Final answer:
The potassium ion (K+) is primarily responsible for maintaining the resting membrane potential in excitable cells, with the sodium-potassium pump further contributing to the maintenance of this potential by actively transporting ions across the cell membrane.
Step-by-step explanation:
The ion that is primarily responsible for maintaining the resting membrane potential of excitable cells is the potassium ion (K+). Potassium is the major intracellular cation, with a high concentration inside the cell compared to the outside.
This disparity in concentration, along with selective membrane permeability that favors potassium movement out of the cell, establishes a negative charge inside the cell relative to the extracellular fluid.
The actions of the sodium-potassium pump are vital for maintaining this potential over the long term. This pump uses ATP to move two K+ ions into the cell and three Na+ ions out.
This movement contributes to the negative charge inside the neuron since more positive charges are expelled than are brought in, resulting in a resting membrane potential of approximately -70 mV.
The energy cost for maintaining the resting potential is high, as a neuron can use up to 50 percent of its ATP for this purpose alone.
Additionally, glial cells, such as astrocytes, play a role in maintaining ionic balance in the extracellular fluid, ensuring proper functioning of the resting membrane potential in neurons within the CNS.