Sensory Receptors
Sensory receptors are specialized to transduce a particular type of stimulus energy into electrical signals. Sensory receptors are classified as chemoreceptors, photoreceptors, or mechanoreceptors depending on the class of stimulus energy that excites them. They transform that energy into an electrical signal that is transmitted along pathways that serve one sensory modality. The insets in each panel illustrate the location of the ion channels that are activated by stimuli.
The olfactory hair cell responds to chemical molecules in the air. The olfactory cilia on the mucosal surface bind specific odorant molecules and depolarize the sensory nerve through a second-messenger system. The firing rate signals the concentration of odorant in the inspired air.
Rod and cone cells in the retina respond to light. The outer segment of both receptors contains the photopigment rhodopsin, which changes configuration when it absorbs light of particular wavelengths. Stimulation of the chromophore by light reduces the concentration of cyclic guanosine 3′, 5′-monophosphate (cGMP) in the cytoplasm, closing cation channels, and thereby hyperpolarizing the photoreceptor.
Adapted, with permission, from Andres and von Düring 1973.