Final answer:
The extracellular concentration of potassium ions (K+) significantly affects the resting membrane potential, with a higher impact than other ions due to the cell's permeability to K+ and the presence of numerous potassium leakage channels.
Step-by-step explanation:
A change in the extracellular concentration of potassium ions (K+) has the greatest effect on the resting membrane potential of neurons and muscle cells. The resting membrane potential is primarily influenced by the movement of potassium ions across the cell membrane due to the cell's high permeability to K+. The resting membrane potential is a negative voltage state where the inside of the cell is approximately -70 millivolts (mV) more negative than the outside. Neurons maintain high concentrations of potassium inside and high concentrations of sodium outside the cell. Numerous potassium leakage channels allow potassium ions to move out of the cell more readily than sodium ions can enter, contributing significantly to the negative charge inside the cell relative to the extracellular fluid.