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stimuli cause action potential, but the subthreshold stimulus does not. The threshold intensity or threshold stimulus is used as an index to evaluate the excitability. The excitability vwies inversely to the threshold intensity, that is, the lower the threshold intensity of a cell, the high the excitability. Threshold Potential Not all membrane depolarization in excitable cells trigger the positive feedback relationship that leads to an action potential. Action potentials occur only when the net mowment of positive charge through ion channels is inward. The threshold potential is the membrane potential at which this occurs, and the stimuli that arc just strong enough to depolarization the membrane to this level are threshold stimuli. The threshold potential of most excitable cell membranes is about 15 mV less negative than the resting membrane potential. Thus, if the resting potential of nerve cell is 70 mV, the threshold potential may be -55 mV. Howewr, at depolarization less than threshold potential, the

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