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Summation (neurophysiology)
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<p>Summation, which includes both spatial summation and temporal summation, is the process that determines whether or not an <a href="/facts/Action_potential/Sz8yCRqx">action potential</a> will be generated by the combined effects of <a href="/facts/Excitatory_postsynaptic_potential/fh6J3sUm">excitatory</a> and <a href="/facts/Inhibitory_postsynaptic_potential/Jfm13Wkn">inhibitory</a> 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 <a href="/facts/Threshold_potential/VTnHFuGo">threshold voltage</a> to trigger an action potential. </p><p><a href="/facts/Neurotransmitter/LlpYFSU3">Neurotransmitters</a> released from the <a href="/facts/Axon_terminal/QhUJg8SH">terminals</a> of a <a href="/facts/Presynaptic/vdMRJCUn">presynaptic</a> <a href="/facts/Neuron/G7CyTVdH">neuron</a> fall under one of <a href="/facts/Neurotransmitter/LlpYFSU3">two categories</a>, depending on the <a href="/facts/Ion_channel/A46LARFH">ion channels</a> gated or modulated by the <a href="/facts/Neurotransmitter_receptor/6a6WnCwT">neurotransmitter receptor</a>. Excitatory neurotransmitters produce <a href="/facts/Depolarization/qGX8lFXE">depolarization</a> of the postsynaptic cell, whereas the <a href="/facts/Hyperpolarization_(biology)/XNH4zPwN">hyperpolarization</a> produced by an inhibitory neurotransmitter will mitigate the effects of an excitatory neurotransmitter. This depolarization is called an EPSP, or an <a href="/facts/Excitatory_postsynaptic_potential/fh6J3sUm">excitatory postsynaptic potential</a>, and the hyperpolarization is called an IPSP, or an <a href="/facts/Inhibitory_postsynaptic_potential/Jfm13Wkn">inhibitory postsynaptic potential</a>. </p><p>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 <i>disfacillitation</i>. A pathway may also be inhibited; removal of inhibitory input constitutes <i>disinhibition</i>, which, if other sources of excitation are present in the inhibitory input, can augment excitation. </p><p>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. </p>