8. The Action PotentialAn action potential is a large and rapid transient increase in membrane potential. It begins with themembrane potential at its resting level (i.e., at the resting potential). There is then an increase inmembrane potential from the resting level to a threshold potential. At this point there is a large and rapidincrease in membrane potential called a depolarisation. The membrane potential reaches a peak and thenbegins to decrease again. This decrease is called a repolarisation. At the end of the repolarisation phase,the membrane potential actually goes below the resting membrane potential. This is called ahyperpolarisation. Membrane potential then rises again to the resting potential level.9The changes in membrane potential during an action potential are the result of Na+ moving into the cellthrough voltage-gated Na+ channels and K+ moving out of the cell through voltage-gated K+ channels.In order for Na+ to move from the extracellular fluid (ECF) through the Na+ channel into the cytosol, itmust pass through two gates within the Na+ channel: first it must move through the activation gate andthen through the inactivation gate. The Na+ channel activation gate is closed when the membranepotential is at the resting level. As the membrane potential increases from the resting potential to thethreshold potential, activation gates in some Na+ channels begin to open. Once the membrane potentialreaches the threshold potential, the activation gates in all Na+ channels open (very rapidly).The Na+ channel activation gate will close again once the membrane potential falls back to the thresholdpotential during the hyperpolarisation phase.The inactivation gate in the Na+ channel is open at the resting membrane potential.As the depolarisation progresses and begins to approach the peak level of depolarization, the inactivationgate begins to close (they close slowly).The Na+ channel inactivation gate will open once more when the membrane potential begins to drop inthe early part of the repolarisation phase (just after the peak membrane potential has been achieved.In order for K+ to leave the cell it must pass through the activation gate in the K+ channel. Note that thereis only one activation gate in the K+ channel; there is no inactivation gate in the K+ channel.The K+ channel’s activation gate is closed at the resting membrane potential. It opens slowly during thedepolarisation phase.The K+ channel activation gate will close once more when the membrane potential begins to drop in theearly part of the repolarisation phase (just after the peak membrane potential has been achieved