Edexcel A Level (IAL) Biology -8.4 The Nerve Impulse- Study Notes- New Syllabus

Conduction of a Nerve Impulse (Action Potential)

🌱 Introduction

A nerve impulse is an electrical signal transmitted along a neurone.
It is caused by the movement of ions across the neurone membrane and involves rapid depolarisation and repolarisation.

🔍 Key Terms

• Resting potential: ~-70 mV, inside of axon is negative relative to outside.
• Depolarisation: Membrane becomes less negative as sodium enters.
• Repolarisation: Membrane returns to resting potential as potassium exits.
• Threshold potential: Minimum stimulus needed to trigger an action potential (~-55 mV).

🧬 Step-by-Step Conduction Along an Axon

1. Resting Potential

• Sodium-potassium pump maintains high Na⁺ outside, high K⁺ inside.
• Membrane is polarised: inside negative, outside positive.
• Axon is ready to fire.

2. Stimulus and Depolarisation

• Stimulus opens sodium channels.
• Sodium ions (Na⁺) rush into the axon.
• Inside becomes less negative → depolarisation.
• If threshold is reached, an action potential is generated.

3. Repolarisation

• Sodium channels close.
• Potassium channels open, K⁺ ions move out of the axon.
• Membrane potential becomes negative again → repolarisation.

4. Hyperpolarisation

• Too many K⁺ ions exit → inside becomes slightly more negative than resting.
• Sodium-potassium pump restores resting potential.

5. Propagation Along the Axon

• Local depolarisation causes adjacent regions of the membrane to depolarise.
• Impulse moves as a wave along the axon.
• In myelinated axons, impulse jumps between nodes of Ranvier → saltatory conduction, faster and more efficient.

📌 Important Points

• Action potential is all-or-nothing: once threshold is reached, full impulse occurs.
• Direction of impulse is from dendrite → axon → axon terminals.
• Myelin sheath increases speed; unmyelinated axons are slower.
• Sodium-potassium pump restores ionic balance after the impulse.

📊 Summary Table