Action potentials are brief, rapid, large changes in the membrane potential in which the potential actually reverses. They are usually initiated as a result of spread from graded potentials.
- Similar to graded potentials, an action potential occurs only at a single point of the membrane BUT
- Action potentials are propagated/conducted along the axon of a neuron WITHOUT losing their strength (nondecremental conduction).
- Unlike graded potentials, they are an ALL-OR-NOTHING phenomenon- occuring only once the cell membrane has reached ‘threshold potential‘ (usually between -55 and -50mV)
- It is characterised by a rapid depolarisation (to +30-50mV) followed just as quickly by a rapid hyperpolarisation (down to -80mV). The resting potential is then restored. This process lasts just 1ms (0.001 secs).
Ion movement and the Action potential
The whole basis of the action potential is based around a change in permeability to sodium and potassium due to the opening/closing of voltage-gated channels.
- As the membrane potential reaches threshold potential, the activation gate of the VG sodium channel opens, allowing sodium to flow into the cell.
- This causes further depolarisation and so more VG sodium channels open… (process continues)
- This makes the membrane around 600 times more permeable to sodium. The membrane potential, therefore, tends towards the equilibrium potential for sodium (+60mV).
- However, in the morphological change that took place to open the activation gates of sodium channels also causes closure of the inactivation gates. This process is much slower than the rapid opening of activation gates, so enough sodium flows across the membrane to depolarise it (to around +30mV) before inactivation gates start to close and the permeability for sodium returns to normal.
- At the point of threshold depolarisation, VG potassium channels also begin to open- this is also a delayed process so the effect of this is seen at the peak of depolarisation, when repolarisation occurs (the membrane becomes 300 times more permeable to potassium and the membrane potential rapidly tends to the equilibrium potential for potassium- -90mV).
- Note that the newly positive intra-cellular region will repel potassium out of the cell (moreso than just by concentration gradient)
- The closure of potassium channels is also a delayed process, so there is also some hyperpolarisation (to -80mV).
- As repolarisation occurs, potassium channels close and sodium channels return to their activated but closed states.
- The Na/K pump returns the membrane potential to its resting state.
see http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter8/animation__voltage-gated_channels_and_the_action_potential__quiz_1_.html for a good animation of the movement of sodium and potassium.
This second image shows the changes in permeability for sodium and potassium during an action potential.