Final answer:
Calcium-induced calcium release (CICR) triggers the release of Ca₂⁺ from the ryanodine receptor channels during the plateau of the action potential, initiated by the binding of ACh to sarcolemma receptors and subsequent depolarization by Na⁺ influx.
Step-by-step explanation:
The release of a larger quantity of Ca₂⁺ from the ryanodine receptor channels during the plateau of the action potential is primarily triggered by a process known as calcium-induced calcium release (CICR). This process occurs during an excitation-contraction coupling in muscle cells. When acetylcholine (ACh) is released from the axon terminal, it binds to receptors on the muscle cell membrane (sarcolemma), leading to the opening of sodium ion channels. This influx of sodium ions (Na⁺) depolarizes the membrane and produces an action potential which travels down the T tubule. The action potential triggers the opening of the ryanodine receptor channels in the sarcoplasmic reticulum, allowing stored Ca₂⁺ to flood into the cytoplasm, which then contributes to muscle contraction.