Sliding Filament Theory of Contraction

When a muscle contracts it decreases in length as a result of the shortening of its individual fibers. Shortening of the muscle fibers, in turn, is produced by shortening of their myofibrils, which occurs as a result of the shortening of the distance from Z line to Z line. As the sarcomeres shorten in length, however, the A bands do not shorten but instead move closer together. The I bands—which represent the distance between A bands of successive sarcomeres—decrease in length (table 12.2).

The thin filaments composing the I band, however, do not shorten. Close examination reveals that the thick and thin filaments remain the same length during muscle contraction.

Asynchronous Muscle BandBands Sarcomere

I I band A band

Sarcomere

I I band A band

Sarcomere

■ Figure 12.6 The striations of skeletal muscles are produced by thick and thin filaments. (a) Electron micrograph of a longitudinal section of myofibrils, showing the banding pattern characteristic of striated muscle. (b) Illustration of the arrangement of thick and thin filaments that produces the banding pattern. The colors used in (a) to depict different bands and structures correspond to the colors of (b).

332 Chapter Twelve

332 Chapter Twelve

Cross Section Sarcomere

Sarcomere

Z disc Z disc

Z disc Z disc

Sliding Filament Theory Steps

■ Figure 12.7 Arrangement of thick and thin filaments in a striated muscle fiber. (a) In a longitudinal section, the thick and thin filaments are seen to form repeating units called sarcomeres. The banding patterns of the sarcomeres are labeled I, A, and H, as shown. A corresponding electron micrograph (53,000x) is shown to the right of the illustration. (b) The three-dimensional structure of the sarcomeres is illustrated. This three-dimensional structure can be seen in a cross section of a myofibril taken through a region of overlaping thick and thin filaments. In the electron micrograph, the arrows point to cross bridges between the thick filaments (dark dots) and thin filaments (light dots). (SR = sarcoplasmic reticulum; M = mitochondria).

Electron micrographs (right) from R. G. Kessel and R. H. Kardon, Tissues and Organs: A Test-Atlas of Scanning Electron Microscopy, 1979, W. H. Freeman & Company.

, Titin filament Thin filament M line Thick filament

Sarcomere

■ Figure 12.8 Titin filaments and M lines. The M lines are protein filaments in the middle of the A bands that join thick filaments together. Titin proteins are elastic proteins of extremely large size that run through the thick filaments, beginning at the M lines and ending at the Z discs. These stabilize the position of each thick filament within the sarcomere and serve as elastic elements that help muscles return to their resting length.

Muscle 333

Table 12.2 Summary of the Sliding Filament Theory of Contraction

1. A myofiber, together with all its myofibrils, shortens by movement of the insertion toward the origin of the muscle.

2. Shortening of the myofibrils is caused by shortening of the sarcomeres— the distance between Z lines (or discs) is reduced.

3. Shortening of the sarcomeres is accomplished by sliding of the myofilaments— the length of each filament remains the same during contraction.

4. Sliding of the filaments is produced by asynchronous power strokes of myosin cross bridges, which pull the thin filaments (actin) over the thick filaments (myosin).

5. The A bands remain the same length during contraction, but are pulled toward the origin of the muscle.

6. Adjacent A bands are pulled closer together as the I bands between them shorten.

7. The H bands shorten during contraction as the thin filaments on the sides of the sarcomeres are pulled toward the middle.

Shortening of the sarcomeres is produced not by shortening of the filaments, but rather by the sliding of thin filaments over and between the thick filaments. In the process of contraction, the thin filaments on either side of each A band slide deeper and deeper toward the center, producing increasing amounts of overlap with the thick filaments. The I bands (containing only thin filaments) and H bands (containing only thick filaments) thus get shorter during contraction (fig. 12.9).

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Responses

  • ADAMO
    Why sliding filament theory important?
    5 years ago
  • HAZEL
    Which regions shorten during muscle contraction?
    5 years ago
  • Pirkko Pekkarinen
    Does a band remain same length during muscle contraction?
    5 years ago
  • alem
    Why do h bands shorten?
    5 years ago
  • kristin eiffel
    Why do bands in sarcomere shorten?
    5 years ago
  • reginard
    Which filament shortens during contraction?
    5 years ago
  • marko
    Does distance between z disks and sarcomere decrease?
    5 years ago
  • saverio
    When a muscle shortens during contraction the myosin filaments shorten the a bands shorten?
    4 years ago
  • daisy
    Why does the I band and the H band shorten during contraction?
    4 years ago
  • Ross
    Do sarcomeres shorten in sliding filament theory of contraction?
    3 years ago
  • isaias
    How does the sliding filament theory related to isometric contraction?
    2 years ago
  • tytti
    Is sarcomere shortening the same as the sliding filament theory?
    1 year ago
  • tony smith
    Do both thick and thin filaments shorten in sliding filament theory?
    11 months ago

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