Explain how spatial and temporal summation at a postsynaptic neurone determine whether an action potential is generated.

IB DP Biology Higher Level (2023 syllabus) — C2.2 Neural signalling · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Spatial summation occurs when several presynaptic neurones release neurotransmitter onto the same postsynaptic neurone at the same time; the individual EPSPs add together.
Temporal summation occurs when one presynaptic neurone fires repeatedly in rapid succession; successive EPSPs arrive before the previous one has decayed, so they accumulate.
Both types of summation produce a graded change in membrane potential at the axon hillock. If the combined depolarisation reaches the threshold, voltage‑gated Na⁺ channels open and an action potential is generated.
If the summed potential does not reach threshold, no action potential occurs, showing that summation integrates the strength and frequency of incoming signals.

Examiner tips

  • Use the terms ‘spatial summation’, ‘temporal summation’, ‘EPSP’, ‘threshold’, ‘voltage‑gated Na⁺ channels’. Show the logical sequence: summation → change in membrane potential → threshold → action potential. Mention that failure to reach threshold means no AP.
  • common_mistakes
  • :
  • Confusing EPSP with IPSP. Saying ‘any depolarisation’ instead of ‘reaches threshold’. Forgetting to mention the axon hillock or voltage‑gated Na⁺ channels.

Mark scheme (4 marks)

  1. Spatial summation occurs when multiple presynaptic neurones release neurotransmitter onto the same postsynaptic neurone simultaneously, with their individual EPSPs combining.
  2. Temporal summation occurs when a single presynaptic neurone fires repeatedly in rapid succession, so successive EPSPs arrive before the previous one has decayed and accumulate.
  3. Both forms of summation produce a graded change in membrane potential at the axon hillock; if the combined depolarisation reaches the threshold potential, voltage-gated sodium channels open.
  4. If the combined postsynaptic potential does not reach threshold, no action potential is generated, illustrating that summation acts as an integrating mechanism allowing the neurone to process the relative strength and frequency of incoming signals.

Key terms in this question

temporal summation

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