Which statement best describes receptor potentials and generator potentials and their relation to action potential generation?

• A. Receptor potentials occur in sensory receptors; generator potentials occur in primary afferent neurons; both are all-or-none.
• B. Receptor potentials occur in sensory receptors; generator potentials occur in primary afferent neurons; both are graded depolarizations that may trigger action potentials if threshold is reached.
• C. Both potentials are all-or-none responses that do not vary with stimulus.
• D. Generator potentials occur in postsynaptic neurons after a chemical synapse.

Answer: (B)

The main idea is that sensory signals start as graded depolarizations rather than spikes. Receptor potentials occur in the sensory receptor cells themselves as they transduce a stimulus, and they are proportional to the stimulus strength. These receptor cells typically release neurotransmitter onto the attached primary afferent neuron, producing a generator potential in the peripheral end of that neuron. The generator potential, like the receptor potential, is a graded depolarization, and if it reaches the threshold of the first-order neuron, it triggers an action potential that travels toward the CNS. This describes why the best statement is that receptor potentials occur in sensory receptors; generator potentials occur in primary afferent neurons; both are graded depolarizations that may trigger action potentials if threshold is reached. The idea that these potentials are all-or-none is incorrect, since both are graded by stimulus strength, and generator potentials do not occur in postsynaptic neurons after a chemical synapse.