Initiation of an action potential.
Most voltage-sensing channels have voltage-sensing ‘paddles’. These paddles consist of charged helices that can move according to the charges of its surroundings. Movements of these voltage-sensors create conformational changes in the channel that leads to their opening/closing.
Action potentials move in one direction.
Na+ channels can also be inactivated. After activation Na+ channels are inactivated for a time (refractory period). In the ‘ball and chain’ model the channel is blocked by a ball-structure which binds in the channel pore. This ensures that the action potential does not propagate backwards. The channel will remain inactivated until the membrane has reached its resting potential.
Repolarisation is also delayed by the hyperpolarisation that occurs from K+ channel flux. This is seen as a further dip in Figure 12.8a in Nelson. K+ voltage gated channels are opened once the action potential depolarisation has occurred. They then remain open until the membrane potential returns to its resting state.
Information taken from: Molecular Cell Biology, Lodish, 6th edn, Chapter 23.
Information and Channel figure taken from: The Voltage Sensor in Voltage-Dependent Ion Channels, F. Bezanilla, 2000, Pysiological Reviews.
1st polarisation figure taken from: http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A6125