<P> Once the membrane potential reaches threshold, an action potential occurs and causes a sharp spike in membrane polarity . There are five phases of an action potential: threshold, depolarization, peak, repolarization, and hyperpolarization . </P> <P> Threshold is when the summation of MEPPs reaches a certain potential and induces the opening of the voltage - gated ion channels . The rapid influx of sodium ions causes the membrane potential to reach a positive charge . The potassium ion channels are slower - acting than the sodium ion channels and so as the membrane potential starts to peak, the potassium ion channels open and causes an outflux of potassium to counteract the influx of sodium . At the peak, the outflux of potassium equals the influx of sodium, and the membrane does not change polarity . </P> <P> During repolarization, the sodium channels begin to become inactivated, causing a net efflux of potassium ions . This causes the membrane potential to drop down to its resting membrane potential of - 100mV . Hyperpolarization occurs because the slow - acting potassium channels take longer to inactivate, so the membrane overshoots the resting potential . It gradually returns to resting potential and is ready for another action potential to occur . </P> <P> During the action potential before the hyperpolarization phase, the membrane is unresponsive to any stimulation . This inability to induce another action potential is known as the absolute refractory period . During the hyperpolarization period, the membrane is again responsive to stimulations but it requires a much higher input to induce an action potential . This phase is known as the relative refractory period . </P>

A mepp is the result of the spontaneous release of a synaptic vesicle containing neurotransmitter