The Structures Found in a Sarcomere
A sarcomere is the basic contractile unit of muscle fiber. Each sarcomere is composed of two main protein filaments—actin and myosin—which are the active structures responsible for muscular contraction. The most popular model that describes muscular contraction is called the sliding filament theory. In this theory, active force is generated as actin filaments slide past the myosin filaments, resulting in contraction of an individual sarcomere. The thick myosin filament contains numerous heads, which when attached to the thinner actin filaments create actin-myosin cross bridges. 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.
In fact, the contractile properties of muscle are a defining characteristic of animals. Animal movement is notably smooth and complex. Dexterous movement requires a change in muscle length as the muscle flexes. This calls for a molecular structure that can shorten along with the shortening muscle. Such requisites are found in the sarcomere. 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.
Is muscle contraction completely understood? Scientists are still curious about several proteins that clearly influence muscle contraction, and these proteins are interesting because they are well conserved across animal species. For example, molecules such as titin, an unusually long and "springy" protein spanning sarcomeres in vertebrates, appears to bind to actin, but it is not well understood. In addition, scientists have made many observations of muscle cells that behave in ways that do not match our current understanding of them. For example, some muscles in mollusks and arthropods generate force for long periods, a poorly understood phenomenon sometimes called "catch-tension" or force hysteresis (Hoyle 1969). Studying these and other examples of muscle changes (plasticity) are exciting avenues for biologists to explore. Ultimately, this research can help us better understand and treat neuromuscular systems and better understand the diversity of this mechanism in our natural world. One important refinement of the sliding filament theory involved the particular way in which myosin is able to pull upon actin to shorten the sarcomere. 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 .
In the final analysis, mutants with specific defects in the hypodermis may help dissect the structure and function of the hemidesmosomal complex in a manner similar to what has been achieved for the muscle dense body. 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.