Biology:Temporal summation
From HandWiki
Temporal summation is an effect generated by a single neuron as a way of achieving action potential. Summation occurs when the time constant is sufficiently long and the frequency of rises in potential are high enough that a rise in potential begins before a previous one ends. The amplitude of the previous potential at the point where the second begins will algebraically summate, generating a potential that is overall larger than the individual potentials. This allows the potential to reach the threshold to generate an action potential.
See also