If a muscle fiber were to suddenly and permanently stop producing ATP the fiber would no longer be able to actively transport calcium out of the cytoplasm (sarcoplasm) and the intracellular calcium concentration would rise. Which of the following would you expect to happen? A. Calcium would be transported to the sarcoplasmic reticulum therefore contractions would cease. B. Myosin would be able to bind to the exposed binding sites on thin filaments but it would not be able to detach. C. The fiber would twitch uncontrollably due to excessive calcium bound to troponin. D. No change would occur in a muscle that was relaxed to begin with.

A muscle fibre will stop contraction immediately ATP is used up. Also, muscle contraction will end as soon as the information from the motor neuron stops. This repolarizes the sarcolemma and T-tubules, thereby closing the voltage-gated calcium channels in the sarcoplasmic reticulum. Calcium ions are then pumped back into the sarcoplasmic reticulum, in order for tropomyosin to cover the binding sites on the actin strands again.