What action occurs when sodium channels open in muscle fibers?

When sodium channels open in muscle fibers, sodium ions rush into the cell. This movement occurs due to the concentration gradient, as there is a higher concentration of sodium outside the cell compared to the inside. The influx of sodium ions causes a rapid depolarization of the muscle cell membrane, an essential step in the process of muscle contraction. This depolarization is what initiates the action potential that ultimately leads to muscle fiber contraction by triggering the release of calcium from the sarcoplasmic reticulum.

The other options do not accurately reflect the physiological process occurring when sodium channels open. For instance, calcium does not rush out; rather, it is primarily the influx of sodium that plays a critical role at this stage. Additionally, potassium leaving the cell and chloride ions entering do not occur as a direct result of the opening of sodium channels in this context. The focus on sodium's role is key to understanding the mechanisms of action potentials and muscle contraction.