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.