The Structures Found in a Sarcomere
In essence, a myosin head is similar to a cocked spring, which on binding with an actin filament flexes and produces a power stroke. The power stroke slides the actin filament past the myosin, resulting in force generation and shortening of an individual sarcomere. Because sarcomeres are joined end to end throughout an entire muscle fiber, their simultaneous contraction shortens the entire muscle.
Upon closer inspection, skeletal muscle tissue gives off a striped appearance, called striation. These “stripes” are given off by a pattern of alternating light and dark bands corresponding to different protein filaments. These stripes are formed by the interlocking fibers that comprise each sarcomere. Tubular fibers called myofibrils are the basic components that form muscle tissue. However, myofibrils themselves are essentially polymers, or repeating units, of sarcomere. Myofibrils are fibrous and long, and made of two types of protein filament that stack on top of each other. Myosin is a thick fiber with a globular head, and actin is a thinner filament that interacts with myosin when we flex.When viewed under a microscope, muscle fibers of varied lengths are organized in a stacked pattern. The myofibril strands, thereby actin and myosin, form bundles of filament arranged parallel to one another. When a muscle in our body contracts, it is understood that the way this happens follows the sliding filament theory. This theory predicts that a muscle contracts when filaments are allowed to slide against each other. This interaction, then, is able to yield contractile force. However, the reason the sarcomere structure is so crucial in this theory is that a muscle needs to physically shorten. Thus, there is a need for a unit that is able to compensate for the lengthening or shortening of a flexing muscle. The sliding filament theory was first posited by scientists who had used high-resolution microscopy and filament stains to observe myosin and actin filaments in action at various stages of contraction. They were able to visualize the physical lengthening of the sarcomere in its relaxed state, and the shortening in its contracted state. Their observations led to the discovery of sarcomere zones.
Scientists have demonstrated that the globular end of each myosin protein that is nearest actin, called the S1 region, has multiple hinged segments, which can bend and facilitate contraction (Hynes et al. 1987; Spudich 2001). The bending of the myosin S1 region helps explain the way that myosin moves or "walks" along actin. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility, and it rotates in concert with the S1 contraction .
For example, it will be interesting to see if this structure has components common to mammalian hemidesmosomes, in particular an α6-β4 integrin complex. Beyond merely dissecting the attachment complexes of muscle and hypodermis, we will need to resolve how these two tissues coordinate and facilitate their behavior during morphogenesis if we are to understand how a fully functional muscle quadrant is established.
Clark, M. Milestone 3 (1954): Sliding filament model for muscle contraction. Muscle sliding filaments. Nature Reviews Molecular Cell Biology 9, s6–s7 (2008) doi:10.1038/nrm2581.
Goody, R. S. The missing link in the muscle cross-bridge cycle. Nature Structural Molecular Biology 10, 773–775 (2003) doi:10.1038/nsb1003-773.
Hoyle, G. Comparative aspects of muscle. Annual Review of Physiology 31, 43–82 (1969) doi:10.1146/annurev.ph.31.030169.000355.
Huxley, H. E. & Hanson, J. Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation. Nature 173, 973–976 (1954) doi:10.1038/173973a0.
Huxley, A. F. & Niedergerke, R. Structural changes in muscle during contraction: Interference microscopy of living muscle fibres. Nature 173, 971–973 (1954) doi:10.1038/173971a0.
Hynes, T. R. et al. Movement of myosin fragments in vitro: Domains involved in force production. Cell 48, 953–963 (1987) Doi:10.1016/0092-8674(87)90704-5.