What is the correct sequence of events from a nerve signal to Ca2+-triggered contraction at the muscle fiber?

What is the correct sequence of events from a nerve signal to Ca2+-triggered contraction at the muscle fiber?

• A. Nerve impulse → acetylcholine release at the neuromuscular junction → ACh receptors depolarize the motor end plate → action potential travels along T-tubules → Ca2+ release from the sarcoplasmic reticulum → Ca2+ binds troponin and initiates cross-bridge cycling.
• B. Nerve impulse → Ca2+ influx into the muscle fiber directly opens calcium channels → Tropomyosin moves → ACh is released.
• C. ACh is released at the NMJ after a motor neuron action potential, but Ca2+ is not involved in activation of the SR.
• D. Ca2+ is released first from the SR and then triggers ACh release at the NMJ.

Answer: (A)