Establishing the potential difference across an axon

• (1) Sodium ions are actively transported out of the axon, whilst potassium ions are actively transported into the axon via sodium-potassium pumps.
• (2) The active transport of sodium ions exceeds the active transport of potassium ions, since 3 Na+ ions move out for every 2 K+ ions.
• (3) Although both sodium and potassium are positive, the outward movement of sodium ions is greater than the inward movement of potassium ions, so there are more Na+ ions in the tissue fluid surrounding the axon than the cytoplasm. However, there are more K+ ions in the cytoplasm than the tissue fluid.
• (4) The differences in Na+ and K+ ion concentration between the tissue fluid and cytoplasm of the axon create an electrochemical gradient.
• (5) Sodium ions naturally diffuse back into the axon, and potassium ions begin to diffuse out of the axon as a result of the electrochemical gradient.
• (6) Most of the gates in the channels that allow potassium ions to move through are open, while most the gates in the channels that permit sodium ion movement are closed.