A long-lasting stimulus whose intensity is increased very gradually fails to generate an action potential in a sensory nerve, whereas the same final intensity reached abruptly does fire one. This failure is best explained by:
- A Depletion of neurotransmitter vesicles at the first node
- B Activation of the sodium-potassium pump raising the membrane potential toward the potassium equilibrium potential
- C Slow depolarization allowing time-dependent inactivation of voltage-gated sodium channels, so threshold is never reached with enough channels available ✓
- D Progressive increase in membrane resistance reducing the local circuit current
Correct answer: C. Slow depolarization allowing time-dependent inactivation of voltage-gated sodium channels, so threshold is never reached with enough channels available
Explanation
Accommodation occurs when a slowly rising stimulus depolarizes the membrane gradually enough that the h-gates (inactivation gates) of voltage-gated sodium channels close before sufficient numbers of m-gates open together. Fewer activatable channels remain when the membrane finally crosses the old threshold value, so the effective threshold rises and no action potential fires. Vesicle depletion is irrelevant because this is an axonal event, not a synaptic one.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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