Which ion enters the muscle fiber through open chemically gated ion channels?

The correct answer is sodium (Na+). In muscle fibers, particularly during the initiation of an action potential, chemically gated ion channels that respond to neurotransmitters are crucial. When a neurotransmitter, like acetylcholine, binds to these receptors on the muscle cell membrane, it causes the channels to open, allowing sodium ions to flow into the muscle fiber.

This influx of sodium ions results in depolarization of the muscle cell membrane, which is a critical step in generating an action potential that then leads to muscle contraction. The depolarization process is essential for the transmission of signals from motor neurons to muscle fibers, effectively translating the neural signal into a mechanical response.

While calcium ions play a significant role in muscle contraction by triggering the interaction between actin and myosin during the excitation-contraction coupling phase, they do not enter the muscle fiber through chemically gated ion channels in the initial signaling process. Potassium ions primarily flow out of the cell during repolarization, and chloride ions usually act to stabilize the membrane potential rather than initiating action potentials. Thus, sodium is the key ion involved in the depolarization process prompted by the binding of a neurotransmitter to chemically gated ion channels in the muscle fiber.