Myosin crossbridge

The structure and arrangement of the actin and myosin filaments in muscle. During muscle contraction the cyclic interaction of myosin crossbridges with actin filaments draws the actin filaments across the myosin filaments. 

The crossbridge cycle in muscle. Myosin crossbridges interact cyclically with binding sites on actin filaments. Note that the energy release step—when ATP is broken down to ADP—recocks the crossbridge head. 

Tropomyosin is thought to lie in the groove formed between the associated actin strands. The sites at which the myosin crossbridges attach are affected by the relationship between tropomyosin and the actin strands. The role of tropomyosin in smooth muscle is completely undefined while in striated muscle it is clearly involved in the activation of contraction. The difference is made clear by the absence from smooth muscle of the protein, troponin, which in striated muscle provides the binding site for the triggering calcium. 

In his theory, A.F. Huxley suggested that a myosin crossbridge attaches to actin and produces an amount of force which is proportional to the amount of strain on the crossbridge, that is to the amount by which it is distorted (Figure 5). He also suggested that crossbridges were only able to attach over a range of preferred distortions. From these basic ideas he derived mathematical expressions for force... 

Figure 7. Length tension relationship. A schematic diagram showing how force varies with sarcomere length, and how this is explained by the relative amount of overlap between the thick and the thin filaments, and hence the numbers of myosin crossbridges in the thick filaments that can interact with actin in the thin filaments.

Explain the functions of the following myosin crossbridges, troponin, tropomyosin, sarcomeres, Z lines, neuromuscular junction, transverse tubules, and sarcoplasmic reticulum... 

In the absence of ATP, myosin crossbridges are unable to release the actin. As a result, the sarcomeres, and therefore the muscle, remain contracted. This phenomenon is referred to as rigor mortis. Following death, the concentration of intracellular calcium increases. This calcium allows the... 

Fast-twitch muscle fibers develop tension two to three times faster than slow-twitch muscle fibers because of more rapid splitting of ATP by myosin ATPase. This enables the myosin crossbridges to cycle more rapidly Another factor influencing the speed of contraction involves the rate of removal of calcium from the cytoplasm. Muscle fibers remove Ca++ ions by pumping them back into the sarcoplasmic reticulum. Fast-twitch muscle fibers remove Ca++ ions more rapidly than slow-twitch muscle fibers, resulting in quicker twitches that are useful in fast precise movements. The contractions generated in slow-twitch muscle fibers may last up to 10 times longer than those of fast-twitch muscle fibers therefore, these twitches are useful in sustained, more powerful movements. 

Upon entering the smooth muscle cell, Ca++ ions bind with calmodulin, an intracellular protein with a chemical structure similar to that of troponin. The resulting Ca++-calmodulin complex binds to and activates myosin kinase. This activated enzyme then phosphorylates myosin. Crossbridge cycling in smooth muscle may take place only when myosin has been phosphorylated. 

Contraction of smooth muscle is significantly slower than that of skeletal muscle. Furthermore, smooth muscle contraction is quite prolonged (3000 msec) compared to that in skeletal muscle (100 msec). The slow onset of contraction as well as its sustained nature is due to the slowness of attachment and detachment of the myosin crossbridges with the actin. Two factors are involved ... 

In smooth muscle, myosin crossbridges have less myosin ATPase activity than those of skeletal muscle. As a result, the splitting of ATP that provides energy to "prime" the crossbridges, preparing them to interact with actin, is markedly reduced. Consequently, the rates of crossbridge cycling and tension development are slower. Furthermore, a slower rate of calcium removal causes the muscle to relax more slowly. 

Figure 13.6 Myosin and actin molecules and myosin crossbridges. Each kind of filament is composed of a different protein myosin in the thick filaments and actin in the thin filaments. In the case of actin, the individual F-actins are more or less spherical but a large number of these combine to produce a long chain, two of which wind around each other, rather like a rope, to produce the thin filament. The myosin molecule is more complex and shaped somewhat like a golf club. To form the thick filament, the shafts aggregate to leave the heads protruding on all sides. These heads form the cross-bridges and are responsible for pulling the thin filaments into the spaces between the thick filaments (see Figure 13.5).

Harford, J. J., and Squire, J. M. (1986). The crystalline myosin crossbridge array in relaxed bony fish muscles. Biophys. J. 50, 145-155. 

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