Summation is the process of adding things up. In the case of nervous system, it is about adding up the effect of multiple stimuli, that are all individually subthreshold, so that together they are suprathreshold and are able to generate an action potential (a response). There are two types of summation: spatial summation and temporal summation that occur between neurones.
Spatial summation is the effect of triggering an action potential in a neuron from one or more presynaptic neurons. This occurs when more than one excitatory postsynaptic potential (EPSP) originates simultaneously and a different part of 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 that exceed the threshold voltage 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.
This happens when the summation of graded potentials originates from one presynaptic neurone or in other words, the signals 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 a suprathreshold EPSP and an action potential will be generated. This is an example of postsynaptic integration.