Action potentials and saltatory conductionEdexcel International A Level Biology: Revision notes
Section 1
The resting potential
At rest, the inside of an axon is negative relative to the outside, at about −70 mV. This is the resting potential. It is maintained by:
- The sodium-potassium pump, which uses ATP to move 3 Na⁺ out for every 2 K⁺ in.
- The membrane being more permeable to K⁺ than Na⁺ (K⁺ leak channels), so K⁺ diffuses out, leaving the inside negative.
The membrane is said to be polarised.
Section 2
Generating an action potential
An action potential is a brief reversal of the potential difference across the membrane, caused by changes in permeability to Na⁺ and K⁺.
- Stimulus: some voltage-gated Na⁺ channels open and Na⁺ diffuses in, so the membrane starts to depolarise.
- Threshold: at about −55 mV many more Na⁺ channels open (positive feedback). Large Na⁺ entry raises the potential to about +40 mV.
- Repolarisation: Na⁺ channels close and voltage-gated K⁺ channels open, so K⁺ diffuses out and the potential falls.
- Hyperpolarisation: K⁺ channels close slowly, so the potential briefly falls below −70 mV, to about −80 mV.
- Recovery: the sodium-potassium pump and K⁺ leak channels restore −70 mV.
The action potential is all-or-nothing: a stimulus below threshold gives no action potential, and larger stimuli do not make a bigger one.
Na⁺ and K⁺ move by facilitated diffusion through voltage-gated channels during the action potential. The pump is not what causes the rise or fall.
Section 3
Refractory period and one-way conduction
After an action potential the Na⁺ channels cannot open for a short time. This refractory period has two effects:
- The action potential can only move forwards, because the region behind it is refractory.
- Action potentials are discrete and separate, and their frequency is limited.
An impulse travels along the axon by local circuits. Depolarisation at one point makes Na⁺ diffuse along the axon to the next region, which is brought to threshold so a new action potential forms there.
Section 4
Myelination and saltatory conduction
In a myelinated axon, the myelin sheath insulates the membrane. Ions can only cross the membrane, and voltage-gated channels are concentrated, at the nodes of Ranvier, the gaps between Schwann cells.
Local circuits flow between nodes, so the action potential jumps from node to node. This is saltatory conduction. It is much faster than the continuous conduction in an unmyelinated axon (up to about 100 m s⁻¹ compared with about 1 m s⁻¹) and uses less energy, as fewer ions need to be pumped back.
Worked example: over 0.50 m, an impulse at 50 m s⁻¹ takes 0.50 ÷ 50 = 0.010 s. At 1.0 m s⁻¹ it takes 0.50 s, which is 50 times longer.
For a speed question: myelin insulates, depolarisation only at nodes, impulse jumps between nodes, so it is faster. Do not say the myelin 'speeds up the ions'.
That's the notes covered.
Carry on to the next subtopic.
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).