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A stimulus opens some sodium channels. Na+ inflow through channels depolarizes membrane. If threshold is reached, it triggers action potential.

A stimulus is a perceptible alteration in the internal or external surroundings of an organism's physical or chemical composition. Sensitivity is the capacity of an organism or organ to recognise external stimuli and to respond appropriately to them. A stimulus is an occasion or any change in the environment that causes an organ or tissue to have a certain functional response. An external or internal trigger is possible.

When you enter a room, you must cross a threshold. A threshold separates one space from another, and you are also on a threshold just before beginning something new. A threshold is a starting or stopping point. The maximum amount of money permitted per transaction is known as a Threshold Amount.

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