What is leakage current

(also: membrane potential)

If you measure the electrical voltage on the axon, you get this result, for example:

As shown here, the internal charge is negative compared to the external one. This potential of around -75mV is called the resting potential.

Cause of the resting potential

The following ions are responsible for the electrical charge on the inside of the membrane:

Mainly outside of the nerve cell:
Sodium cations (Na +)
Chloride anions (Cl-)

Mainly within the nerve cell:
Potassium cations (K +)
Protein anions (A-)

However, due to the selective permeability of the membrane, all ions behave differently:

  • The chloride and potassium channels are open during the resting potential so that Cl- and K + ions can diffuse.
  • The sodium channels are closed. Nevertheless, few sodium ions get through the membrane. This small influx is also called leakage current.
  • The rather large protein anions do not get through the membrane.

Since the potassium ions have a higher concentration gradient inside, they flow out of the axon, thus leaving a negative charge inside the axon.
However, if more and more potassium ions have diffused outwards, an opposite charge gradient builds up, with the result that ultimately no more potassium ions would diffuse outwards. This would create an equilibrium between the concentration and charge gradient.
However, the rest potential becomes more and more positive over time due to the Na + leakage currents into the interior. The sodium-potassium pump works against this: it conveys the sodium ions that have gotten inside due to the leakage current to the outside and at the same time transports the potassium ions that have flown outwards back inside.
As a result, the concentration gradient is not evened out and thus the resting potential remains.