Action potentials and saltatory conductionEdexcel International A Level Biology: Flashcards
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What is the resting potential of an axon?
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- What is the resting potential of an axon?
- About −70 mV, with the inside negative relative to the outside.
- How does the sodium-potassium pump help maintain the resting potential?
- It uses ATP to move 3 Na⁺ out and 2 K⁺ in, making the inside more negative.
- What causes depolarisation?
- Voltage-gated Na⁺ channels open and Na⁺ diffuses into the axon.
- What is the threshold potential?
- About −55 mV, the level at which many more Na⁺ channels open and an action potential is triggered.
- Why is the action potential described as 'all-or-nothing'?
- Below threshold there is none, and above threshold its size is always the same.
- What causes repolarisation?
- Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out.
- What is hyperpolarisation?
- The potential falling below the resting value (about −80 mV) because K⁺ channels close slowly.
- What is the refractory period?
- A short time after an action potential when the Na⁺ channels cannot open, so no new action potential can start.
- Why do impulses travel in only one direction?
- The region behind is in its refractory period, so only the region ahead can depolarise.
- What are local circuits?
- Flows of ions along the axon between a depolarised region and the next region, bringing it to threshold.
- What are nodes of Ranvier?
- Gaps in the myelin sheath where the axon membrane is exposed and depolarisation can occur.
- What is saltatory conduction?
- The action potential jumping from node to node along a myelinated axon.
- Why is saltatory conduction faster?
- Depolarisation occurs only at the nodes, so the whole membrane does not have to depolarise.
- Give two factors that affect the speed of conduction.
- Myelination and axon diameter (also temperature).
Exam questions on Action potentials and saltatory conduction
- A neuroscientist stimulates an axon in a laboratory and records the potential difference across its membrane. At rest the inside of the axon is at −70 mV relative to the outside. After a stimulus the potential difference rises rapidly to +40 mV and then falls back to −70 mV.Explain why the inside of the axon is negative relative to the outside at rest.2 marks
- Tetrodotoxin (TTX), a poison found in pufferfish, blocks the voltage-gated sodium ion channels in axon membranes. A different laboratory chemical blocks only the voltage-gated potassium ion channels.Explain why a person poisoned with TTX becomes paralysed.2 marks
- Two axons of the same diameter and 0.50 m long are compared. Axon A is myelinated and conducts impulses at 50 m s⁻¹. Axon B is unmyelinated and conducts impulses at 1.0 m s⁻¹.Explain why axon A conducts impulses faster than axon B.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).