Which of the following is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber?
1) Tropomyosin shifts and unblocks the cross-bridge binding sites.
2) Calcium is released and binds to the troponin complex.
3) The sarcoplasmic reticulum is depolarized when a wave of depolarization moves from the neuromuscular junction into the cell's interior by way of the transverse tubules.
4) The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.
5) An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane at the neuromuscular junction.

Question

Quizplus · Accepted Answer

The correct sequence for excitation and contraction of a skeletal muscle fiber is as follows: 
1) An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane at the neuromuscular junction (step 5). 
2) This depolarization spreads along the transverse tubules, reaching the sarcoplasmic reticulum and causing the release of calcium ions (step 3). 
3) Calcium ions bind to the troponin complex, which causes tropomyosin to shift and unblock the cross-bridge binding sites on the thin filaments (step 2). 
4) The myosin heads bind to the exposed binding sites on the actin molecules of the thin filaments, ratcheting them across the thick filaments and contracting the muscle fiber (step 4). 
Therefore, the correct sequence is 5 → 3 → 2 → 1 → 4, which is option E.

Which of the Following Is the Correct Sequence That Describes