A muscle fiber relaxes when:

a. the concentration of Ca2+ in the cytosol returns to resting levels.

b. the supply of ATP is exhausted.

c. Ca2+ flood the cytosol.

d. acetylcholine is released from the axon terminal and the sarcolemma depolarizes.

Video transcript

Hi, everyone. Let's take a look at this practice problem together. Relaxation of muscle fibers is regulated by which element. The answer options are A phosphorus B copper C, iron and D calcium. So let's review the purpose of each element in the human body. Option. A phosphorus recall that this element is important for the formation of bones and teeth. So we can eliminate option A option B copper copper helps make red blood cells and it also plays an important role in the nervous and immune systems. It doesn't regulate muscle fibers so we can eliminate option B as well. Option C iron recall that iron is especially important for producing hemoglobin and hemoglobin is used by our red blood cells. So it is also an incorrect answer choice. The correct answer is calcium. So how does muscle fiber contraction and relaxation work? Let's talk about it. Recall that muscle fiber contraction or relaxation is produced by the degree of overlapping between the thin and thick elements. Thus, the more overlap there is there's more cross ridges that can occur between the filaments and more muscle tension is generated. Now, the amount of overlapping that can occur between the filaments and thus cross bridge formation is controlled by regulatory proteins. These regular regulatory proteins are controlled by calcium. So, let's talk about that. When calcium binds regulatory proteins cross bridges between the thin and thick filaments can occur. And m muscle tension is generated when calcium is not present or bound to the regulatory proteins, the regulatory proteins prevent thin and thick muscle overlapping. So the muscle is relaxed. So again, the correct answer is option D calcium. All right, everyone. I hope you found this helpful and I'll see you soon for the next practice problem.