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Threshold Potentials

Each cell's membrane is held at a specific resting electric potential, which is the potential difference between the interior and exterior of the cell. Almost all plasma membranes have an electrical potential across them that is due to the membrane permeability for potassium, sodium, calcium and chloride.

The resting potential of neurons is of particular importance, since changing the membrane potential of a neuron can result in an action potential. The resting membrane potential of neurons is about -70 mV, which means that the interior of the neuron is 70 mV less than external of the cell. A change in membrane potential which brings the neuron's membrane potential above the threshold value of -55 mV will result in an action potential. However, any change in membrane potential that results in a membrane potential less than -55 mV will not result in an action potential.

A graph with membrane potential in millivolts on the y axis and time on the x axis. A green dashed line at minus 55 millivolts represents the threshold of excitation. At time 0 the resting potential is a minus 70 millivolts. Next is the depolarisation phase, the intracellular sodium ions increase, and the membrane potential increases rapidly from minus 70 millivolts to 30 millivolts. Then in the repolarization phase, the membrane potential decreases rapidly from 30 millivolts to minus 70 millivolts. Finally, there is the hyperpolarization phase, the extracellular potassium ions increase, and the membrane potential drops below minus 70 millivolts before gradually rising back up to minus 70 millivolts.

Figure 1: Action potential steps

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Action potential