Summation (neurophysiology)

[[Neurotransmitter]]s released from the [[axon terminal|terminals]] of a [[presynaptic]] [[neuron]] fall under one of [[Neurotransmitter#Excitatory and inhibitory|two categories]], depending on the [[ion channel]]s gated or modulated by the [[neurotransmitter receptor]]. Excitatory neurotransmitters produce [[depolarization]] of the postsynaptic cell, whereas the [[hyperpolarization (biology)|hyperpolarization]] produced by an inhibitory neurotransmitter will mitigate the effects of an excitatory neurotransmitter.{{cite book|last1=Coolen |last2=Kuhn |last3=Sollich|title=Theory of Neural Information Processing Systems|year=2005|publisher=Oxford University Press|location=London, UK}} This depolarization is called an EPSP, or an [[excitatory postsynaptic potential]], and the hyperpolarization is called an IPSP, or an [[inhibitory postsynaptic potential]].

[[Neurotransmitter]]s released from the [[axon terminal|terminals]] of a [[presynaptic]] [[neuron]] fall under one of [[Neurotransmitter#Excitatory and inhibitory|two categories]], depending on the [[ion channel]]s gated or modulated by the [[neurotransmitter receptor]]. Excitatory neurotransmitters produce [[depolarization]] of the postsynaptic cell, whereas the [[hyperpolarization (biology)|hyperpolarization]] produced by an inhibitory neurotransmitter will mitigate the effects of an excitatory neurotransmitter.{{cite book|last1=Coolen |last2=Kuhn |last3=Sollich|title=Theory of Neural Information Processing Systems|year=2005|publisher=Oxford University Press|location=London, UK}} This depolarization is called an EPSP, or an [[excitatory postsynaptic potential]], and the hyperpolarization is called an IPSP, or an [[inhibitory postsynaptic potential]].

Older models of neurons treated them mostly as simple integrators that just add up signals without much going on. [[Dendrites]] change that picture a lot. Modern studies point out they are not passive at all. They act like active parts that handle computations on their own. It seems like through voltage-gated channels, they manage this nonlinear summation. That allows for some local processing right there in the dendrites. Signals do not even make it to the [[axon hillock]] until after that step. The idea is dendrites do real work before everything else kicks in. Traditional views missed how dynamic that is.{{Cite journal |last=Stuart |first=Greg J. |last2=Spruston |first2=Nelson |date=2015-12 |title=Dendritic integration: 60 years of progress |url=https://www.nature.com/articles/nn.4157 |journal=Nature Neuroscience |language=en |volume=18 |issue=12 |pages=1713–1721 |doi=10.1038/nn.4157 |issn=1546-1726}}{{Cite journal |last=Moore |first=Jason J. |last2=Ravassard |first2=Pascal M. |last3=Ho |first3=David |last4=Acharya |first4=Lavanya |last5=Kees |first5=Ashley L. |last6=Vuong |first6=Cliff |last7=Mehta |first7=Mayank R. |date=2017-03-24 |title=Dynamics of cortical dendritic membrane potential and spikes in freely behaving rats |url=https://www.science.org/doi/10.1126/science.aaj1497 |journal=Science |volume=355 |issue=6331 |pages=eaaj1497 |doi=10.1126/science.aaj1497}}

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.

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.