In neurophysiology, which of the following statements about graded potentials in neurons is not true?

Introduction / Context:
Graded potentials are local changes in membrane potential that occur in neurons and other excitable cells. They are important for initiating and modulating action potentials. Understanding their properties helps explain how information is integrated in the nervous system. This question asks which statement about graded potentials is not true.

Given Data / Assumptions:

• Graded potentials occur in dendrites, cell bodies, and receptor endings.
• They are different from action potentials in size, duration, and conduction.
• They can be excitatory or inhibitory and can summate.
• We assume standard descriptions from human physiology.

Concept / Approach:
A key property of graded potentials is that they decrease in amplitude as they spread from the site of origin due to passive spread and leak of current. They are local and short lived. They can occur on receptor endings and postsynaptic membranes and can be called receptor or postsynaptic potentials. They can summate spatially and temporally to influence whether the neuron reaches threshold for an action potential. Therefore, any statement saying that graded potentials increase in amplitude as they move away from the stimulus is incorrect.

Step-by-Step Solution:
Step 1: Recall that graded potentials are local changes whose magnitude varies with stimulus strength. Step 2: Understand that as graded potentials spread over the membrane, their amplitude decays with distance because the membrane is leaky and there is no regeneration like in action potentials. Step 3: Examine option A, which notes that graded potentials can form on receptor endings or dendrites; this is true. Step 4: Examine option B, which says they increase in amplitude as they move away from the stimulus point. This contradicts the known property that they decrease, so it is not true. Step 5: Option C correctly states that graded potentials are short lived local changes in membrane potential. Step 6: Option D correctly notes that graded potentials on postsynaptic membranes are called postsynaptic potentials. Step 7: Option E correctly states that graded potentials can summate in time and space.

Verification / Alternative check:
Textbook figures of graded potentials show a localized depolarization that gradually diminishes in amplitude with distance from the site of stimulation. This pattern is often drawn as a decreasing height curve. Only action potentials maintain or regenerate their amplitude as they propagate. Laboratory recordings using microelectrodes also confirm that graded potentials decay with distance.

Why Other Options Are Wrong:
Options A, C, D, and E are true statements. Graded potentials indeed occur on receptor endings and dendrites, are short lived and local, can be called postsynaptic potentials when they occur at synapses, and are capable of summation. Option B is the only statement that contradicts basic principles by stating that amplitude increases with distance, which never happens in normal graded potentials.

Common Pitfalls:
Students may confuse graded potentials with action potentials, which are regenerative and maintain their amplitude along the axon. Another pitfall is overlooking the concept of decremental conduction, where the signal diminishes as it spreads. Remember that graded potentials are designed to integrate local inputs, while action potentials are designed for long distance transmission.

Final Answer:
The statement that is not true is They increase in amplitude as they move farther away from the stimulus point.