EPSP, IPSP, and Summation
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An excitatory postsynaptic potential depolarizes toward threshold, per Bio 2823; an inhibitory one hyperpolarizes away; summation at the axon hillock combines inputs into one firing decision.
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Multiple weak EPSPs arriving close together can summate into a signal strong enough to cross threshold, even if no single input could.
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IPSPs can cancel EPSPs — the net summed voltage determines whether the neuron fires.