The development of action potential
Something that slightly increases the membrane potential from -90mV towards zero, the increased potential will cause the opening of some potential gate sodium channels.
Up to this point, we have explained the changes in sodium and potassium permeability across the cell membrane, as well as the development of action potentials, but we have not explained the generation of action potentials.
A positive feedback loop opens the sodium channel
First, if the membranes of the nerve fibres remain intact, no action potentials occur in normal nerves. However, if something were to slightly increase the membrane potential from -90mV towards zero, the increased potential would cause some potential gate sodium channels to open. This allows the rapid inward flow of sodium ions, causing a further increase in the membrane potential, leading to the opening of the potential gate, and allowing more sodium to flow into the nerve cell.
This process is a positive feedback loop, once the feedback is strong enough, the process continues until all potential gates are activated (open). Then, within a fraction of a millisecond, the increased membrane potential causes the closure of the sodium channels and the opening of the potassium channels, and the action potential rapidly ends.
Threshold of action potential
The action potential will not occur until the increase in membrane potential is large enough to produce the positive feedback described above.
This occurs when the amount of sodium ions entering the nerve becomes larger than the number of potassium ions coming out. A sudden increase in membrane potential of 15-30 mV is required. Thus, a sudden increase in the membrane potential in a large nerve fibre from -90 mV up to about -65 mV usually causes an explosive development of the action potential. A level of -65 mV is said to be the threshold for stimulation.