Summation
There are two types of summation: spatial summation and temporal summation that occur between neurones.
Spatial summation[1]
This occurs when more than one excitatory postsynaptic potential (EPSP) originates simultaneously and a different part on the neurone. If all of the EPSPs are subthreshold then an action potential will not be fired once they reach the neurone individually. However if they all stimulate the neurone simultaneously at the trigger zone, then the subthreshold EPSP’s will sum up to create a suprathreshold which will then generate an action potential.
Another form of spatial summation is that which involve an inhibitory postsynaptic potential (IPSP) which reaches the neurone at the same time as the multiple EPSPs and the sum of the IPSP and EPSP’s (the summed potential) is subthreshold so no action potential is generated as the IPSP has diminished the EPSPs. Spatial summation in this example is known as postsynaptic inhibition.
Temporal summation[2]
This happens when the summation of graded potentials originates from one presynaptic neurone or in other words, the signals are overlap reaching a postsynaptic neurone. If a subthreshold EPSP reaches the neurone then no action potential will be generated however, if multiple subthreshold EPSPs reach the neurone trigger zone close enough together in time then the two subthreshold EPSPs will sum up to cause an suprathreshold EPSP and an action potential will be generated. This is an example of postsynaptic integration.