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Initiation of Action Potential
The neuron is a specialized cell that is capable of generating an action potential on the cell membrane of the axon hillock. It does this by using special voltage-gated ion channels that respond to changes in voltage across the membrane of the cell. There are two types of voltage-gated channels, one for sodium and one for potassium. At the resting membrane potential of about -70 millivolts, these two voltage-gated ion channels are closed. During stimulation, when the stimulus has reached or exceeded the threshold value of around -45 millivolts, this change in voltage will signal the voltage-gated sodium channels to open up. An influx of sodium ions down their electrochemical gradient will cause the inside of the cell to become much more positive than the outside. This will cause the cell membrane to reverse polarity and this period is known as depolarization. As the voltage increases, the opening of the sodium channels causes even more sodium channels to open up. The permeability of sodium channels is now greater than the permeability of potassium channels. Eventually the cell membrane will reach a voltage of + 45 mV. This will signal the cell to inactive the sodium channels and open the potassium channels. This stage is known as the depolarization period. As the potassium channels are open, the potassium will move down its electrochemical gradient and to the outside the cell. This will cause a decrease in the amount of positive charge on the inside and eventually this will shift the polarity of the membrane back to normal. Since the potassium at this point is slightly more permeable than normal, the voltage of the membrane will drop slightly below the normal resting membrane potential. This stage is known as hyperpolarization. To return the membrane to the normal potential, the cell must use the sodium-potassium ATPase pumps, which move 3 sodium ions to the outside and 2 potassium ions to the inside (against the electrochemical gradient). Action potentials are all-or-nothing, meaning that a certain stimulus is needed to achieve the action potential. It also means that increasing the stimulus will not change the amplitude (height) of the action potential. But increasing the stimulus will increase the frequency of the action potential. There are two types of refractory periods - absolute refractory and relative refractory. During absolute refractory, no amount of stimulus will be able to generate another action potential because the sodium voltage-gated channels are either open or inactivated. However, during the relative refractory period, some of the sodium voltage-gated channels are being recovered from the inactivated phase and therefore a higher-than-normal stimulus can generate another action potential.
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