Muscle contraction actin-based

The contraction of muscles from all sources occurs by the general mechanism described above. Muscles from different organisms and from different cells and tissues within the same organism may have different molecular mechanisms responsible for the regulation of their contraction and relaxation. In all systems, plays a key regulatory role. There are two general mechanisms of regulation of muscle contraction actin-based and myosin-based. The former operates in skeletal and cardiac muscle, the latter in smooth muscle. 

Calcium is known to be an important key regulator for contractile activities of both skeletal and smooth muscles. There are two general mechanisms of regulation of muscle contraction actin based and myosin based. 

The musculature is what makes movements possible. In addition to the skeletal muscles, which can be contracted voluntarily, there are also the autonomically activated heart muscle and smooth muscle, which is also involuntary. In all types of muscle, contraction is based on an interplay between the proteins actin and myosin. 

Smooth muscles have molecular structures similar to those in striated muscle, but the sarcomeres are not aligned so as to generate the striated appearance. Smooth muscles contain a-actinin and tropomyosin molecules, as do skeletal muscles. They do not have the troponin system, and the fight chains of smooth muscle myosin molecules differ from those of striated muscle myosin. Regulation of smooth muscle contraction is myosin-based, unlike striated muscle, which is actin-based. However, like striated muscle, smooth muscle contraction is regulated by Ca. ... 

Microfilaments of F actin traverse the microvilli in ordered bundles. The microfila-ments are attached to each other by actin-as-sociated proteins, particularly fimbrin and vil-lin. Calmodulin and a myosin-like ATPase connect the microfilaments laterally to the plasma membrane. Fodrin, another microfila-ment-associated protein, anchors the actin fibers to each other at the base, as well as attaching them to the cytoplasmic membrane and to a network of intermediate filaments. In this example, the microfilaments have a mainly static function. In other cases, actin is also involved in dynamic processes. These include muscle contraction (see p. 332), cell movement, phagocytosis by immune cells, the formation of microspikes and lamellipo-dia (cellular extensions), and the acrosomal process during the fusion of sperm with the egg cell. 

In all the actin-based motility systems, and especially skeletal, cardiac and smooth muscle, contraction is initiated primarily by an increase in cytoplasmic [Ca +]. However, the major differences in histology and function of these muscle types are associated with great variety in how contraction is controlled. Smooth muscle especially differs from the model presented for skeletal muscle. Skeletal muscle is activated by Ca + released from SR in response to sarcolemmal action potentials. Ca + exerts its effect by binding to troponin on the thin filaments, which reverses the tropomyosin inhibition of cross-bridge formation. 

Figure 4.4. Model of muscle contraction based on sliding filaments [ 2 ] (a) The bands H and I are shortened during contraction the length of thick and thin filaments remain constant the arrows indicate the myosin and actin-G polarity (b) Schematic representation of the interactions between myosin head and thin filaments during contraction.

The initial event in the contraction of skeletal muscles is generally assumed to be the binding of calcium ions to troponin C, one of three subunits in the protein complex, troponin, on the actin thin filament. Troponin C (MW 18,000) contains 46 carboxylic groups out of a total of 159 amino acids (26). Based on primary structure analogies between parvalbumin, the crystal structure of which is known, and troponin C (TnC), Kretsinger and Barry have predicted the location and general structure of four... 

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