# Explain how summation at a postsynaptic neuron allows the nervous system to integrate information from multiple sources.

> IB DP Biology Standard Level (2023 syllabus) — C2.2 Neural signalling · Explain · 4 marks

> A postsynaptic neuron in the central nervous system receives simultaneous input from several presynaptic neurons, some of which release excitatory neurotransmitters and others inhibitory neurotransmitters.

## Mark scheme (4 marks)

1. Excitatory postsynaptic potentials (EPSPs) depolarise the postsynaptic membrane, bringing it closer to the threshold potential.
2. Inhibitory postsynaptic potentials (IPSPs) hyperpolarise the postsynaptic membrane, moving it further from the threshold potential.
3. Summation involves the algebraic addition of all EPSPs and IPSPs; spatial summation combines inputs from multiple presynaptic neurons simultaneously, and/or temporal summation combines repeated inputs from the same neuron in rapid succession.
4. An action potential is generated only if the net summated potential reaches or exceeds the threshold potential, allowing the postsynaptic neuron to act as an integrating centre that produces an output proportional to the balance of excitatory and inhibitory inputs.

## Key terms

- [summation](https://www.gradenine.co.uk/glossary/summation)

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