IMAT 2015 Q24 [Reflex Response]

The following events occur during a reflex response to a person placing a hand on a hot object.

  1. Myosin binding sites on actin filaments uncovered.
  2. ADP and phosphate ion released from myosin head.
  3. Sodium voltage­-gated channels open.
  4. Myosin head detaches from binding site on actin.
  5. Calcium ions released by sarcoplasmic reticulum.

Which of the following options places four of these events in the correct order (from left to right)?

A. 3, 5, 4, 2
B. 5, 2, 1, 4
C. 1, 2, 4, 3
D. 5, 1, 2, 4
E. 3, 1, 4, 2

The answer to this question is choice B.

If you notice in the choices, there are only 4 out of 5 numbers included among the sequence given. You need to notice these things first, as to not waste time mistakenly thinking that all numbers given in the question are relevant. Thus, to become efficient in answering this question, you will need to odd-one-out first. To start doing that, try skimming numbers 1-5, and you will realize that this relates to the sequence involved in muscle contraction. Out of the numbers, #3 seems to be out of place, as it is not directly related to the steps of muscle contraction, as the main ion directly involved would be calcium, and not sodium. This then leaves choices B and D as your only remaining considerations.

The correct sequence for this question is 5 → 1 → 2 → 4. Let us rationalize each statement:

  1. Statement 5: “Calcium ions released by sarcoplasmic reticulum.” This is the first step since if you recall, after the action potential has induced the release of acetylcholine to the motor end plate, this will subsequently induce depolarization, which will spread through the muscle fiber. This depolarization will cause the sarcoplasmic reticulum to release its calcium ion stores.

  2. Statement 1: “Myosin binding sites on actin filaments uncovered.” The calcium ion that was released from the sarcoplasmic reticulum will now play a role in making sure myosin can bind to actin. Calcium will bind to troponin, which reconfigures a complex made of tropomyosin and effectively removes its blockage of the myosin binding sites on the actin. This exposure of binding sites will allow myosin heads to attach.

  3. Statement 2: “ADP and phosphate ion released from myosin head.” When bound ATP is hydrolyzed by ATPase into ADP and phosphate ion, the myosin head will go into a “cocked” position. This means that while ADP and phosphate are attached, there is potential energy that is stored, which can be used for the contraction. Now that the myosin binding sites are exposed and the myosin head is attached via a cross-bridge, ADP and phosphate ion are released, where the energy released with this induces a conformational change called the “power stroke” which is responsible for the muscle contraction.

  4. Statement 4: “Myosin head detaches from binding site on actin.” This cycle of muscle contraction is ready to repeat again, but the myosin head needs to detach from actin first. This is possible due to the attachment of new ATP, which breaks the cross-bridge between actin and myosin.

Thus, with the sequence in mind, the only correct choice for this question would be choice D.