Question

Elevated cytosolic Ca++ is a major signal that triggers contraction in all types of muscle (skeletal, cardiac, smooth).  Explain the role of elevated cytosolic Ca++ in triggering the contraction of skeletal muscle, and then state one very significant difference for each of cardiac and smooth muscle contraction.

Answer 1

Role of Ca+2 in muscle contraction:

During resting stage, troponin-1 is lightly bound to actin and the tropomyosin molecules are located in the groove between the strands of actin filaments in such a way that they block the myosin binding sites on actin. Thus, during resting stage, no actin-myosin cross-bridges are formed. Thus, the troponin-tropomyosin complex so-called relaxing proteins which inhibit the interaction between actin and myosin. When activation takes place, the Ca2+ ions released into the cytosol from the terminal cisterns of the sarcoplasmic reticulum get attached to troponin-C subunit of the protein troponin. It results in a conformation change which causes the tropomyosin molecule to move laterally, uncovering the binding sites on the actin molecules for head of the myosin molecules. Seven myosin binding sites on the actin filament are uncovered for each molecule of troponin that binds a Ca2+ ion. Thus the cross-bridge cycle is switched on (initiated) by the lateral movement of the tropomyosin.

Difference between skeletal muscle and smooth muscle contraction:

'in skeletal muscle the calcium gets attached to troponin and then it helps in removing the tropomyosin cover and exposing the myosin binding site on actin.
'but in smooth muscle there is no troponin and it get contracted by sliding of myosin fibre and actin filaments over each other. This process is assisted by calmodulin.

Difference between skeletal muscle and cardiac muscle contraction:

'in skeletal muscle the source of calcium is the t-tubules of sarcoplasmic reticulum. Presence of action potential all causes opening of the gates of the the t tubules and causes flux of calcium.

but in case of cardiac muscle, depolarisation of cardiac muscle membrane causes opening of cali]cium channel and calcium em]nters the cells. This calcium initiates release of more calcium from the sarcoplasmic reticulum and thus increase the concentration of calcium within the cardiac cells.