What characterizes the membrane of an unstimulated muscle fiber?

The membrane of an unstimulated muscle fiber is characterized as electrically polarized. This means that there is a difference in electrical charge across the membrane, with the inside of the fiber being negatively charged relative to the outside. This polarization is a result of the distribution of ions, primarily sodium (Na+) and potassium (K+) ions, which are maintained by the action of the sodium-potassium pump and other ion channels.

In a polarized state, the concentration of potassium ions is higher inside the cell, while sodium ions are more concentrated outside. As a result, when the muscle fiber is at rest, there is a stable membrane potential that is critical for the initiation of action potentials when the muscle fiber is stimulated.

Other options describe incorrect features of the membrane during the resting state. While there is indeed a difference in charge, a positive charge on the inside does not occur in this state, and the membrane is not completely permeable to sodium ions; it has selective permeability that allows for the resting potential to be maintained. Additionally, the concentration of sodium and potassium ions is not equal, as the resting potential relies on the distinct gradients of these ions across the membrane.