The Sliding Filament Theory Explains Muscle Contraction

The structure of skeletal muscle provides important clues to the mechanism of contraction. The width of the A bands (thick-filament areas) in striated muscle remains constant, regardless of the length of the entire muscle fiber, while the width of the I bands (thin-filament areas) varies directly with the length of the fiber. At the edges of the A band are fainter bands whose width also varies. These represent material extending into the A band from the I bands. The spacing between Z lines also depends directly on the length of the fiber. The lengths of the thin and thick myofilaments remain constant despite changes in fiber length.

The sliding filament theory proposes that changes in overall fiber length are directly associated with changes in the overlap between the two sets of filaments,- that is, the thin filaments telescope into the array of thick filaments. This interdigitation accounts for the change in the length of the muscle fiber. It is accomplished by the interaction of the globular heads of the myosin molecules (crossbridges, which project from the thick filaments) with binding sites on the actin filaments. The crossbridges are the sites where force and shortening are produced and where the chemical energy stored in the muscle is transformed into mechanical energy. The total shortening of each sarcomere is only about 1 |xm, but a muscle contains many thousands of sarcomeres placed end to end (in series). This arrangement has the effect of multiplying all the small sarcomere length changes into a large overall shortening of the muscle (Fig. 8.8). Similarly, the amount of force exerted by a single sarcomere is small (a few hundred micronewtons), but, again, there are thousands of sarcomeres side by side (in parallel), resulting in the production of considerable force.

The effects of sarcomere length on force generation are summarized in Figure 8.9. When the muscle is stretched beyond its normal resting length, decreased filament overlap occurs (3.65 |xm and 3.00 |xm, Fig. 8.9). This limits the

Least overlap

Least overlap

Moderate overlap

Moderate overlap

The multiplying effect of sarcomeres placed in series. The overall shortening is the sum of the shortening of the individual sarcomeres.

Most overlap

The multiplying effect of sarcomeres placed in series. The overall shortening is the sum of the shortening of the individual sarcomeres.

amount of force that can be produced, since a shorter length of thin filaments interdigitates with A band thick filaments and fewer crossbridges can be attached. Thus, over this region of lengths, force is directly proportional to the degree of overlap. At lengths near the normal resting length of the muscle (i.e., the length usually found in the body), the amount of force does not vary with the degree of overlap (2.25 |xm and 1.95 |xm, Fig. 8.10) because of the bare zone (the H zone) along the thick filaments at the center of the A band (where no myosin heads are present).

Sliding Filament Theory

Effect of filament overlap on force generation. The force a muscle can produce depends on the amount of overlap between the thick and thin filaments because this determines how many crossbridges can interact effectively. (See text for details.)

Over this small region, further interdigitation does not lead to an increase in the number of attached crossbridges and the force remains constant.

At shorter lengths, additional geometric and physical factors play a role in myofilament interactions. Since muscle is a "telescoping" system, there is a physical limit to the amount of shortening. As thin myofilaments penetrate the A band from opposite sides, they begin to meet in the middle and interfere with each other (1.67 ^m, Fig. 8.9). At the extreme, further shortening is limited by the thick filaments of the A band being forced against the structure of the Z lines (1.27 ^m, Fig. 8.9).

The relationship between overlap and force at short lengths is more complex than that at longer lengths, since more factors are involved. It has also been shown that at very short lengths, the effectiveness of some of the steps in the excitation-contraction coupling process is reduced. These include reduced calcium binding to troponin and some loss of action potential conduction in the T tubule system. Some of the consequences for the muscle as a whole are apparent when the mechanical behavior of muscle is examined in more detail (see Chapter 9).

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • Nadine
    What theory explains how muscle contract?
    5 years ago
  • milla
    What is the sliding filament theory Explain.?
    5 years ago
  • KY
    How can muscle imbalances change the normal pattern of the sliding filament theory?
    5 years ago
  • Amanuel Alem
    Does sliding filament mechanism lead to whole muscle function?
    4 years ago
  • arsi lampinen
    What is sliding filament hypothesis explains the process of contraction of skeletal muscle?
    9 months ago

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