Why are action potentials usually conducted in one direction?
(A) Ions can flow along the axon in only one direction.
(B) The brief refractory period prevents reopening of voltagegated
Na+ channels.
(C) The axon hillock has a higher membrane potential than the
terminals of the axon.
(D) Voltage-gated channels for both Na+ and K+ open in only
one direction.
The correct answer and explanation is :
The correct answer is:
(B) The brief refractory period prevents reopening of voltage-gated Na⁺ channels.
Explanation:
Action potentials are usually conducted in one direction—from the axon hillock to the axon terminals—because of the refractory period of voltage-gated sodium (Na⁺) channels. The refractory period consists of two phases:
- Absolute Refractory Period:
- After an action potential is initiated, the Na⁺ channels open and then quickly become inactivated.
- During this period, no new action potential can be generated, regardless of stimulus strength.
- This ensures that the action potential does not travel backward.
- Relative Refractory Period:
- This follows the absolute refractory period, during which some Na⁺ channels begin to reset, but the membrane is still hyperpolarized due to K⁺ efflux.
- A stronger-than-normal stimulus is required to trigger a new action potential.
How This Prevents Backward Conduction:
When an action potential propagates along an axon, the local depolarization opens voltage-gated Na⁺ channels in the adjacent region of the membrane. However, the region of the membrane behind the action potential is in its refractory period, meaning the Na⁺ channels are inactivated and cannot reopen immediately. This prevents the signal from traveling backward and ensures a unidirectional propagation toward the axon terminals.
Why Other Answers Are Incorrect:
- (A) Incorrect: Ions (Na⁺ and K⁺) can diffuse in both directions, but action potential propagation depends on channel inactivation, not ion flow direction.
- (C) Incorrect: The axon hillock initiates action potentials due to a high concentration of Na⁺ channels but does not determine direction.
- (D) Incorrect: Voltage-gated channels can open in both directions, but refractory periods prevent bidirectional conduction.