As stated previously, muscles cells and neurons are able to display what is called an action potential. An action potential is a mechanism that involves the opening and closing of the Na+ activation gate, Na+ inactivation gate, and K+ activation gate. All these "gates" are voltage-gated channels, which are located on the cell membrane as well as the neuron's axon hillock. These voltage-gated channels respond to the electrical potentials within a cell and can trigger an action potential to occur (Wibbels, 2008). These channels are triggered by the influx of Na+ cations into the cell. The influx of Na+ increases the potential of the cell to a certain potential that triggers the action potential. This certain potential is called the threshold potential. As stated previously, the normal electrical potential in the ICF is about -70millivolts. As the Na+ cations flow in, the potential increases and when it reaches the threshold potential, the "all or nothing" mechanism occurs. How does this happen? The ligand-gated Na+ channels located on the cell membrane have receptors that receive a stimulus that will create an influx of Na+ cations into the cell to cause it to reach threshold potential. This, as stated before, will trigger the Na+ activation gates of the voltage-gated channels to rapidly open and create an influx of Na+ ions to flow in and cause the cell to undergo depolarization. The Na+ inactivation gates are much slower to close, which allows for more Na+ to enter into the cell. When the cell potential reaches threshold, the K+ voltage-gated channels also become activated and slowly opens and fully opens at about +30mV. At this point, the cell undergoes repolarization and becomes more negative as the K+ ions begin to be pushed out of the cell. But the K+ gates close so slow that it hyperpolarizes the cell and then it finally closes at about -70mV. After the K+ gate and Na+ inactivation gates are closed, the Na+/K+ pump restores the original distribution of the two ions in the ICF and ECF. (Wibbels and Vickery, 2007).
The stage after the action potential has occurred is called the refractory period. This makes the cell very difficult or impossible to have another action potential. Why? The main reason for this is because the Na+ inactivation gates have closed and will not open once they have closed. If all gates have closed, this is the absolute refractory period, which makes it impossible for the membrane to undergo another action potential. But sometimes not all the cells Na+ inactivation gates have closed completely which allows a small amount of Na+ molecules to still pass through into the cell. The membrane must have a very large stimulus in order to create an action potential. This is called the relative refractory period. This occurs right after the absolute refractory period and only lasts for a few milliseconds. Both the absolute and relative refractory periods account for the cell's inability to have another, immediate action potential. (Wibbels, 2008).
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