Question

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 5→3→2→1→4
b 1→2→3→4→5
c 2→1→3→5→4
d 2→3→4→1→5

Explanation:

• First, the motor neuron action potential leads to acetylcholine release (step 5).
• Then, the action potential propagates down transverse tubules (step 3).
• Next, calcium is released and binds (step 2).
• After that, tropomyosin shifts (step 1).
• Finally, myosin cross - bridge cycles occur (step 4).

Answer:

A. \(5
ightarrow3
ightarrow2
ightarrow1
ightarrow4\)