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Action potential is a brief reversal of membrane potential where the membrane potential changes from -70mV to +30mV. When the membrane potential of the axon hillock of a neuron reaches threshold, a rapid change in membrane potential occurs in the form of an action potential.
This moving change in membrane potential has three phases. First one is depolarization which is followed by repolarization and a short period of hyper polarization. All these three events, happen over a short period of few milliseconds.
The depolarization, also called the rising phase, is caused when positively charged sodium ions (Na+) suddenly rush through open voltage gated sodium channels into a neuron. As additional sodium rushes in, the membrane potential actually reverses its polarity. During this change of polarity the membrane actually develops a positive value for a moment (+40 millivolts).
The repolarizaton, or falling phase, is caused by the slow closing of sodium channels and the opening of voltage gated potassium channels. As a result, the membrane permeability to sodium declines to resting levels. As the sodium ion entry declines, the slow voltage gated potassium channels open and potassium ions rush out of the cell. This expulsion acts to restore the localized negative membrane potential of the cell.
The hyper polarization is a phase where some potassium channels remain open, and sodium channels reset. There is a period of increased potassium permeability which results in excessive potassium efflux before the potassium channels close. This results in hyper polarization as seen in a slight dip following the spike.
The propagation of action potential is independent of of stimulus strength, but is dependent on refractory periods. The period from the opening of the sodium channels until the sodium channels begin to reset is called the absolute refractory period. During this period, the neuron cannot respond to another stimulus, no matter how strong.
Source: Boundless. “The Action Potential and Propagation.” Boundless Anatomy and Physiology. Boundless, 08 Aug. 2016. Retrieved 28 Sep. 2016 from https://www.boundless.com/physiology/textbooks/boundless-anatomy-and-physiology-textbook/overview-of-the-nervous-system-11/neurophysiology-113/the-action-potential-and-propagation-615-8485/