The nerve impulse
The nerve impulse
- An impulse is a change in the voltage across the neurone's membrane.
- At rest the inside is negative; an impulse flips it briefly positive.
- A fatty sheath can make it travel far faster.
The resting potential
- When no impulse passes, the inside is negative compared with the outside — the resting potential.
- It is kept this way by a pump that moves sodium out and potassium in, and by the membrane being more permeable to potassium.
Practice
At the resting potential, the inside of the neurone is:
The resting potential keeps the inside negative, maintained by the Na⁺/K⁺ pump and K⁺ permeability.
The action potential
- A stimulus opens sodium channels → Na⁺ rushes in → inside briefly positive (depolarisation).
- Then potassium channels open → K⁺ leaves → the membrane returns to negative (repolarisation).
- A short refractory period follows: sodium channels can't reopen, restoring rest and limiting how often impulses fire.
Practice
Depolarisation during an action potential is caused by:
Sodium channels open and Na⁺ floods in, reversing the charge (depolarisation); then K⁺ leaves to repolarise.
Practice
The refractory period is important because it:
Sodium channels cannot reopen briefly, resetting the neurone and setting a maximum impulse frequency.
Faster: myelin
- Some axons are wrapped in a fatty myelin sheath that the impulse cannot cross.
- So the impulse jumps from one gap (node) to the next — saltatory conduction — travelling much faster.
Practice
Saltatory conduction makes impulses faster because the impulse:
The impulse cannot cross the myelin, so it jumps node to node — much faster than along a bare axon.
You've got it
Key idea
- resting potential: inside negative (Na⁺ pumped out, K⁺ in; membrane leaks K⁺)
- action potential: Na⁺ in → depolarisation (+); K⁺ out → repolarisation (−)
- refractory period: Na⁺ channels can't reopen → limits impulse frequency
- myelin → saltatory conduction (jumps node to node) → much faster