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.
The sensory receptor may be a specialized portion of the plasma membrane, a whole cell associated with a neuron ending, or a group of such cells.
The sensory receptors involved in taste and smell contain receptors that bind to specific chemicals. Odor receptors in olfactory receptor neurons, for example, are activated by interacting with molecular structures on the odor molecule. Similarly, taste receptors (gustatory receptors) in taste buds interact with chemicals in food to produce an action potential.
Other receptors such as mechanoreceptors and photoreceptors respond to physical stimuli. For example, photoreceptor cells contain specialized proteins such as rhodopsin to transduce the physical energy in light into electrical signals. Some types of mechanoreceptors fire action potentials when their membranes are physically stretched.
The sensory receptor functions as the first component in a sensory system.
Sensory receptors respond to specific stimulus modalities. The stimulus modality to which a sensory receptor responds is determined by the sensory receptor's adequate stimulus.
The sensory receptor responds to its stimulus modality by initiating sensory transduction. This may be accomplished by a net shift in the initial states of a receptor(see a picture of these putative states  with the biophysical description - link ).
by adequate stimulus
A sensory receptor's adequate stimulus is the stimulus modality for which it possesses the adequate sensory transduction apparatus. Adequate stimulus can be used to classify sensory receptors:
- Ampullae of Lorenzini respond to electromagnetic fields and temperature
- Baroreceptors respond to pressure
- Chemoreceptors respond to chemical stimuli
- Hydroreceptors respond to changes in humidity
- Mechanoreceptors respond to mechanical stress or mechanical strain
- Nociceptors respond to damage to body tissues leading to pain perception
- Osmoreceptors respond to the osmolarity of fluids (such as in the hypothalamus)
- Photoreceptors respond to light
- Proprioceptors provide the sense of position
- Thermoreceptors respond to temperature, either heat, cold or both
Sensory receptors can be classified by location:
- Cutaneous receptors are sensory receptors found in the dermis or epidermis.
- Muscle spindles contain mechanoreceptors that detect stretch in muscles.
Somatic sensory receptors near the surface of the skin can usually be divided into two groups based on morphology:
- Free nerve endings characterize the nociceptors and thermoreceptors and are called thus because the terminal branches of the neuron are unmyelinated and spread throughout the dermis and epidermis.
- Encapsulated receptors consist of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning.
Different sensory receptors are innervated by different types of nerve fibers. Muscles and associated sensory receptors are innvervated by type I and II sensory fibers, while cutaneous receptors are innervated by Aβ, Aδ and C fibers.