Question

Distinguish the contractile units of cardiac and smooth muscle cells at the molecular level..

Answer 1

Contractile unit of cardiac and smooth muscle cells at molecular level

In cardiac muscle cells, sarcomere is the functional unit of the muscle tissue .During muscle contraction and relaxation long fibrous proteins slide over each other.Two most important unit within the sarcomere is myosin and actin.Myosin contains fibrous long tail and globular head which binds with the actin filament. Along with that myosin head also bind with ATP.Actin and myosin together forms the microfibril as a molecular structure through out the muscle cell.When ATP gets bound to myosin ,it dissociate themselves with actin filament of microfibril and causes muscle contraction.In order to execute the mucle contraction,when action potential arises between sarcomere,it facilitates influex of calcium ion via activation of T-tubule. The calcium ion in cytoplasm causes binding with cardiac troponin-c which displace the troponin from complex away from actin binding site and binds to myosin therefore contraction occurs.

In smooth muscle cell,the substantial portion of cytoplasm is occupied by myosin and actin helps in contraction of muscle like cardiac muscle cells. Smooth muscles contains calmodulin inplace of troponin.other than calmodulin,tropomyosin ,calponin and caldesmon also significantly contribute the functionality of smooth muscle.Tropomyosin in smooth muscle has seven actin monomers and distributed throughout the thin filament of muscle. calponin functions as a long bearing molecule inside the muscle tissue.caldesmon helps in tethering actin,myosin and tropomyosin in muscle cell thus maintains the tension within.