myofilaments in a contracting muscle fiber. Note that all myofilaments are the same length as before, but they overlap to a greater extent.

Which band narrows or disappears when muscle contracts?

Figure 11.4 Muscle Striations and Their Molecular Basis.

(a) Five myofibrils of a single muscle fiber, showing the striations in the relaxed state. (b) The overlapping pattern of thick and thin myofilaments that accounts for the striations seen in figure a. (c) The pattern of myofilaments in a contracting muscle fiber. Note that all myofilaments are the same length as before, but they overlap to a greater extent.

Which band narrows or disappears when muscle contracts?

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  • kyra
    Which band narrows or disappears when muscle contracts?
    8 years ago

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