Question

Smooth muscle, like striated muscle, utilizes Ca2+ as a signal molecule that can allow contraction to occur. However, smooth muscle does not have troponin or tropomyosin. Therefore, Ca2+ must interact with other proteins. Describe these relationships.

Answer 1

Skeletal muscle

• In skeletal muscle calcium binds to troponin c and induces a conformational change in the troponin molecule.
• This result in the shifting of troponin I from the actin filament allowing tropomyosin to move deeper into the actin groove unmasking the myosin binding sites.
• The exposure of binding sites allows myosin head to interact with actin and engage in cross-bridge cycling. thus the troponin nd tropomyosin complex regulates the contarction

where as in smooth muscle

• The changes in sarcoplasmic calcium ion concentration plays a major role in muscle contraction.
• The strength of the stimulus acting on the smooth muscle fiber can be graded.Accordingly the change in cytosolic calcium concentration may be minute,more or very high,producing different degrees of contraction.
• The cytosolic calcium concentration is increased mainly by entry of calcium from the interstitial fluid or to some extent by its release from cytoplasmic calcium stores.
• However,the concentration of calcium in cytosol decreases due to the binding of calcium to calmodulin,pumping back of calcium to the SR and calcium efflux.