The short answer
How a neuron fires an all-or-nothing electrical spike — and the ion movements behind every phase.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Every thought and movement rides on a tiny electrical spike. A neuron sits at about −70 mV, then briefly flips its voltage in under a millisecond — an all-or-nothing pulse called the action potential. It's all about ions moving through gated channels.
All-or-nothing
If a stimulus pushes the membrane past threshold (~−55 mV), a full action potential fires — always the same size. A bigger stimulus doesn't make a bigger spike; it makes them fire more often. Below threshold, nothing fires.
One action potential, in phases
Resting (−70 mV)
The Na⁺/K⁺ pump keeps Na⁺ high outside and K⁺ high inside. The inside is negative.
Depolarisation
A stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes IN, flipping the inside positive (toward +30 mV).
Repolarisation
Na⁺ channels close and K⁺ channels open; K⁺ flows OUT, restoring the negative inside.
Refractory period
A brief overshoot and reset during which the neuron can't easily fire again — which forces the signal to travel one way.
Worked example
A toxin blocks the voltage-gated sodium (Na⁺) channels of a neuron. What happens to the neuron's ability to fire an action potential?
Check yourself
The rapid rising phase (depolarisation) of an action potential is caused mainly by:
Key takeaways
- Resting potential ≈ −70 mV, maintained by the Na⁺/K⁺ pump.
- Reach threshold (~−55 mV) → a full, all-or-nothing action potential fires.
- Depolarisation = Na⁺ IN; repolarisation = K⁺ OUT.
- Stimulus strength is coded by FREQUENCY of firing, not spike size.
- Block Na⁺ channels and the neuron can't fire — the basis of many anaesthetics.
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