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.

Verified step by step guidance

1. Step 1: Understand the question. The question is asking why action potentials, which are electrical signals in neurons, are usually conducted in one direction along the axon of a neuron.
2. Step 2: Review the options. Option a suggests that ions can only flow in one direction along the axon. This is not correct because ions can flow in both directions across the cell membrane, but the direction of the net flow of ions (which determines the direction of the action potential) is determined by the distribution of ion channels and the electrochemical gradient.
3. Step 3: Consider option b. This option suggests that the refractory period, which is a brief period after an action potential during which the neuron cannot fire another action potential, prevents the reopening of voltage-gated Na+ channels. This is correct. After an action potential, there is a refractory period during which the Na+ channels are inactivated. This prevents the action potential from traveling backwards.
4. Step 4: Evaluate the remaining options. Option c suggests that the axon hillock has a higher membrane potential than the terminals of the axon. This is not correct because the membrane potential is the same throughout the neuron. Option d suggests that voltage-gated channels for both Na+ and K+ open in only one direction. This is not correct because these channels can open in both directions, but the direction of ion flow (and thus the direction of the action potential) is determined by the electrochemical gradient.
5. Step 5: Choose the correct answer. Based on the above analysis, the correct answer is option b: The brief refractory period prevents reopening of voltage-gated Na+ channels.

Key Concepts

Action Potentials

Action potentials are rapid, temporary changes in the membrane potential of a neuron, allowing for the transmission of electrical signals along the axon. They are initiated when a neuron's membrane depolarizes past a certain threshold, leading to the opening of voltage-gated sodium channels, which allows Na+ ions to flow into the cell, further depolarizing the membrane.

Refractory Period

The refractory period is a brief phase following an action potential during which a neuron cannot fire another action potential. This occurs due to the inactivation of sodium channels and the opening of potassium channels, which repolarizes the membrane. The absolute refractory period ensures that action potentials only travel in one direction along the axon, as the previous segment is temporarily unresponsive.

Axon Hillock

The axon hillock is the region of a neuron where the axon begins and is critical for the initiation of action potentials. It has a higher density of voltage-gated sodium channels compared to other parts of the neuron, making it more likely to reach the threshold for firing an action potential. This anatomical feature contributes to the unidirectional propagation of signals along the axon.