Final answer:
Muscle contraction begins with the release of acetylcholine at the neuromuscular junction, causing voltage-gated sodium channels to open and an action potential to spread along the muscle fiber.
Step-by-step explanation:
To start a muscle contraction, a nerve signal is sent to the muscle fiber, then an "all or none voltage-gated sodium channel" opens, depolarizing the cell membrane, and causing the contraction.
This depolarization is a change in voltage of the muscle fiber's membrane, which occurs in response to the neurotransmitter acetylcholine (ACh) being released from the motor neuron at the neuromuscular junction.
Upon the arrival of ACh, it binds to ACh receptors on the muscle fiber, causing sodium channels to open.
Then, sodium ions flow into the muscle cell, triggering an action potential that rapidly spreads along the entire membrane, further promoting the excitation-contraction coupling necessary for muscle contraction.