Questions: Which of the following best defines graded potential? - Large, all-or-none changes in membrane potential. - Small, localized changes in membrane potential that vary in amplitude. - A rapid and brief change in membrane potential that travels along the axon. - Depolarization of the membrane potential caused by the opening of voltage-gated channels. - Sustained changes in membrane potential that occur in response to neurotransmitters.

The answer is the second one: Small, localized changes in membrane potential that vary in amplitude.

Explanation for each option:

1. Large, all-or-none changes in membrane potential: This describes an action potential, not a graded potential. Action potentials are characterized by their all-or-none nature, meaning they either occur fully or not at all.

2. Small, localized changes in membrane potential that vary in amplitude: This is the correct definition of a graded potential. Graded potentials are changes in membrane potential that are confined to a small region of the plasma membrane and can vary in size depending on the strength of the stimulus.

3. A rapid and brief change in membrane potential that travels along the axon: This describes an action potential. Action potentials are rapid, brief, and propagate along the axon.

4. Depolarization of the membrane potential caused by the opening of voltage-gated channels: While this can occur during a graded potential, it is more characteristic of the initiation of an action potential, where voltage-gated sodium channels open.

5. Sustained changes in membrane potential that occur in response to neurotransmitters: This is not an accurate description of graded potentials. Graded potentials are typically transient and not sustained over long periods.

In summary, graded potentials are small, localized changes in membrane potential that vary in amplitude, making the second option the best definition.