Which cells detect a stimulus in a reflex arc?

In a reflex arc, the cells that detect a stimulus are known as receptors (or sensory nerve cells).12

These specialized cells act as biological transducers, converting energy from the external or internal environment into electrical impulses. Receptors may be individual nerve endings or massed together to form sense organs, such as the eye or ear. Once a receptor detects a stimulus and reaches a sufficient threshold of strength, it generates a nerve impulse that is transmitted via afferent nerve fibers to the central nervous system, initiating the reflex response.31