What Happens to the Actin and Myosin Filaments When a Muscle Contracts?

How do muscles contract? What molecules are necessary for a tissue to alter its shape?

Muscle is a specialized contractile tissue that is a distinguishing characteristic of animals. Changes in muscle length support an exquisite array of animal movements, from the dexterity of octopus tentacles and peristaltic waves of Aplysia feet to the precise coordination of linebackers and ballerinas. What molecular mechanisms give rise to muscle contraction? The process of contraction has several key steps, which have been conserved during development across the bulk of animals

What Is a Sarcomere?

When muscle cells are viewed nether the microscope, one can run across that they contain a striped pattern (striations). This pattern is formed by a series of basic units called sarcomeres that are arranged in a stacked pattern throughout muscle tissue (Figure 1). In that location can be thousands of sarcomeres within a unmarried muscle cell. Sarcomeres are highly stereotyped and are repeated throughout muscle cells, and the proteins within them can change in length, which causes the overall length of a muscle to change. An individual sarcomere contains many parallel actin (thin) and myosin (thick) filaments. The interaction of myosin and actin proteins is at the core of our electric current agreement of sarcomere shortening. How does this shortening happen? It has something to do with a sliding interaction between actin and myosin.

Figure i: A gastrocnemius muscle (calf) with striped pattern of sarcomeres

The view of a mouse gastrocnemius (dogie) muscle under a microscope. The sarcomeres are artifically colored green, and appear equally stacked horzontal stripes of similar lengths. (White calibration bar = 25 microns.)

© 2008 Nature Publishing Group Llewellyn, M. E. et al. Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans. Nature 454, 784-788 (2008). All rights reserved.

The Sliding Filament Theory

In 1954, scientists published two groundbreaking papers describing the molecular basis of muscle contraction. These papers described the position of myosin and actin filaments at diverse stages of contraction in muscle fibers and proposed how this interaction produced contractile force. Using high-resolution microscopy, A. F. Huxley and R. Niedergerke (1954) and H. E. Huxley and J. Hanson (1954) observed changes in the sarcomeres as muscle tissue shortened. They observed that one zone of the repeated sarcomere arrangement, the "A band," remained relatively constant in length during contraction (Figure 2A). The A band contains thick filaments of myosin, which suggested that the myosin filaments remained central and constant in length while other regions of the sarcomere shortened. The investigators noted that the "I band," rich in thinner filaments made of actin, changed its length along with the sarcomere. These observations led them to propose the sliding filament theory, which states that the sliding of actin past myosin generates muscle tension. Because actin is tethered to structures located at the lateral ends of each sarcomere chosen z discs or "z bands," any shortening of the actin filament length would result in a shortening of the sarcomere and thus the musculus. This theory has remained impressively intact (Figure 2B).

Effigy two: Comparison of a relaxed and contracted sarcomere

(A) The basic organization of a sarcomere subregion, showing the centralized location of myosin (A band). Actin and the z discs are shown in red. (B) A conceptual diagram representing the connectivity of molecules within a sarcomere. A person standing between 2 bookcases (z bands) pulls them in via ropes (actin). Myosin (M) is analogous to the person and the pulling artillery. (z bands are also called z discs.)

© Nature Publishing Grouping A) adapted from Huxley, A. F. & Niedergerke, R. Structural Changes in Muscle During Contraction: Interference Microscopy of Living Musculus Fibres. Nature 172, 971-973 (1954). B) © Nature Educational activity All rights reserved.

An Analogy for Sliding Filaments in a Sarcomere Shortening Event

Imagine that y'all are standing between ii large bookcases loaded with books. These big bookcases are several meters autonomously and are positioned on track then that they can be hands moved. Y'all are given the task of bringing the two bookcases together, simply you are limited to using only your arms and two ropes. Standing centered between the bookcases, y'all pull on the two ropes — one per arm — which are tied securely to each bookcase. In a repetitive way, you pull each rope toward you, regrasp information technology, and so pull again. Eventually, every bit you progress through the length of rope, the bookcases move together and approach you. In this example, your arms are like to the myosin molecules, the ropes are the actin filaments, and the bookcases are the z discs to which the actin is secured, which make up the lateral ends of a sarcomere. Similar to the way you would remain centered between the bookcases, the myosin filaments remain centered during normal muscle contraction (Figure 2B).

What Are Cross Bridges, and How Do They Relate to Sliding Filaments?

One important refinement of the sliding filament theory involved the item fashion in which myosin is able to pull upon actin to shorten the sarcomere. Scientists accept demonstrated that the globular end of each myosin protein that is nearest actin, chosen the S1 region, has multiple hinged segments, which can curve and facilitate wrinkle (Hynes et al. 1987; Spudich 2001). The bending of the myosin S1 region helps explain the mode that myosin moves or "walks" forth actin. The slimmer and typically longer "tail" region of myosin (S2) also exhibits flexibility, and it rotates in concert with the S1 contraction (Figure 3A).

The movements of myosin appear to be a kind of molecular dance. The myosin reaches frontwards, binds to actin, contracts, releases actin, and then reaches forward again to demark actin in a new wheel. This process is known as myosin-actin cycling. As the myosin S1 segment binds and releases actin, it forms what are chosen cross bridges, which extend from the thick myosin filaments to the sparse actin filaments. The wrinkle of myosin's S1 region is chosen the power stroke (Figure 3). The ability stroke requires the hydrolysis of ATP, which breaks a high-energy phosphate bail to release energy.

Figure 3: The ability stroke of the swinging cross-bridge model, via myosin-actin cycling

Actin (red) interacts with myosin, shown in globular form (pink) and a filament form (black line). The model shown is that of H. E. Huxley, modified to indicate bending (curved arrow) near the middle of the elongated cross bridge (subfragment 1, or S1) which provides the working stroke. This bending propels actin to the correct approximately ten nanometers (x nm step). S2 tethers globular myosin to the thick filament (horizontal yellow line), which stays in place while the actin filament moves. Modified from Spudich (2001).

© 2001 Nature Publishing Group Spudich, J. A. The myosin swinging cross-bridge model. Nature Reviews Molecular Cell Biology 2, 387-392 (2001). All rights reserved.

Specifically, this ATP hydrolysis provides the free energy for myosin to become through this cycling: to release actin, change its conformation, contract, and echo the process over again (Effigy iv). Myosin would remain bound to actin indefinitely — causing the stiffness of rigor mortis — if new ATP molecules were not available (Lorand 1953).

2 central aspects of myosin-actin cycling use the energy fabricated available by the hydrolysis of ATP. Offset, the action of the reaching myosin S1 head uses the energy released afterward the ATP molecule is broken into ADP and phosphate. Myosin binds actin in this extended conformation. Second, the release of the phosphate empowers the contraction of the myosin S1 region (Figure 4).

Figure 4: Illustration of the cycle of changes in myosin shape during cross-span cycling (1, 2, 3, and four)

ATP hydrolysis releases the energy required for myosin to do its job. AF: actin filament; MF myosin filament. Modified from Goody (2003).

© 2003 Nature Publishing Group Goody, R. S. The missing link in the muscle cantankerous-bridge cycle. Nature Structural Biology 10, 773-775 (2003). All rights reserved.

What Regulates Sarcomere Shortening?

Calcium and ATP are cofactors (nonprotein components of enzymes) required for the contraction of muscle cells. ATP supplies the energy, as described to a higher place, but what does calcium practice? Calcium is required by two proteins, troponin and tropomyosin, that regulate muscle contraction by blocking the bounden of myosin to filamentous actin. In a resting sarcomere, tropomyosin blocks the binding of myosin to actin. In the above analogy of pulling shelves, tropomyosin would go in the way of your paw every bit information technology tried to hold the actin rope. For myosin to bind actin, tropomyosin must rotate around the actin filaments to expose the myosin-binding sites. In 1994, William Lehman and his colleagues demonstrated how tropomyosin rotates by studying the shape of actin and myosin in either calcium-rich solutions or solutions containing low calcium (Lehman, Craig, & Vibertt 1994). Past comparison the activity of troponin and tropomyosin nether these two conditions, they plant that the presence of calcium is essential for the wrinkle mechanism. Specifically, troponin (the smaller poly peptide) shifts the position of tropomyosin and moves it away from the myosin-bounden sites on actin, effectively unblocking the binding site (Figure v). Once the myosin-binding sites are exposed, and if sufficient ATP is present, myosin binds to actin to brainstorm cross-bridge cycling. And then the sarcomere shortens and the muscle contracts. In the absence of calcium, this bounden does not occur, so the presence of free calcium is an important regulator of muscle contraction.

Figure v: Troponin and tropomyosin regulate contraction via calcium binding

Simplified schematic of actin backbones, shown equally grayness chains of actin molecules (assurance), covered with shine tropomyosin filaments. Troponin is shown in blood-red (subunits not distinguished). Upon binding calcium, troponin moves tropomyosin away from the myosin-binding sites on actin (lesser), effectively unblocking it. Modified from Lehman et al. (1994).

© 1994 Nature Publishing Group Lehman, West., Craig, R. & Vibert, P. Ca²⁺-induced tropomyosin movement in Limulus thin filaments revealed past iii-dimensional reconstruction. Nature 368, 65-67 (1994). All rights reserved.

Unresolved Questions

Is muscle contraction completely understood? Scientists are still curious well-nigh several proteins that clearly influence musculus 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" poly peptide spanning sarcomeres in vertebrates, appears to bind to actin, but it is non well understood. In improver, scientists have made many observations of muscle cells that behave in means that do not match our current understanding of them. For example, some muscles in mollusks and arthropods generate forcefulness for long periods, a poorly understood phenomenon sometimes called "catch-tension" or forcefulness hysteresis (Hoyle 1969). Studying these and other examples of muscle changes (plasticity) are exciting avenues for biologists to explore. Ultimately, this research can aid us better understand and treat neuromuscular systems and better understand the diverseness of this mechanism in our natural world.

Summary

Muscle wrinkle provides animals with great flexibility, allowing them to move in exquisite ways. The molecular changes that upshot in muscle contraction have been conserved across evolution in the majority of animals. Past studying sarcomeres, the basic unit decision-making changes in musculus length, scientists proposed the sliding filament theory to explain the molecular mechanisms behind muscle wrinkle. Within the sarcomere, myosin slides along actin to contract the muscle fiber in a process that requires ATP. Scientists accept too identified many of the molecules involved in regulating muscle contractions and motor behaviors, including calcium, troponin, and tropomyosin. This research helped us learn how muscles can change their shapes to produce movements.

References and Recommended Reading

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Clark, Thousand. Milestone 3 (1954): Sliding filament model for muscle contraction. Musculus sliding filaments. Nature Reviews Molecular Prison 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:x.1038/nsb1003-773.

Hoyle, G. Comparative aspects of muscle. Annual Review of Physiology 31, 43–82 (1969) doi:x.1146/annurev.ph.31.030169.000355.

Huxley, H. East. & 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 forcefulness production. Cell 48, 953–963 (1987) Doi:10.1016/0092-8674(87)90704-5.

Lehman, Westward., Craig, R. & Vibertt, P. Ca2+-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction. Nature 368, 65–67 (1994) doi:10.1038/368065a0.

Lorand, L. "Adenosine triphosphate-creatine transphosphorylase" every bit relaxing factor of muscle. Nature 172, 1181–1183 (1953) doi:10.1038/1721181a0.

Spudich, J. A. The myosin swinging cross-bridge model. Nature Reviews Molecular Jail cell Biology 2, 387–392 (2001) doi:x.1038/35073086.

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