8.1.4Unit 6: Practical Skills in Biology II
The Nerve Impulse — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 38 of 48 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Resting Potential
Value
- Value The inside of a resting axon is about −70 mV relative to the outside, so the membrane is polarised.
Sodium–potassium pump
- Sodium–potassium pump Uses ATP to move 3 Na⁺ out and 2 K⁺ in, which sets up concentration gradients for both ions.
Potassium leakage
- Potassium leakage Open potassium channels let K⁺ diffuse out faster than Na⁺ can enter, leaving the inside more negative.
Permeability
- Permeability The membrane is far more permeable to K⁺ than to Na⁺ at rest, and voltage-gated channels are closed.
Part 2 of 5Generating an Action Potential
1
A stimulus opens some sodium ion channels, so Na⁺ diffuses in and makes the inside less negative.
2
If the membrane reaches the threshold of about −55 mV, voltage-gated sodium channels open and many more Na⁺ ions rush in.
3
The potential reverses to about +30 mV, which is depolarisation.
4
The sodium channels close and voltage-gated potassium channels open, so K⁺ diffuses out and the inside becomes negative again, which is repolarisation.
5
The membrane briefly overshoots to a more negative value, called hyperpolarisation, before the pump restores the resting potential.
Part 3 of 5Properties of the Impulse
Properties of the Impulse
All or nothing
- All or nothing If the threshold is not reached no action potential occurs, and all action potentials have the same size.
Refractory period
- Refractory period The membrane cannot be stimulated again straight away, so impulses are discrete and travel in one direction.
Propagation
- Propagation Na⁺ diffuses along the axon and depolarises the next section, creating a moving wave of depolarisation.
Intensity
- Intensity A stronger stimulus produces a higher frequency of action potentials, not larger ones.
Part 4 of 5Worked Example
Question
- A resting axon at −70 mV is stimulated.
- Sodium ions enter and the potential rises to −58 mV.
- State whether an action potential occurs, then calculate the total potential change from the resting value to the peak of +30 mV.
- Threshold is about −55 mV; −58 mV is more negative than this, so the threshold has not been reached.
- No voltage-gated channels open, so there is no action potential.
- Change to peak = 30 − (−70) = 100 mV.
Answer
- No action potential occurs;
- a full action potential changes the potential by 100 mV.
Part 5 of 5Exam Checklist & Tips
8.1.4 – Key Points for the Exam
- Resting potential The potential difference across the membrane of an unstimulated neurone, about −70 mV.
- Threshold potential The membrane potential at which voltage-gated sodium channels open and an action potential starts.
- Depolarisation Reversal of the membrane potential as sodium ions enter.
- Repolarisation Return of a negative internal potential as potassium ions leave.
- Refractory period The time after an action potential when the membrane cannot respond to a new stimulus.
Common Mistakes
- Saying an impulse is a flow of electrons along the axon like a current in a wire.
- Stating that a stronger stimulus produces a bigger action potential.
- Forgetting that potassium ions move out during repolarisation.
- Confusing resting potential, which is negative inside, with the peak of the action potential, which is positive.
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