allow for the high-frequency firing that mammalian neurons require. Areas with dense Kv3 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 Kv4 channels are associated with the primary repolarization conductance following the depolarization period of a neuron. When the Kv4 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 Ca2+ ions entering the cell. When large quantities of Ca2+ ions enter the cell due to extended repolarization periods, the neuron may die, leading to the development of stroke or seizures. The Kv1 channels are found to contribute to repolarization of pyramidal neurons, likely associated with an upregulation of the Kv4 channels. The Kv2 channels were not found to contribute to repolarization rate as blocking these channels did not result in changes in neuron repolarization rates. Repolarization of atria cells Another type of K+ channel that helps to mediate repolarization in the human atria is the SK channel, which are K+ channels which are activated by increases in Ca2+ concentration. "SK channel" stands for a small conductance calcium activated potassium channel, and the channels are found in the heart. SK channels specifically act in the right atrium of the heart, and have not been found to be functionally important in the ventricles of the human heart. The channels are active during repolarization as well as during the atrial diastole phase when the current undergoes hyperpolarization. Specifically, these channels are activated when Ca2+ binds to calmodulin (CaM) because the N-lobe of CaM interacts with the channel's S4/S5 linker to induce conformational change. When these K+ channels are activated, the K+ ions rush out of the cell during the peak of its action potential causing the cell to repolarize as the influx of Ca2+ ions are exceeded by K+ ions leaving the cell continuously. Ventricular repolarization In the human ventricles, repolarization can be seen on an ECG (electrocardiogram) via the J-wave (Osborn), ST segment, T wave and U wave. Due to the complexity of the heart, specifically how it contains three layers of cells (endocardium, myocardium and epicardium), there are many physiological changes effecting repolarization that will also affect these waves. Apart from changes in the structure