How Does My Brain Know About Sensations on My Tongue ?
 Harold Rodinsky
DePaul University
January, 2005

Based on the article "Making Sense of Taste" by David V. Smith and Robert F. Margolskee
Scientific American, March 2001
 

"How do cells on the tongue register the sensations of sweet, salty,

sour and bitter? Scientists are finding out—and discovering

how the brain interprets these signals as various tastes "
 

Flavor is a complex mixture of sensory input composed of taste (gustation), smell (olfaction) and the tactile sensation of food as it is being chewed. Although people may use the word “taste” to mean “flavor,” in the strict sense it (taste) is applicable only to the sensations arising from specialized taste cells in the mouth.
 

 Scientists generally describe human taste perception in terms of four qualities: salty, sour, sweet and bitter. Another category that may also be present is  umami, elicited by glutamate, an amino acids that make up the proteins in meat, fish and legumes. Glutamate also serves as a flavor enhancer in the form of the additive monosodium glutamate (MSG).
 

After we begin chewing the salivary glands begin producing enzymes that predigest the food we are eating.	The tongue is covered with specialized receptors that we refer to as taste buds	the receptors on the tongue transduce the chemical sensation into an electrical signal (action potential) which travels along neural pathways to the brain.

 Scientists generally describe human taste perception in terms of four qualities: salty, sour, sweet and bitter. Another category that may also be present is  umami, elicited by glutamate, an amino acids that make up the proteins in meat, fish and legumes.

General layout of the tongue	General sketch of a taste cell	The general layout of taste receptors on the tongue as derived from research in the early 19th century.

TASTE DETECTORS
       1)  Taste cells are components of  taste buds, which are located on the tongue and soft palate.
       2) Taste buds on the tongue are located within papillae, the tiny projections that give the tongue its velvety appearance.
       3) Fungiform ("mushroomlike") papillae on the front part of the tongue and around edge of the tongue
       4) Circumvallate ("wall-like") papillae  are located at the back of the tongue are roughly 12 larger taste bud-containing papillae called the
       5) Taste buds are  structures that have between 50 and 100 taste cells
       6) Microvilli is a fingerlike projections  that poke through the opening top of the taste bud called the taste pore.
       7) Chemicals from food termed tastants dissolve in saliva and contact the taste cells through the taste pore.
       8)  the food chemicals interact either with proteins on the surfaces of the taste cells known as taste receptors
       9) or with pore-like proteins called ion channels.
     10) These interactions cause electrical changes in the taste cells that trigger them to send chemical signals that ultimately result in impulses to the brain.
     11) Taste cells, like neurons, normally have a net negative charge internally and a net positive charge externally.
     12) Such depolarization causes the taste cells to release  neurotransmitters, which prompt neurons connected to the taste cells to relay electrical
           messages.

How Specialized are Taste Detectors?
       1) There is not always a one-to-one correlation between taste quality and chemical class, particularly for bitter and sweet tastants.
       2) Many carbohydrates are sweet, for instance, but some are not.
       3) Different types of chemicals can evoke the same sensation:  chloroform and the artificial sweeteners aspartame and saccharin taste sweet even
          though their chemical structures have nothing in common with sugar.
       4) The compounds that elicit salty or sour tastes are less diverse and are typically ions that directly through ion channels
       5) Those compounds responsible for sweet and bitter tastes bind to surface receptors that trigger a sequence of signals to the cells' interiors that
          ultimately results in the opening and closing of ion channels.
       6) In 1992 Margolskee  Susan K. McLaughlin and Peter J. McKinnon identified a key member of this sequence. They named the molecule "gustducin"
          because of its similarity to transducin, a protein in retinal cells that  transduce light hitting the retina into an electrical impulse that
          constitutes vision.
       7) Gustducin and transducin are both so-called G-proteins, which are found on the undersides of many different types of receptors
          these proteins are regulated by a chemical called guanosine triphosphate, GTP.
       9) When the right tastant molecule binds to a taste cell receptor, (like a key in a lock) it prompts the sub-units of gustducin to split apart and carry
          out biochemical reactions that  open and close ion channels and make the cell interior more positively charged. (remember it was negative!)
      10) The stimuli that the brain interprets as the basic tastes--salty, sour, sweet, bitter and, possibly, umami--are registered via a series of chemical
          reactions in the taste cells of the taste buds.
     11) The five biochemical pathways underlying each taste quality are depicted here in separate taste cells solely for clarity.
      
 Is each cell so specialized  it only "recognizes" one tastant?
      1) Scientists have gone back and forth on whether individual neurons are "tuned" to respond only to a single tastant such as salt or sugar--and
          therefore signal only one taste quality--or whether the activity in a given neuron contributes to the neural representation of more than one taste.
       2) Studies show that both peripheral and central gustatory neurons typically respond to more than one kind of stimulus. Although each neuron
          responds most strongly to one tastant, it usually also generates a response to one or more other stimuli with dissimilar taste qualities.
       3) How then can the brain represent various taste qualities if each neuron responds to many different-tasting stimuli?
       4) Many researchers believe it can do so only by generating unique patterns of activity across a large set of neurons.

Research
      1) The first electrophysiological studies of gustatory sensory neurons was done in the early 1940s by Carl Pfaffmann of Brown University
       2)  He demonstrated that peripheral neurons are not specifically responsive to stimuli representing a single taste quality but instead
          record a spectrum of tastes.
       3) Pfaffmann suggested that taste quality might be represented by the pattern of activity across gustatory neurons because
          the activity of any one cell was ambiguous.
       4)  In the 1970s and 1980s several scientists began to accumulate data indicating that individual neurons are turned maximally for one taste.
       5) They interpreted this as activity in a particular type of cell represented a given taste quality-- they called this
           labeled-line hypothesis. (vs. across neurons pattern)
       6) According to this  activity in neurons that respond best to sugar would signal "sweetness," activity in those that respond best to acids
          would signal "sourness".
       7)  Smith, Van Buskirk, Travers and Bieber, (1983) demonstrated that the same cells that others had interpreted as labeled lines
         actually defined the similarities and differences in the patterns of activity across taste neurons.
       8) This suggested that the same neurons were responsible for taste-quality representation, whether they were viewed
          as labeled lines or as critical parts of an across-neuron pattern.
       9) These investigators further demonstrated that the neural distinction among stimuli of different qualities depended on the
          simultaneous activation of different cell types, much as color vision depends on the comparison of activity across photoreceptor cells in the eye.
     10) These and other considerations have led us to favor the idea that the patterns of activity are key to coding taste information.
     11) Scientists now know that things that taste alike evoke similar patterns of activity across groups of taste neurons.
     12) What is more, they can  use multivariate statistical analysis to plot the similarities in the patterns elicited by various tastants.
     13) Taste researchers have generated such comparisons for gustatory stimuli from the neural responses of hamsters and rats.
     14) These correspond very closely to similar plots generated in behavioral experiments, from which scientists infer which stimuli
          taste alike and which taste different to animals.
     15) Such data show that the across-neuron patterns contain sufficient information for taste discrimination.
     16) Because taste neurons are so widely responsive, neurobiologists must compare the levels of activity of a range of neurons to get an idea
          of what sensation they are registering.
     17) No single neuron type alone is capable of discriminating among stimuli of different qualities,
          because a given cell can respond the same way to disparate stimuli, depending on their relative concentrations.
     18) In this sense, taste is like vision, in which three types of photoreceptors respond to light of a broad range of wavelengths
          to allow us to see the myriad hues of the rainbow.
     19) It is well known that the absence of one of these photoreceptor pigments disrupts color discrimination,
          and this disruption extends well beyond the wavelengths to which that receptor is most sensitive .
          That is, discrimination between red and green stimuli is disrupted when either the "red" or the "green" photopigment is absent.

THE "TASTE MAP"
      1) One of the most dubious and misleading "facts" about taste--and one that is commonly reproduced in textbooks
          is the oft-cited  "tongue map" showing large regional differences in sensitivity across the human tongue.
       2)  These maps indicate that sweetness is detected by taste buds on the tip of the tongue, sourness on the sides, bitterness
          at the back and saltiness along the edges.
      3) Taste researchers have known for many years that these tongue maps are wrong. The maps arose early in the 20th century
          as a result of a misinterpretation of research reported in the late 1800s, and they have been almost impossible to purge from the literature.
      4) In reality, all qualities of taste can be elicited from all the regions of the tongue that contain taste buds.

So what does it all mean it terms of flavor experience?
     1) Sensory information from taste cells is critical for helping us to detect and respond appropriately to needed nutrients.
       2) The sweet taste of sugars, for example, provides a strong impetus for the ingestion of carbohydrates.
       3) Taste signals also evoke physiological responses, such as the release of insulin, that aid in preparing the body to use the nutrients effectively.
       4) Humans and other animals with a sodium deficiency will seek out and ingest sources of sodium.
       5) Evidence also indicates that people and animals with dietary deficiencies will eat foods high in certain vitamins and minerals.
       6) Just as important as ingesting the appropriate nutrients is not ingesting harmful substances.
          The universal avoidance of intensely bitter molecules shows a strong link between taste and disgust.
       7) Toxic compounds, such as strychnine and other common plant alkaloids, often have a strong bitter taste.
           In fact, many plants have evolved such compounds as a protective mechanism against foraging animals.
       8) The sour taste of spoiled foods also contributes to their avoidance
       9) All animals, including humans, generally reject acids and bitter-tasting substances at all but the weakest concentrations.
     10) The intense reactions of pleasure and disgust evoked by sweet and bitter substances appear to be present at birth and
           to depend on neural connections within the lower brain stem.
     11) Animals with their forebrains surgically disconnected and anencephalic human newborns (those lacking a forebrain) show
          facial responses normally associated with pleasure and disgust when presented with sweet and bitter stimuli, respectively.
     12) The strong link between taste and pleasure—or perhaps displeasure—is the basis of the phenomenon of taste-aversion learning.
     13) Animals, including humans, will quickly learn to avoid a novel food if eating it causes, or is paired with, gastrointestinal
         distress.
     14) Naturally occurring or experimentally induced taste-aversion learning can follow a single pairing of tastant
        and illness, even if there is a gap of many hours between the two.
     15) One side effect of radiation treatments and chemotherapy in cancer patients is loss of appetite; much of this is caused
          by conditioned taste aversions resulting from the gastrointestinal discomfort produced by these treatments.
     16) This mechanism has also made it extremely difficult to devise an effective poison for the control of rats,
          which are especially good at making the association between novel tastants and their physiological consequences.