For the following five events, which is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber?
1) Tropomyosin shifts, exposing the myosin-binding sites on actin.
2) Calcium is released from the sarcoplasmic reticulum and binds to the troponin complex.
3) An action potential is propagated down the transverse tubules.
4) Cycles of myosin cross-bridge formation and breakdown cause the thin filaments to slide toward the center of the sarcomere.
5) An action potential in a motor neuron causes the axon to release acetylcholine, which triggers an action potential in a muscle fiber.
A) 1 → 2 → 3 → 4 → 5
B) 2 → 1 → 3 → 5 → 4
C) 2 → 3 → 4 → 1 → 5
D) 5 → 3 → 2 → 1 → 4

Question

Quizplus · Accepted Answer

The correct sequence starts with an action potential in a motor neuron triggering the release of acetylcholine (5), which then leads to an action potential in a muscle fiber that is propagated down the transverse tubules (3). This action potential triggers the release of calcium from the sarcoplasmic reticulum (2), which binds to the troponin complex causing tropomyosin to shift and expose the myosin-binding sites on actin (1). Finally, cycles of myosin cross-bridge formation and breakdown slide the thin filaments toward the center of the sarcomere, causing muscle contraction (4).

For the Following Five Events, Which Is the Correct Sequence