A threshold is the minimum intensity level that a stimulus must have in order to trigger the rapid depolarization of an action potential. 

Brandeis University

The reduction of the electrical potential of the membrane. During depolarization, 
voltage-gated Na+ ion channels begin to open, thus letting some Na+ ions pass through the membrane. The action potential is sparked when the rushing in of the positive Na+ ions into the neuron reduces the electrical potential to around -40 mV.

Depolarization

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

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