Final answer:
An influx of sodium ions causes depolarization of the muscle cell membrane, leading to an action potential that spreads along the membrane. This event triggers the release of calcium ions and results in muscle cell contraction.
Step-by-step explanation:
The greater influx of sodium ions results in depolarization of the membrane. Then, contraction of the muscle cell occurs.
This process begins when a nerve impulse prompts sodium channels to open, allowing sodium ions to enter the muscle fiber.
Due to the higher concentration of sodium ions outside the cell, they rush in, which is primarily driven by their concentration gradient.
As these positively charged ions accumulate inside, they change the internal voltage of the cell, making it less negative relative to the outside.
This change is referred to as depolarization, and as the membrane depolarizes, an action potential rapidly spreads along the membrane and through structures called T-tubules.
This triggers the release of calcium ions from the sarcoplasmic reticulum, initiating the process of excitation-contraction coupling, ultimately causing the muscle cell to contract.