Skeletal muscle fiber contraction begins when?

The correct answer is: An electrical impulse from a motor neuron reaches the NMJ.

Skeletal muscle contraction is initiated by an electrical impulse. The command is sent from the brain in the form of an action potential. The spinal cord then transmits this signal to the neuromuscular junction (NMJ). Here, the action potential causes calcium ions to be released into the NMJ, which in turn causes the release of acetylcholine. Acetylcholine then binds to receptors on the muscle cell membrane, causing a conformational change that leads to the opening of sodium channels. This influx of sodium ions creates an action potential in the muscle cell, which in turn initiates muscle contraction. So the correct answer is: An electrical impulse from a motor neuron reaches the NMJ.