Resting Potential Sodium-potassium ion pump creates concentration gradients across the membrane Potassium ions diffuse out of the cell down the potassium ion concentration gradient, making the outside of the membrane positive and the inside negative The electrical gradient will pull potassium ions back into the cell At -70mV potential difference, the two gradients counteract each other and there is no net movement of potassium ions
Action Potential What causes action potential? The change in the potential difference across the membrane causes a change in the shape of the Na+ gate, opening some of the voltage-dependent sodium ion channels As the sodium ions flow in, depolarisation increases, triggering more gates to open once a certain potential difference threshold is reached, thus increasing depolarisation (positive feedback) There is no way of controlling the degree of depolarisation of the membrane Action potentials are either there or they are not (all-or-nothing) Action potential is caused by changes in the permeability of the cell surface membrane to Na+ and K+ channels At the resting potential, these channels are blocked by gates preventing the flow of ions through them Changes in the voltage across the membrane cause the gates to open, and so they are referred to as voltage-dependent gated channels Depolarisation
The voltage-dependent Na+ channels spontaneously close and Na+permeability of the membrane returns to its usual very low level
Voltage-dependent K+ channels open due to the depolarisation of the membrane
Potassium ions move out of the axon, down the electrochemical gradient, and the inside of the cell once again becomes more negative than the outside
This is the falling phase of the oscilloscope trace
Restoring the resting potential The membrane is now highly permeable to potassium ions, and more ions move out than occurs at resting potential, making the potential difference more negative than the normal resting potential (hyperpolarisation) The resting potential is re-established by closing of the voltage-dependent K+ channels and potassium ion diffusion into the axon.