(c) the generation and transmission of nerve impulses in mammals

The idea

In mammals, a resting neurone keeps its inside negative relative to the outside (about −70 mV) because the sodium–potassium pump moves 3 Na⁺ out for every 2 K⁺ in, and the membrane is far more permeable to K⁺ than to Na⁺, so K⁺ leaks out. A stimulus above threshold opens voltage-gated Na⁺ channels: Na⁺ rushes in, the membrane depolarises to about +40 mV, then Na⁺ channels close and K⁺ channels open, repolarising and briefly hyperpolarising the membrane before the pump restores the resting potential. This local depolarisation triggers the next patch of membrane, so the action potential travels as a wave. In myelinated axons the impulse jumps between nodes of Ranvier (saltatory conduction), which is faster. The all-or-nothing law means a stronger stimulus gives more frequent impulses, not bigger ones, and the refractory period enforces one-way travel.

What earns the marks

• Resting potential is maintained by the Na⁺/K⁺ pump moving 3 Na⁺ out and 2 K⁺ in, plus greater K⁺ permeability, giving an inside-negative membrane.
• Depolarisation results from voltage-gated Na⁺ channels opening and Na⁺ diffusing in, reversing the membrane polarity.
• Repolarisation follows when Na⁺ channels close and K⁺ channels open, allowing K⁺ to leave the axon.
• The action potential is all-or-nothing; stimulus strength is coded by impulse frequency, not amplitude.
• Myelination speeds transmission by saltatory conduction between nodes of Ranvier.
• The refractory period prevents the impulse reversing direction along the axon.

Common mistakes and how to put them right

• Thinking a bigger stimulus makes a taller action potential.

  Put it right: The response is all-or-nothing, so intensity is encoded by frequency of impulses.

• Believing the pump fires continuously during the spike.

  Put it right: The pump mainly restores ion gradients after the impulse; the spike depends on channel opening.

• Confusing depolarisation with repolarisation.

  Put it right: Depolarisation is Na⁺ entering (membrane becoming positive inside); repolarisation is K⁺ leaving.

• Assuming myelin carries the current itself.

  Put it right: The impulse jumps between nodes of Ranvier; the myelin insulates.

Exam tips

• Learn the sequence resting → depolarisation → repolarisation → hyperpolarisation → restoration as an ordered chain.
• Quote the 3 Na⁺ out : 2 K⁺ in ratio and link it to the inside-negative resting potential.
• When asked why conduction is faster, name saltatory conduction and nodes of Ranvier explicitly.