Which is a characteristic of an action potential? They are graded. They are decremental. They propagate in one direction. The are reversible.

The characteristic of an action potential is that they propagate in one direction.

Here's a step-by-step explanation:

1. An action potential is a brief electrical charge that travels down the axon of a neuron. It is the way how neurons communicate with each other.

2. The action potential is initiated at the axon hillock, a specialized part of the cell body (or soma) of a neuron that connects to the axon.

3. Once initiated, the action potential travels down the axon, away from the cell body towards the axon terminals. This is a one-way journey, meaning it propagates in one direction.

4. The action potential cannot go back towards the cell body because the part of the neuron it has just passed through is in a refractory period, a short period of time immediately after an action potential during which the neuron is unable to generate another action potential.

5. Therefore, the statement "They propagate in one direction" is a characteristic of an action potential. The other options are incorrect: action potentials are not graded or decremental, and they are not reversible.