Question

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 voltage-gated 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.

Short answer

(A) The statement“Ions can flow along the axon in only one direction” is false.

(B) The statement “The brief refractory period prevents reopening of voltage-gated Na⁺ channels” is true.

(C) The statement “The axon hillock has a higher membrane potential than the terminals of the axon” is false.

(D) The statement “Voltage-gated channels for both Na⁺ and K⁺ open in only one direction” is false.

Step-by-step solution

Neuron

The term neuron is used for denoting the main functional cells of the nervous system. These are responsible for creating a medium of communication between the body part and the units of the nervous system.Structurally, a neuron comprises a cell body and two types of extensions (dendrites and axons).

Explanation of option ‘(A)’

The membranous covering of the axon contains several ion channels that are regulated by fluctuations in the membrane’s electrical potential. Some of these channels transport ions out of the axon, while some of the channels facilitate ion influx. Inside the axon, ions can flow in all directions.

Thus, the flow of ions is not restricted to any one direction.

Therefore, the given statement is false.

Explanation of option ‘(B)’

After the propagation of action potential, the axonal membrane hyperpolarizes to a highly negative electric potential (a value lower than the resting membrane potential).

The negativity of the membrane prevents it from depolarizing again and inhibits the backflow of the action potential. This short duration at which the membrane cannot be depolarized irrespective of the signal’s strength is denoted as the refractory period. It facilitates the unidirectional flow of action potential.

Thus, the refractory period prevents the membrane from being depolarized again for a short while and facilitates forward propagation of the action potential.

Therefore, the given statement is true.

Explanation of option ‘(C)’

The axon hillock has the highest value of membrane potential when an incoming signal depolarizes it. After this, the membrane gets hyperpolarized and then returns to the value of resting membrane potential, which has the same value for the axon hillock and the axon terminal.

Thus, the membrane of the axon hillock is not always higher than the axon terminal.

Therefore, the given statement is false.

Explanation of option ‘(D)’

The voltage-gated ion channels have a set direction for the flow of ions; some allow inward flow, while others allow outward flow. This directional flow is activated or inactivated through differences in membrane potential.

So the unidirectional flow of action potential is regulated by the ion channels’ activation or deactivation and not the direction in which these ion channels allow ion movement.

Thus, the activation and deactivation of ion channels regulate action potential propagation.

Therefore, the given statement is false.