| Muscle contraction: The thick and thin filaments
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| The sarcomere may shorten by as much as 70% in length during muscle contraction (Fig. 19.2; see also Fig. 19.1). The main sarcomere components producing the shortening are the thick and thin filaments. The thick filament is composed of myosin protein, and the thin filament is mainly made up of actin, with associated tropomyosin and the troponin family proteins. Thick and thin filaments extend in opposite directions from both sides of the M- and Z-lines, respectively, and overlap and slide past one another during the contractile process (Fig. 19.2). The M- and Z-lines are, in effect, base plates for anchoring the filaments. In smooth muscle thick and thin filaments are anchored at structures called dense bodies that are further anchored by intermediate filaments. In striated muscle increased thick-thin filament overlap during contraction causes the H-zone (myosin only) and I-bands (actin only) to decrease. Although all three muscle types contain actin and myosin proteins, each muscle type expresses tissue specific protein types or isoforms; the cardiac actin and troponins differ slightly from those in skeletal muscle.
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