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== Sliding Filament Model of Contraction == The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.[[File:Actin-tropomyosin-troponin.png|756x756px|alt=|center|thumb|The Actin-Tropomyosin-Troponin complex. In the resting state, tropomyosin, held in place by troponin, blocks the site on actin to which myosin binds. However, in the presence of Ca++, it binds to troponin, and when troponin binds Ca++ it changes conformation, moving tropomyosin away from the myosin binding site on actin.]]Key points * The sarcomere is the region in which sliding filament contraction occurs. * During contraction, myosin myofilaments ratchet over actin myofilaments contracting the sarcomere. * Within the sarcomere, key regions known as the I and H band compress and expand to facilitate this movement. * The myofilaments themselves do not expand or contract Cross-bridge cycle * The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle. During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament. * At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains constant. * The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant<ref name=":3">[https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.4%3A_Muscle_Contraction_and_Locomotion/38.4C%3A_Sliding_Filament_Model_of_Contraction Sliding Filament Model of Contraction]. (2020, August 15). Retrieved March 7, 2021, from: https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/38%3A_The_Musculoskeletal_System/38.4%3A_Muscle_Contraction_and_Locomotion/38.4C%3A_Sliding_Filament_Model_of_Contraction (last accessed 7.3.2021)</ref>.

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