Action potentials are usually propagated in only one direction along an axon because :

Options

A. (a) nodes of Ranvier only conduct in one direction
B. (b) brief refractory period prevents a depolarization from occurring in the direction from which the impulse came
C. (c) axon hillock has a higher membrane potential than the tips of the telodendria
D. (d) both sodium and potassium voltage-sensitive gates open in one direction

Explanation

**Step-by-Step Solution:**

1. **Understanding Action Potentials**: Action potentials are electrical signals that propagate along the axon of a neuron. They involve a rapid change in membrane potential due to the movement of ions across the neuron's membrane.

2. **Resting Potential**: At rest, the inside of the neuron is negatively charged compared to the outside. This resting potential is maintained by the sodium-potassium pump, which actively transports sodium ions out of the cell and potassium ions into the cell.

3. **Depolarization Phase**: When a stimulus occurs, sodium channels open, allowing sodium ions to rush into the neuron. This influx of positive ions causes the inside of the neuron to become more positive (depolarization).

4. **Repolarization Phase**: After depolarization, potassium channels open, allowing potassium ions to flow out of the neuron. This outflow of positive ions causes the inside of the neuron to become negative again (repolarization).

5. **Refractory Period**: After an action potential, there is a brief refractory period during which the sodium channels cannot reopen. This means that no new action potential can be initiated in the region where the previous action potential just occurred.

6. **Direction of Propagation**: Because of the refractory period, the action potential can only propagate in one direction—away from the point of origin. The area behind the action potential is in a refractory state and cannot depolarize again immediately.

7. **Conclusion**: Thus, the reason action potentials are propagated in only one direction along an axon is due to the brief refractory period that prevents depolarization from occurring in the direction from which the impulse came.

**Final Answer**: The action potentials are usually propagated in only one direction along an axon because the brief refractory period prevents depolarization from occurring in the direction from which the impulse came.

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