Summation
Joe Wolfensohn
Teacher
Recall Questions
This topic requires prior knowledge of synapses from GCSE and generating an action potential. You can test your knowledge on these below.
What is a synapse?
A junction between two neurones where the electrical signal is converted to a chemical signal using neurotransmitters.
What is an action potential?
A rapid, temporary change in membrane potential that transmits nerve impulses along a neurone.
What is the threshold potential?
The minimum depolarisation required to trigger an action potential, typically around –55 mV.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains summation or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Temporal Summation
Definition:
- Multiple impulses from the same presynaptic neurone arrive in quick succession, more neurotransmitters are released into the synaptic cleft. Their effects add up (summate) leading to more depolarisation of the post synaptic neurone and exceeding threshold to trigger an action potential.
Key Concept:
- One pre-synaptic neurone firing repeatedly and rapidly.
- Individual impulses may be too weak alone, but together they can reach threshold in the post synaptic neurone.
Spatial Summation
Definition:
- Multiple presynaptic neurones release neurotransmitters at the same time onto one postsynaptic neurone. More neurotransmitters are released into the synapse and their combined effect may be enough to trigger an action potential.
Key Concept:
- Many neurones converging onto one.
- Useful when information from different sources needs integration (e.g., reflex control or sensory input).
Inhibitory Synapses
Definition:
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Synapses where neurotransmitters hyperpolarise the postsynaptic membrane, making it harder to reach the threshold potential and less likely for an action potential to be generated.
It Works:
- Neurotransmitter binds to receptors that open Cl⁻ (chloride) or K⁺ (potassium) channels.
- Cl⁻ ions enter or K⁺ ions leave → membrane becomes more negative (hyperpolarised).
- Moves potential further from threshold (making it very unlikely that enough sodium ions would enter to generate an action potential)
- Results in an inhibitory postsynaptic potential (IPSP).
Effect:
- Inhibits action potentials.
Key Terms
- IPSP: Inhibitory postsynaptic potential – hyperpolarisation that moves neurone further from threshold.
- Temporal Summation: Repeated firing of one presynaptic neurone in quick succession.
- Spatial Summation: Simultaneous firing from multiple presynaptic neurones.
- Inhibitory Synapse: Synapse that prevents the generation of action potentials by hyperpolarising the membrane.
No answer provided.
Exam Tips
Be precise with terminology – use "depolarise" and "hyperpolarise" instead of "stimulate" or "slow down".
State clearly whether the threshold is reached or not.
No answer provided.
Explain how temporal and spatial summation allow action potentials to be generated in postsynaptic neurones. (4 marks)
- In temporal summation, one presynaptic neurone releases neurotransmitters multiple times in quick succession.
- In spatial summation, multiple presynaptic neurones release neurotransmitters at the same time.
- The combined release of neurotransmitters increases the chance of reaching the threshold.
- If the total depolarisation reaches threshold, an action potential is triggered in the postsynaptic neurone.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!