Summation

When an afferent neuron fires a sequence of action potentials in a rapid succession, then the initial psps produced in the postsynaptic cell will not have time to decay before the next psps enter. Therefore the successive psps summate over time. This is known as the temporal summation. When sufficient, the temporal summation will cause the postsynaptic cell to reach the firing threshold.

The summing of postsynaptic potentials produced at individual points on the neuron surface is known as the spatial summation which is as shown in figure. When a sufficient number of excitatory synapses are activated in relation to inhibitory ones then the cell will fire.

Figure: Spatial summation. In each situation the upper trace is the summed response of the two lower epsps produced at synapses: (a) a long way apart, (b) close altogether.

However the temporal and spatial summation is explained as individual processes, both take places altogether as a neuron is stimulated and it is their combined effect that dictates whether it will fire. The exact details of how summation works depends on the neuron’s geometry as this determines exactly how all the synaptic potentials set up on the cell decay as they spread towards the axon hillock. The frequency with which the cell fires, and how long it fires, is determined by the amplitude and period of the depolarization of the axon hillock membrane.