The concentration gradient of ions across the membrane and the membrane permeability to each type of ion determines the resting potential of a neuron.
All neurons exhibit a resting potential, meaning there is an electrical potential difference (voltage) between the inside of the neuron and the extracellular space. The membrane potential is the difference in total charge between the inside and outside of the cell. High concentrations within the cell maintain potassium ions (K+), while high concentrations outside of the cell maintain the sodium ions (Na+). The cell possesses potassium and sodium ion channels that allow the two ions to diffuse down their concentration gradient. However, the neurons have far more potassium leakage channels than sodium leakage channels. Therefore, potassium diffuses out of the cell at a much faster rate than in sodium. More cations leaving the cell than entering it causes the interior of the cell to be negatively charged relative to the outside of the cell. Once established, the actions of the sodium-potassium pump help to maintain the resting potential.
There is high extracellular calcium as well, which also contributes to the membrane potential.
MCAT Official Prep (AAMC)
Biology Questions Pack, Vol. 1 Passage 11 Question 74
• When the neuronal membrane is at rest, the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.
• Potassium ions diffuse out of the cell at a much faster rate than sodium ions because neurons have many more potassium leakage channels than sodium leakage channels.
• Sodium-potassium pumps move two potassium ions inside the cell as three sodium ions are pumped out to maintain the negatively-charged membrane inside the cell; this helps maintain the resting potential.
ion channel: a protein complex or single protein that penetrates a cell membrane and catalyzes the passage of specific ions through that membrane
cation: a positively charged ion
membrane potential: the difference in electrical potential across the enclosing membrane of a cell
resting potential: the resting membrane potential of a neuron is about -70 mV. at rest, there are relatively more sodium ions outside the neuron and more potassium ions inside the neuron
neurons: a basic working unit of the brain
extracellular space: the space outside of a cell
sodium-potassium pump: powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient