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Which process must occur for a muscle to be able to relax?

A. Myosin and actin must attach to each other.
B. The amount of ATP in the neuromuscular junction must increase.
C. The concentration of calcium in the myofibril must decrease.
D. A nerve must initiate an action potential in the muscle fiber.

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gmb444
Someone asked this question already and it was answered by @mrsbabyjacob and verified by an expert. Here’s there answer:

“Relaxation occurs when stimulation of the nerve stops. Calcium is then pumped back into the sarcoplasmic reticulum breaking the link between actin and myosin. Actin and myosin returned to their unbound state causing the muscle to relax.”

Another person said they thought the answer was A.

Just don’t want you to accidentally waste your points when there are answers already :)
Riia

Answer:

C. The concentration of calcium in the myofibril must decrease.

Explanation:

As soon as the stimulus which causes myofibril to contract stops, its time for muscle to relax.  

A low calcium level in the sarcoplasm is the utmost requirement of muscle relaxation otherwise sliding of myosin will continue and muscle will keep on showing contraction. In order for muscle contraction, calcium ion i.e. Ca²⁺ must come out of the sarcoplasmic reticulum so for relaxation reverse process occurs i.e. Ca²⁺ is pushed back to the sarcoplasmic reticulum. There are many pumps known as "calcium pumps" which facilitate this process. Now, the concentration of Ca²⁺ becomes comparatively low in the sarcoplasm which causes troponin C to detach from Ca²⁺. Their detachment causes actin and myosin to detach from each other too which finally leads to muscle relaxation.