Question

Which of the following statements correctly describes why a series of closely spaced action potentials causes a sustained contraction rather than a series of closely spaced twitches?

a. Ca2+ ions are released quickly from troponin, keeping the Ca2+ concentration in the cytosol high between closely spaced action potentials.
b. Release of Ca2+ from the sarcoplasmic reticulum through channels is slow compared to the uptake of Ca2+ into the SR via ATP-dependent pumps, resulting in Ca2+ slowly trickling into the sarcomeres between closely spaced action potentials.
c. When a series of action potentials is closely spaced, there is not sufficient time for Ca2+ uptake into the sarcoplasmic reticulum between action potentials, and Ca2+ remains bound to troponin throughout the series.
d. Fewer Ca2+ ions are released from the sarcoplasmic reticulum as a result of several closely spaced action potentials than as a result of a single action potential.

Answer 1

The correct answer is - option C.

Step-by-step explanation:

A series of closely spaced action potentials cause the sustained muscle to contract which is only possible if in the sarcomere of muscles the has high Ca2+ during the complete contraction cycle.

If the Ca2+ level is lower due to the sufficient Ca2+ were taken up into the sarcoplasmic reticulum in the sarcomere below the range between each action potential, the muscle would relax because the myosin-binding sites will be covered by tropomyosin.

The Ca2+ channels in the SR are able to move the ions more rapidly than the Ca2+ pumps can move in SR so this will cause the concentration of calcium to always high which leads to the binding of tropomyosin and therefore, the myosin-binding sites would be exposed until the contraction is achieved.