Spatial vs Temporal Summation
The mechanism responsible for the integration of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), or both in the postsynaptic neuron is referred to as Summation. Since, an individual EPSP has very small effect on the postsynaptic membrane potential, it is not enough to reach the threshold level, thus producing an action potential is impossible. Therefore, in order to reach threshold limit, several EPSPs must occur either one after another repeatedly or several EPSPs at the same time. Depending on the ways EPSPs occurring, there are two forms of summation, namely; temporal summation and spatial summation. These two forms occur concurrently to regulate the membrane potential under certain physiological conditions.
Spatial Summation
Spatial summation is the additive effect of EPSPs or ISPSs simultaneously originating from different presynaptic neurons on the membrane potential of the postsynaptic neuron. This involves multiple synapses that are active simultaneously. The algebraic summation of potentials from different inputs on the dendrites is considered in this summation. Summation of EPSPs allows the potential to reach an action potential, and summation of IPSPs prevents the cell from achieving an action potential.
Temporal Summation
Temporal summation is the additive effects of sequential multiple EPSPs or IPSPs originating from a single presynaptic neuron on the membrane potential of the postsynaptic neuron. This involves single synapsis that is active repeatedly. Temporal summation occurs when the time duration is sufficiently long, and frequency of rises in potentials are high enough to reach the action potential.
What is the difference between Spatial and Temporal Summation?
• Spatial summation involves multiple synapses, whereas temporal summation involves single synapse.
• In temporal summation, EPSPs occur rapidly one after another while, in spatial summation, all the ESPSs occur at the same time.
• Unlike the spatial summation, temporal summation depends on the time duration of which EPSPs occur, and the frequency of rises in potential.