Introduction

The initiation and propagation of action potentials in neurons are core to nervous system communication. Physiologically, the action potential often originates at the axon hillock, a specialized region of the neuron, and travels toward the axon terminal. The unique properties of the axon hillock and axonal membrane govern both the site of action potential generation and the directionality of its conduction. This article evaluates key explanations for this location and direction specificity.

Statement Analysis

A. The membrane of axon hillock has highest threshold for the generation of action potential

• This statement is incorrect. The axon hillock has the lowest threshold among neuronal membranes due to a high density of voltage-gated Na+ channels, making it the most excitable region for initiating action potentials.

B. The membrane of axon hillock contains large numbers of voltage-gated Na+ channels and that makes it more excitable

• This statement is correct. The high density of voltage-gated sodium channels at the axon hillock allows it to respond swiftly to depolarizations generating the action potential.

C. The propagating action potential in the middle of the axon cannot generate another action potential in the direction of the cell body since a large fraction of voltage-gated Na+ channels in the preceding portion is voltage inactivated

• This statement is correct. Sodium channels behind the action potential wave are in the inactivated state (absolute refractory period), preventing the action potential from moving backward.

D. As the number of voltage-gated Na+ channels is less in the preceding portion of axonal membrane, the propagating action potential in the middle of the axon cannot generate another action potential in the direction of cell body

• This statement is incorrect. The inactivation state of Na+ channels, not merely their quantity, is responsible for unidirectional propagation.

Explaining the Physiological Basis

• The axon hillock serves as the trigger zone due to its low threshold and high Na+ conductance, making it the preferred starting point for action potentials.study+1

• Following initiation, the action potential travels down the axon by opening adjacent Na+ channels sequentially, each segment following an all-or-none response profile.

• After activation, Na+ channels enter an inactivated state, making the region temporarily unresponsive to new stimuli—the absolute refractory period.wikipedia+1

• This refractoriness ensures unidirectional conduction from the hillock to the axon terminal by preventing backward excitation.taylorandfrancis+1

Correct Combination of Statements

The correct choice is:

(2) B and C