Sensory receptors

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.

Location
The sensory receptor may be a specialized portion of the plasma membrane, or a separate cell associated with a neuron ending.

Functions
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.

Classification 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:


 * Chemoreceptors respond to chemical stimuli
 * Mechanoreceptors respond to mechanical stress or mechanical strain
 * Thermoreceptors respond to temperature, either heat, cold or both
 * Photoreceptors respond to light
 * Baroreceptors respond to pressure
 * Osmoreceptors respond to the osmolarity of fluids (such as in the hypothalamus)
 * Proprioceptors provide the sense of position
 * Nociceptors respond to noxious or potentially noxious stimuli
 * Hydroreceptors respond to changes in humidity

Classification by location
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.

Classification by morphology
Somatic sensory receptors near the surface of the skin can usually be divided into to 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 comprise of the remaining types of cutaneous receptors. Encapsulation exists for specialized functioning.