Explain how summation at a postsynaptic neuron allows the nervous system to integrate information from multiple sources.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Excitatory postsynaptic potentials (EPSPs) depolarise the postsynaptic membrane, bringing it closer to the threshold potential.
Inhibitory postsynaptic potentials (IPSPs) hyperpolarise the postsynaptic membrane, moving it further from the threshold potential.
Summation is the algebraic addition of all EPSPs and IPSPs. Spatial summation combines inputs from several presynaptic neurons acting at the same time, while temporal summation combines repeated inputs from the same neuron in rapid succession.
An action potential is produced only when the net summed potential reaches or exceeds the threshold, so the postsynaptic neuron acts as an integrating centre that outputs a signal proportional to the balance of excitatory and inhibitory inputs.
Inhibitory postsynaptic potentials (IPSPs) hyperpolarise the postsynaptic membrane, moving it further from the threshold potential.
Summation is the algebraic addition of all EPSPs and IPSPs. Spatial summation combines inputs from several presynaptic neurons acting at the same time, while temporal summation combines repeated inputs from the same neuron in rapid succession.
An action potential is produced only when the net summed potential reaches or exceeds the threshold, so the postsynaptic neuron acts as an integrating centre that outputs a signal proportional to the balance of excitatory and inhibitory inputs.
Examiner tips
- Use the exact terms EPSP, IPSP, spatial and temporal summation. Show that the net potential must reach threshold to fire. Explain that the neuron integrates multiple inputs by algebraic addition.
Common mistakes
- Confusing depolarisation with hyperpolarisation. Forgetting to mention that the action potential is only triggered when threshold is reached. Mixing up spatial and temporal summation definitions.
Mark scheme (4 marks)
- Excitatory postsynaptic potentials (EPSPs) depolarise the postsynaptic membrane, bringing it closer to the threshold potential.
- Inhibitory postsynaptic potentials (IPSPs) hyperpolarise the postsynaptic membrane, moving it further from the threshold potential.
- 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.
- 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 in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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