The axon hillock has the lowest threshold for action potential generation in a neuron. Which biophysical property accounts for this?
- A Highest density of voltage-gated Na+ channels and lowest ratio of K+ to Na+ channel density at the hillock ✓
- B Largest diameter ensuring lowest internal resistance for depolarization
- C Absence of myelin sheath at the hillock exposing maximum membrane surface area to depolarizing current
- D Proximity to the nucleus ensuring rapid transcription of ion channel mRNA for rapid channel replacement
Explanation
The axon hillock has the highest density of voltage-gated sodium channels (Nav1.6 in particular) in the entire neuron, combined with a low density of stabilizing K+ channels, giving it the most negative threshold for firing. Graded postsynaptic potentials (EPSPs and IPSPs) summate at the hillock; if net depolarization reaches threshold here, an action potential is initiated. Option B (axon diameter) affects conduction velocity, not threshold. Option C is partially relevant but not the primary biophysical explanation. Option D is irrelevant to excitability threshold.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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