A polarised axon membrane
When a neurone is not transmitting an impulse, there is a potential difference across its axon membrane. This is the resting potential.
The resting membrane potential is approximately −70 mV, meaning that the inside of the axon is about 70 mV more negative than the outside.
This potential difference is established by the combined effects of the sodium-potassium pump and the membrane being more permeable to potassium ions than to sodium ions.
Sodium-potassium pump
- The pump is a carrier protein in the axon membrane.
- It uses ATP, so the movement of ions is by active transport.
- For each cycle, 3 Na+ are transported out of the axon and 2 K+ are transported in.
- This helps produce a high sodium ion concentration outside and a high potassium ion concentration inside the axon.
Differential permeability
- Potassium ion channels are open at rest, so K+ can leave the axon by facilitated diffusion.
- Sodium ion channels are closed at this point, restricting the movement of Na+ back into the axon.
- As positive potassium ions move out, the inside becomes more negative relative to the outside.
- This difference in permeability to Na+ and K+ is differential membrane permeability.
Active transport by the sodium-potassium pump creates unequal Na+ and K+ concentrations on the two sides of the membrane. Because K+ can then diffuse out through open channels while Na+ movement inward is restricted, charge becomes more positive outside than inside and the membrane settles at its resting potential.
Exam Tip: In an answer about resting potential, identify the K+ pathway as an open, non-voltage-gated channel. Use voltage-gated potassium channel specifically when describing the later action-potential sequence.