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Cardiac Mustlt*: Structure

MUSCLE PHYSIOLOGY

FlCURF 3.7 SCHEMA OF STRUCFURt OF CARDIAC MUSCLE

Cardiac musde, like skeletal musde, is slriated in appearance, reflect mg the presence o i Ihe regularly arrangcd actin and myosin fila-mcnts. Cardiac musde fibers are branched and olectrically coupled to one another by gap junctions, which are locatcd in the intercalated discs. Tłu* plasma membranę of the cardiac celi Imyocyte) con-tains transverse (T) tubules like skeletal musde. However, the sar-coplasmic reticulum is not as elaborat? as in skeletal musde, and dyads (rather than triads) are formed between the T tubules and sar-coplasmic reticulum.

bccitation-contraction coupling in cardiac musde is similar to that oł skeletal musde, as is the process of cross-bridge formation and cycling. Although intracdlular Ca^;* levels control contraction in both skeletal and cardiac musde, there are sonie important differences. Skeletal musde contraction Ls dependent on the release of Ca '* from the sarcoplasinit reticulum, and changes in extraccllular |Ca-' | do not apprecinbly alter the strength of the skeletal musde contraction. In contrast. Ca entry into the cardiac myocyte from the extiaccllular fluid is necessary for the release of Ca-'* from the sarcoplasmic reticulum, and removal of extraccllular Ca- * will decrease the force of contraction. The ratę and force of contraction of cardiac myocytes can be im reased by 0-adrenergic agonists.

/3-Adrenergic agonists do not have these effec ts on skeletal musde.

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