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 and how it is maintained
