text: Following the action potential, characteristically generated by the influx of Na through voltage gated Na channels, there is a period of repolarization in which the Na channels are inactivated while K channels are activated.  Further study of K channels shows that there are four types which influence the repolarization of the cell membrane to re-establish the resting potential.  The four types are K1, K2, K3 and K4. The K1 channel primarily influences the repolarization of the axon.  The K2 channel is characteristically activated slower.  The K4 channels are characteristically activated rapidly.  When K2 and K4 channels are blocked, the action potential predictably widens. The K3 channels open at a more positive membrane potential and deactivate 10 times faster than the other K channels. These properties allow for the high-frequency firing that mammalian neurons require. Areas with dense K3 channels include the neocortex, basal ganglia, brain stem and hippocampus as these regions create microsecond action potentials that requires quick repolarization.
  Utilizing voltage-clamp data from experiments based on rodent neurons, the K4 channels are associated with the primary repolarization conductance following the depolarization period of a neuron.  When the K4 channel is blocked, the action potential becomes broader, resulting in an extended repolarization period, delaying the neuron from being able to fire again.  The rate of repolarization closely regulates the amount of Ca ions entering the cell.  When large quantities of Ca ions enter the cell due to extended repolarization periods, the neuron may die, leading to the development of stroke or seizures.
  The K1 channels are found to contribute to repolarization of pyramidal neurons, likely associated with an upregulation of the K4 channels.  The K2 channels were not found to contribute to repolarization rate as blocking these channels did not result in changes in neuron repolarization rates.