5.3.3 Resting Potentials

• Neurones transmit electrical impulses, which travel rapidly along the neurone cell surface membrane from one end of the neurone to the other
• In a resting axon (one that is not transmitting impulses), the inside of the axon always has a negative electrical potential compared to the outside of the axon
  • This is called the resting potential

• This potential difference (when there are no impulses) is usually about -70mV
  • The inside of the axon has an electrical potential about 70mV lower than the outside

• Two factors contribute to establishing and maintaining the resting potential:
  • The active transport of sodium ions and potassium ions
  • Differential membrane permeability

The active transport of sodium ions and potassium ions

• Carrier proteins called sodium-potassium pumps are present in the membranes of neurones
• These pumps use ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions that they actively transport in
• This means that there is a larger concentration of positive ions outside the axon than there are inside the axon
• The movement of ions via the sodium-potassium pumps establishes an electrochemical gradient

A differential membrane permeability

• The cell-surface membrane of neurones has selective protein channels that allow sodium and potassium ions to move across the membrane by facilitated diffusion
• The protein channels are less permeable to sodium ions than potassium ions
• This means that potassium ions can diffuse back down their concentration gradient, out of the axon, at a faster rate than sodium ions

The resting potential is maintained by the net movement of positively charged ions out of the axon