Structure that recognises a stimulus in the internal or external environment of an organism. In response to stimuli, it creates graded potentials or action potentials (a nerve signal) in the nerve cell to which it belongs, or in an adjacent one.
In a sensory system, a sensory receptor is a structure that recognizes a stimulus in the internal or external environment of an organism. In response to stimuli the sensory receptor initiates sensory transduction by creating graded potentials or action potentials in the same cell or in an adjacent one.