- The sodium channels in the excitable tissues have
voltage regulated gates, in fact they have two
gates that resond to different voltage changes.
In the resting cell, one gate (the most external
one) is closed while the other is open.
- In the case of the potassium channels, there are
in fact two channels. On type lack gates, and are
always open while the others have voltage
- Thus, in the resting cell, the membrane is more
permeable to potassium and thus has a negative
- Once the cell membrane is adequately stimulated
and the voltage has reached a certain level (Threshold
Level, e.g. -55mV), the external gates
open up. and an inflow of sodium ensues. This
causes the membrane potential to become more
positive and more sodium gates are opened.
- This results in an explosive influx of sodium,
and the mebrane potential at that point, becomes
nearer to the equilibrium potential for sodium
(+35mV). This is called the Depolarisation
Phase - the membrane potential is
- At this voltage, the internal sodium gate closes
up stopping the inflow of sodium. At the same
time the voltage gated potassium channels open up
and an eflux of potassium follows. This results
in a change of potential to a more negative one.
This phase is called the Repolarisation
- Due to the opening up of the potasium channels,
the negative membrane potentail can overshoot the
resting membrane potential. This part is called
- The internal sodium gates, open up only after the
external gates have closed down (below -55mV).
This difference in the closing and opening
voltage for the internal gates, ensures that the
sodium channels are not open during the