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The All-or-None Law and Stimulus Intensity

A public GRE Psychology question on how a neuron encodes a stronger stimulus, and why the answer cannot be a bigger action potential.

Question

A sensory neuron in the skin responds to a light touch and also to a much firmer press. Both stimuli are well above the neuron's threshold of excitation. Compared with the response to the light touch, the response to the firmer press is best described as involving

  1. a.action potentials of larger amplitude
  2. b.a higher frequency of action potentials
  3. c.action potentials that travel faster along the same axon
  4. d.a more negative resting potential between impulses
  5. e.a longer absolute refractory period after each impulse

Correct answer

B. a higher frequency of action potentials

Full reasoning

  1. 1Both stimuli exceed threshold, so the question is not about whether the neuron fires but about how the difference between them is represented.
  2. 2The all-or-none law fixes the amplitude of every action potential in a given neuron, which removes spike size as a possible code for intensity.
  3. 3The two variables left free are how often the neuron fires and how many neurons fire, so a stronger stimulus produces a higher firing rate and broader recruitment.
  4. 4Conduction velocity and refractory duration are properties of the axon and its channels, not of the stimulus, so neither can carry intensity information either.

Why each choice is right or wrong

Choice A

This treats the action potential like a graded potential. Graded postsynaptic potentials do vary in size with input strength, but the action potential itself is all-or-none: once threshold is crossed it fires at full magnitude regardless of how far past threshold the stimulus pushed the membrane.

Choice B

Correct. Because spike size is fixed by the all-or-none law, the nervous system codes intensity in the rate of firing — and in how many neurons are recruited — rather than in the size of any single impulse.

Choice C

This confuses stimulus intensity with axon properties. Conduction velocity along a given axon is set by its diameter and its myelination, both fixed for that neuron, so a firmer press cannot speed the impulse up.

Choice D

This has the sign of the effect backwards. A more negative interior is hyperpolarization, which moves the membrane further from threshold and makes firing less likely; an intense stimulus depolarizes the membrane instead.

Choice E

The absolute refractory period is a fixed property of the membrane's ion channels, and lengthening it would lower the maximum firing rate — the opposite of what a stronger stimulus produces.

Related formula

Intensity coding under the all-or-none law

stronger stimulus -> more action potentials per second and more neurons recruited; spike amplitude unchanged

Assumptions: Applies to action potentials; graded postsynaptic potentials do vary in size with input strength.

Related topic and practice

Neurons, Neurotransmission, and the Nervous SystemCreate a free account to practice

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Sources

Authored for this public explainer registry. It has no database question ID and is not copied from a protected or official exam bank.

  1. Psychology 2e, Section 3.2: Cells of the Nervous System — OpenStax. Accessed 2026-08-15. OpenStax textbook content is CC BY-NC-SA 4.0; attribute and avoid verbatim reuse beyond short cited references.
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