Explain how a graded receptor potential in a sensory receptor is converted into action potentials transmitted along a sensory neuron.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sensory receptors convert stimuli into electrical signals. The intensity of a stimulus is encoded in the pattern of action potentials sent to the central nervous system.
Model answer (4 marks)
A stimulus opens ion channels in the receptor membrane, producing a graded receptor potential whose magnitude depends on stimulus intensity.
If the receptor potential reaches or exceeds the threshold at the trigger zone (axon hillock), an action potential is initiated.
Because action potentials are all‑or‑nothing, the stimulus intensity is encoded by the frequency of action potentials rather than their size.
The action potentials propagate along the sensory neuron to the CNS by local depolarisation of adjacent membrane, or by saltatory conduction in myelinated fibres.
If the receptor potential reaches or exceeds the threshold at the trigger zone (axon hillock), an action potential is initiated.
Because action potentials are all‑or‑nothing, the stimulus intensity is encoded by the frequency of action potentials rather than their size.
The action potentials propagate along the sensory neuron to the CNS by local depolarisation of adjacent membrane, or by saltatory conduction in myelinated fibres.
Examiner tips
- Use the exact terms ‘graded receptor potential’, ‘threshold’, ‘trigger zone’, ‘all‑or‑nothing’, and ‘frequency encoding’.
- Show the sequence: stimulus → receptor potential → threshold → action potential → propagation.
- Mention myelinated vs unmyelinated conduction if space allows.
Mark scheme (4 marks)
- A stimulus causes ion channels in the receptor membrane to open, producing a graded (receptor) potential whose magnitude depends on stimulus intensity.
- If the receptor potential reaches/exceeds the threshold value at the trigger zone / axon hillock, an action potential is initiated.
- Action potentials are all-or-nothing, so stimulus intensity is encoded by the frequency of action potentials rather than their size/amplitude.
- Action potentials propagate along the sensory neuron to the CNS by local depolarisation of adjacent membrane / saltatory conduction in myelinated neurons.
Key terms in this question
graded receptor potential · action potential
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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