Muscle contraction question from TBR CBT 4

[bleach121]

This question is from TBR CBT 4:

Look at the four labeled regions of the curve below, which represents the four steps of the muscle contraction cycle according to the sliding-filament model. Consider only those regions where the actin thin filaments of the sarcomere overlap each other to some degree. In which region is there the LEAST amount of overlap of the actin thin filaments with each other?

I thought that the maximum tension occurs when the sarcomere is the shortest, because this is when the overlap between actin filaments and myosin filaments is the greatest. However, the graph shows that the maximum tension occurs during region III, and according to the explanation, The actin thin filaments do not overlap each other at all in Regions III and IV of the curve. So if the actin filaments do not overlap at III, then how could there be maximal tension?

The answer is (II)

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[sillyjoe]

This question is referring to the length-tension relationship of muscles. This fact is used as supporting evidence for the sliding filament hypothesis of the sarcomere. If you look at a the below image you can see that there is an ideal range for the length of the sarcomere to maximize the force/tension created. The reason for this is because tension is created by the cross-bridging of myosin onto actin causing the stationary actin to move inward and the length between the Z-lines shorten. When the length of the sarcomere is too long, there is minimal cross bridging and thus minimal force. When the length is too short, there is overlap of the actin filaments coming from either end of the Z-line. Since the myosin heads can only move in one direction on the actin due to the polarity of the actin, there is a large amount of myosin heads that cannot attach to actin to generate force. Note at the point farthest on the left in the sarcomere there is overlap of the actin coming from either end of the Z-line and some of myosin heads cannot attach to actin and therefore cannot create the cross-bridging that causes force generation.

The question says:

Consider only those regions where the actin thin filaments of the sarcomere overlap each other to some degree.

That means that you cannot consider any other option than the 2 left most sarcomeres on the graph below. Of those two points where there is SOME overlap, point II (in the question) has the least amount of overlap of the actin filaments (red below).

It may also be helpful to watch this Khan Academy video for clarification: