OCR A Level Biology

Revision Notes

5.3.3 Resting Potentials

Resting Potentials

  • Neurones transmit electrical impulses, which travel extremely quickly 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 outside the axon
    • This is called the resting potential
  • This potential difference when there are no impulses is usually about -70mV (ie. 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 of an axon (1)_1, downloadable AS & A Level Biology revision notesThe resting potential of an axon (2)_1, downloadable AS & A Level Biology revision notes

The resting potential of an axon and how it is maintained

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