ABSTRACT
Adaptation resembles a learning process that is indispensable for "intelligent" signal processing. Sensory cells translate environmental signals into the universal language of the central nervous system: action potentials, the frequency of which is interpreted by the brain as a sensation. To this end they express specific receptors for light, temperature, touch, sound waves, gravity, smell, and taste as well as, in certain animals, for electricity and magnetism. The receptors for sweet, umami, and bitter tastes interact with G-proteins rather than with ion channels; they belong to the G-protein-coupled receptor superfamily of heptahelical transmembrane proteins. Modern methods for receptor analysis combine bioinformatics and protein expression strategies. By such an approach, for instance, the putative role of polycystins in sour taste processing has been elucidated. Particularly sensitive for mechanical stimuli of blood flow and blood pressure are vasoendothelial cells, which respond by production of blood pressure-regulating factors such as prostacyclin, nitric oxide, endothelin, ATP, growth factors, and other signaling substances.
