Bitterness receptor locations

These basic taste sensations arise at relatively specialised receptors located in different places of the mouth. Sweet substances are perceived primarily at the tip of the tongue, salty substances in defined areas of the upper surface of the tongue, sour substances to the sides and bitter substances in the root of the tongue and soft palate. Taste receptors also respond to other stimuli (tastes) that are incorporated virtually in the whole oral cavity ... 

In taste studies, sucrose is usually taken as a reference standard in the sensory evaluation of sweetness and caffeine is generally used as the reference material for bitterness. However, sour and salty tastants modulate taste-receptor function by direct effect on specific ion channels in the membrane, while sweet and bitter tasting compounds seem to bind to closely located receptors which are coupled to a guanidine-nucleotide binding protein (G-protein). The perception of their tastes proceeds through a transduction mechanism involving G-protein and a second messenger system (Kinnamon, 1988). 

The human tongue is studded with small conical bumps, or papillae, which house the taste buds. Each taste bud consists of receptor cells, and extending from each receptor cell are taste hairs with receptor sites for flavor molecules. The receptor cells can distinguish only four general flavors sweet, salty, bitter, and sour. The areas of response to these tastes are located on specific parts of the tongue. 

Since sweet natural products often co-occur in complex mixtures with bitter- and/or neutral-tasting analogs, it is germane to mention briefly some of the presently held views on the mechanism of sweet taste reception. Although sweet taste chemoreception is thought to be mediated by proteinaceous receptor sites located on the microvillus membrane of taste cells of the... 

A qualitative theory on the relationship between bitterness and chemical structures of bitter Rabdosia diterpenoids has been proposed 143). To be bitter a substance must have at least one bitter unit it consists of a hard acid and a hard base which are located within 1.5 A of each other so that intramolecular hydrogen-bonding is possible. Cleavage of this hydrogen bond and concomitant formation of a new hydrogen bond to the receptor site are responsible for bitterness 150). For instance, isodonal (71) which possesses an a-orientated 11-OH is very bitter, while trichodonin (70), its 11 P-epimer, is not. In bitter isodonal, the distance between the 11-hydroxy proton, the donor proton, and the 6-aldehydic oxygen, the proton acceptor, is ca. 1 A, while in tasteless trichodonin it is ca. 3 A. 

Over 100 years ago, it was determined that taste cells in the taste buds of different papilla located across the tongue respond to more than one type of stimulus [11]. Although each neuron may respond more strongly to one tastant, it wiU also respond to unlike taste properties. Also, it is thought that no single taste cell contains receptors for both bitter and sweet. Each taste receptor cell is connected through a network of cellular activities to a sensory neuron that travels to the brain. A single sensory neuron may be connected to several taste cells each within different taste buds [7]. 

The sense of taste, also known as gustation, consists of taste receptors on the tongue, which then translate chemical reactions into electrical impulses that are transmitted by a series of nerves to the brain stem. The brain stem is located in the rear of the brain, and connects the brain to the spinal cord. The brain stem also regulates the central nervous system. The main taste sensations are sweet, sour, salty, and bitter, along with a newer sensation described as umami (Japanese for savory). These taste sensations, combined with olfactory input and touch sensations in our mouth, give us the sensory input that we describe as flavor [18]. 

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