Summation (neurophysiology)

Summation, which includes both spatial summation and temporal summation, is the process that determines whether or not an action potential will be generated by the combined effects of excitatory and inhibitory signals, both from multiple simultaneous inputs (spatial summation), and from repeated inputs (temporal summation). Depending on the sum total of many individual inputs, summation may or may not reach the threshold voltage to trigger an action potential.^[1]

Neurotransmitters released from the terminals of a presynaptic neuron fall under one of two categories, depending on the ion channels gated or modulated by the neurotransmitter receptor. Excitatory neurotransmitters produce depolarization of the postsynaptic cell, whereas the hyperpolarization produced by an inhibitory neurotransmitter will mitigate the effects of an excitatory neurotransmitter.^[2] This depolarization is called an EPSP, or an excitatory postsynaptic potential, and the hyperpolarization is called an IPSP, or an inhibitory postsynaptic potential.

The only influences that neurons can have on one another are excitation, inhibition, and—through modulatory transmitters—biasing one another's excitability. From such a small set of basic interactions, a chain of neurons can produce only a limited response. A pathway can be facilitated by excitatory input; removal of such input constitutes disfacillitation. A pathway may also be inhibited; removal of inhibitory input constitutes disinhibition, which, if other sources of excitation are present in the inhibitory input, can augment excitation.

When a given target neuron receives inputs from multiple sources, those inputs can be spatially summated if the inputs arrive closely enough in time that the influence of the earliest-arriving inputs has not yet decayed. If a target neuron receives input from a single axon terminal and that input occurs repeatedly at short intervals, the inputs can summate temporally.