IMAT 2015 Q29 [Sarcomeres]

The diagram shows a single sarcomere in a relaxed state.


Which answer describes the changes when the sarcomere contracts?

A. 1 gets smaller and 2 gets smaller.
B. 1 gets smaller and 2 does not change.
C. 1 gets longer and 2 gets smaller.
D. 1 does not change and 2 gets wider.
E. 1 gets longer and 2 gets wider.

The answer to this question is choice B, 1 gets smaller, and 2 does not change.

The diagram given in this question is the sliding filament model of contraction, and it would be useful to be oriented to the parts of this diagram or model (only those that are relevant to this question, other information regarding terms like the H band, A band and I band will not be discussed here to avoid confusion.)

  • What is the thick black horizontal line? That is the thick filament, or the myosin. If you recall in the cycle of contraction, there are myosin heads that attach to actin, wherein the energy released along with the release of ADP and Pi is responsible for the “power stroke” or the change in configuration of myosin heads that pulls on the actin and “contracts” the sarcomere.
  • What is the THIN black horizontal line? That is the thin filament, or the actin. The actin is what myosin binds to when tropomyosin is not covering the myosin-binding sites.

What exactly happens when muscles contract?

  • As a reminder, let’s summarize the process of muscle contraction as brief as we can. When calcium is released from the sarcoplasmic reticulum, it binds to troponin, which induces tropomyosin to move out of the myosin binding sites so that myosin heads can bind. If you can recall, myosin has its own ATPase that hydrolyzes ATP into ADP and Pi, which puts the myosin head into a “cocked” position. If the myosin is attached to the actin, when the ADP and Pi are released, this changes the configuration of the myosin head and pulls on the actin, effectively contracting the muscle. A new ATP binds to the myosin and the cycle repeats.

How is this now then relevant to this question/interpreting the diagram?

  • One very important concept that you should remember is that the length of the filaments do not change. It is the fact that they are “sliding” against one another during contraction that the sarcomere length as a whole is able to shorten in size! This explains the name for this diagram, which is the “Sliding Filament Model”. Connect this to your knowledge that the myosin merely “pulls” on the actin, but ultimately, no lengths change in size.
  • Let’s look at #1: The label #1 is pertaining to the whole sarcomere, and based on what has been discussed above, it makes sense that when filaments slide on another, it is because myosin is pulling on actin, which will be enough to decrease the size of the whole sarcomere.
  • Let’s look at #2: The label #2 is pertaining to the actin filament or the thin filament. As discussed earlier as well, filaments DO NOT change in size during contraction, but they only slide on each other.

It now makes sense why choice B is the correct answer.