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
What do you mean by not propagate backwards? I thought if the axon of a neuron was stimulated, then the action potential would propagate in both directions, but when the signal got to the head, the signal stopped, because the signal could only be transmitted at the other end of the axon. But I agree, the inactivating head stops a propagating signal already moving in one direction from changing direction.
ReplyDeleteIt could not propagate backwards as the propagation of a signal requires depolarisation and with deactivated channels this depolarisation is not obtainable. Without depolarisation to the threshold value electrotonus is the result.
ReplyDeleteIn the process of an action potential, there are three main events that occur.
ReplyDeleteDepolarization - where the potential of the cell has gone from -70mV to -50mV and has started undergoing the action potential. During this steep rise in the potential, the Na+ channels open up and the Na+ ions start moving through the cell membrane.
Repolarization - as the depolarization continues climbing, the K+ ion channels then open up and the sudden flux of K+ ion movement causes the depolarization to slow to a halt, and then the potential of the cell starts to become more negative again. As this turn around occurs, the Na+ ion channels will close off.
Hyperpolarization - where the K+ ion channels have closed off, and the potential of the cell continues to decrease to around 100mV before again rising and levelling off at the standard -70mV. Once this action has been achieved, the cell can undergo another action potential.
Note that once a cell has started undergoing repolarization, another action potential cannot occur until hyperpolarization has occured. However, if the cell is still in the depolarization phase, compound action potentials are capable of occuring.