The primary sensory neuron may also be the ______________.

• A. synapse
• B. interneuron
• C. motor neuron
• D. receptor

Answer: (D)

The main idea is that the peripheral ending of a primary sensory neuron can function as the sensory receptor itself. In many somatic pathways, the first-order neuron is a pseudounipolar cell whose peripheral process ends in a receptor field and is responsible for detecting a stimulus and converting it into a receptor potential that triggers action potentials. Because the neuron’s peripheral terminal does the detecting and transducing, the neuron itself can be considered the receptor.

A synapse is just where neurons communicate, an interneuron is a neuron inside the CNS between sensory and motor pathways, and a motor neuron carries commands to muscles. These roles don’t describe the initial detection of a stimulus, which is why they aren’t correct here.