If the action potential is an all-or-nothing phenomeon, how does the nervous system signal differences in intensity?

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An action potential in the nervous system is the electrical potential of the neuron briefly activating to transmit a signal across the cell.  The colloquial term is that the neuron is "firing".  These action potentials are described as "all-or-none", since the action potential is a set quantity regardless of the strength of the signal.  In other words, the potential is either "on" or "off", there are no degrees of "on".  The way that the nervous system is able to transmit intensity is not with the strength of a signal but with its frequency.  A single neural signal is not very strong.  But if enough of them fire in a rapid amount of time, the effect will be cumulative and will communicate a more intense signal in the nervous system.

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