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Chemical Senses I - Guistation

The Basic Tastes

  • There are 5 basic tastes:

    • Saltiness

    • Sourness

    • Sweetness

    • Bitterness

    • Umami

      • Savory taste of glutamate, monosodium glutamate

  • Are there taste receptors beyond the basic 5?

    • Yes, likely

    • Some recent research thinks we can taste & have specific receptors for:

      • Fat

      • Carbonation (likely from sour taste cells)

      • Calcium

      • Water

  • Chemistry of substances can vary while taste is the same

    • Sugars are less sweet than artificial sweeteners (1:10,000)

    • Bitter substances can be simple ions or complex molecules

  • How do we taste so much with only 5 basic types of taste?

    • Each food activates diff combination of basic taste - uniqueness

    • Distinct flavor due to combined taste and smell

      • e.g. w/o combined taste & smell onion can be mistaken for apple

    • Texture and temperature also play important role

      • Pain sensations - spicy food

The organs of Taste

  • Tongue

    • Tip is most sensitive to sweetness

    • Back is most sensitive to bitterness

    • Sides are most sensitive to saltiness and sourness

    • Papillae: small bumps on tongue shaped like ridges

      • Each one has one to hundreds of tastebuds (only visible with a microscope)

      • Each tastebud has 50-150 taste receptor cells 

      • Each papilla has multiple types of taste receptors specialized for different categories of taste

  • Palate

  • Pharynx

    • Odors from food pass by here into nasal cavity > detected by olfactory receptors

  • Epiglottis

  • Threshold concentration: the minimum level of a substance required for it to be perceived by taste receptors - too low concentration = no taste

    • At concentrations just above threshold papillae tend to be sensitive to only one basic taste

    • At increased concentrations, papillae will become less selective

      • Question: what happens when sick because you cannot taste, is it because sense of smell is gone or because of receptors not sensing taste?

Taste Receptor Cells

  • Apical end: chem sensitive part of a taste receptor cell, small membrane region

    • Microvilli: thin extensions that project into the taste pore

    • Taste pore: small opening on the surface of the tongue where the taste cell is exposed to contents of the mouth

  • Taste receptor cells are not neurons but do form synapses with endings of gustatory afferent axons near bottom of taste bud

    • Also make electrical & chem synapses onto some of the basal cells

      • Some basal cells synapse onto sensory axons & form simple information-processing circuit within each taste buds

  • Lifespan of one taste cell is about 2 weeks

    • Nerve cut = degeneration

  • Chem activates receptor cell → membrane depolarizes (voltage shift = receptor potential) → action potential 

  • In any case of depolarization Ca2+ channels will open → Ca2+ enter cytoplasm → NTMs release → post synapse fires AP which communicate taste signal into brain

    • Basic transmission from taste cell → sensory axon

  • Sour & salty → serotonin onto gustatory axons

  • Sweet, bitter, & umami release ATP as NTM

  • Evidence from recent studies suggests most taste receptor cells respond primarily to one of 5 basic tastes

    • e.g. cells 1 & 3 in figure above

    • Each gustatory axon in the figure is influenced by several tastes but each has bias 

Mechanisms of Taste Transduction

  • Transduction: process by which an environmental stimulus causes an electrical response in a sensory receptor cell

  • Nature of transduction mechanism determines specific sensitivity of a sensory system

    • e.g. because eyes have photoreceptors we can see, if mouth had photoreceptors then we might see with our mouth

    • Some sensory systems have single basic type of receptor cell that uses one transduction mechanism (e.g. auditory system)

    • Taste uses several diff processes & each basic taste uses one or more of mechanisms

  • Taste stimuli (tastants) may

    • Salt and Sour: Directly pass through ion channels

    • Sour: Bind to & block ion channels

    • Bitter, Sweet, Umami: Bind to G-protien coupled receptors in the membrane → activate second system messenger systems → opening of ion channels

Saltiness

  • In relatively low concentrations (10-150 mM) tastes good, but higher concentrations are not good tasting

    • To detect low concentrations taste cells use Na+ selective channel

      • Common in other epithelial cells

      • Blocked by drug amiloride

      • Insensitive to voltage & generally stays open (unlike sodium gated ion channel)

      • Taste chicken soup → Na+ rises outside the receptor cell → gradient for Na+ across the membrane is made steeper → Na+ diffuses down concentration gradient → inward current causes membrane to depolarize (receptor potential) → voltage gate sodium & calcium channels open near the synaptic vesicles → release of NTM onto gustatory afferent axon

1) salt 2) sour

Sourness

  • Foods taste sour bc of high acidity

  • Acids, such as HCl, dissolve in water & generate hydrogen ions → protons causative agents of acidity & sourness

  • Protons can affect the taste receptors in diff ways from either inside or out

    • Likely H+ can bind & block special K+ selective channels

      • When K+ permeability is decreased membrane depolarizes

    • H+ may also activate or permeate special type of ion channel from superfamily of transient receptor potential (TRP) channels

      • Cation current through TRP channels can depolarize sour receptor cells

Bitterness

  • Transduction process relies on two families of taste receptor proteins, T1R & T2R

  • Subtypes of T1R & T2R are G-protein coupled receptors that are similar to the ones that detect NTMs

    • Evidence protein receptors for bitter, sweet & umami are dimers

      • Dimers: two proteins affixed to one another

  • Bitter substances are detected by the 25 types of T2 receptors in humans

  • Bitter receptors are poison detectors & because we have so many can detect various poisons

  • Since each taste cell can send only one type of signal to afferent nervous, a chem that can bind to one of 25 bitter receptors will trigger the same response as a diff chem that binds to another of its bitter receptors

    • The only important message brain receives is that a bitter chem is bad and shouldn’t be truster

    • Nervous system does not distinguish between one type of bitter substance from another

  • Receptors use second messenger pathway to carry signal to gustatory afferent axon

    • Sweet & umami use the same second messenger pathway

  • The pathway

    • Tastant binds to a bitter (or sweet/umami) receptor activating the G-protien

    • Enzyme phospolipase C is stimulated increase production of intracellular messenger IP3

    • IP3 activates special type of ion channel unique to taste cells causing it to open

    • Na+ enters and depolarizes the taste cell

    • IP3 also triggers release of Ca2+

    • Ca+ triggers release of NTMs in unusual way

      • Lacks transmitter-filled presynaptic vesicles

    • Special membrane channel allows ATP to flow out of the cell where ATP acts as transmitter

    • ATP activates purinergic receptors on postsynaptic gustatory axons

Sweetness

  • Many diff sweet tastants, some natural, some artificial

    • All detected from the same taste receptor

  • Sweet receptors resemble bitter receptors bc also dimers

    • Requires two members of T1R receptor family: T1R2 & T1R3

    • If either are missing or mutate animal may not perceive sweetness

  • Chems binding to T1R2 + T1R3 receptor activate same 2nd messenger system that bitter receptors activate

    • Why don’t we confuse bitter with sweet?

    • Reason is bitter receptor proteins & sweet receptor proteins are expressed in diff taste cells & in turn connect to different gustatory axons

      • Activity of diff gustatory axons reflects chem sensitivities of taste cells that drive them so messages about sweet & bitter are delivered to CNS along diff transmission lines

Umami

  • Transduction process is identical to sweetness w/one exception - umami receptor is composed of T1R1 & T1R3

    • Sweet & umami share T1R3 but T1R1 determines the umami taste

    • When T1R1 is removed from mice they cannot taste glutamate & other amino acids but can still sense sweet things

    • Bats do not have functioning T1R1 receptor & cannot taste amino acids (cannot taste sweet & umami)

  • Has the same messenger pathway as sweet & bitter

    • What don’t we confuse taste of amino acids w/sweet or bitter?

    • Taste cells selectively express only one class of taste receptor protein

      • Umami specific taste cells stimulate gustatory axons that deliver messages of umami

Central Taste Pathways

  • Main flow of taste info

    • Taste buds → gustatory axons → brain stem → thalamus → cerebral cortex

  • Three cranial nerves carry primary gustatory axons & bring taste info to the brain

    • Anterior two thirds of the tongue & palate → facial nerve

    • Posterior third of tongue → glossopharyngeal nerve

    • Regions around throat (glottis, epiglottis, pharynx) → vagus nerve

  • Cranial nerve taste axons enter brain stem, bundle together, and synapse within gustatory nucleus as part of solitary nucleus in medulla

    • From gustatory nucleus, taste pathways diverge

    • Neurons of the gustatory nucleus synapse on subset of small neurons in the VPM nucleus (deals w/sensory info from head) → VPM sends axons to the primary gustatory cortex

  • Taste pathways to thalamus & cortex are primarily ipsilateral to cranial nerves that supply them

  • Experience of taste is presumably mediated by cerebral cortex

  • Gustation is important to basic behaviors of feeing & digestion

    • Involved with swallowing, salvation, gagging, vomiting, digestion, & respiration

The Neural Coding of Taste

  • Basic idea of a coding system for taste

    • One way to imagine taste coding: each unique taste (sweet, salty, chocolate, mango, beef, etc.) could have its own special receptor in the tongue.

    • Each receptor would connect to its own set of axons → neurons in the brain → up to the cortex.

    • In this “labeled line” idea, chocolate activates only “chocolate” neurons, sweet activates only “sweet” neurons, etc.

    • Example: chocolate ice cream would fire “chocolate” and “sweet” cells strongly, but not “salty” or “banana” cells.

  • Labeled Line Hypothesis

    • This is the idea that each taste has a dedicated line from the tongue to the brain.

    • It’s simple and rational, but not exactly how the brain really works.

  • How taste receptor cells actually behave

    • Some taste receptor cells are specific: they mostly respond to one category (sweet, bitter, or umami).

    • Others are broadly tuned: they respond to more than one category (e.g., salt and sour).

    • This makes the information coming from a single taste receptor ambiguous (uncertain).

  • Primary taste axons (the “wires” to the brain)

    • Even less specific than receptor cells.

    • They often collect signals from multiple receptor cells at once.

    • Example: one axon might get input from a “sour-preferring” cell and a “salt-preferring” cell → making the axon respond to both.

  • Why cells are broadly tuned

    • A single receptor cell can have two different detection mechanisms, so it responds to more than one taste.

    • Multiple receptor cells can send signals into the same axon → combining their information.

    • In the brain, this continues: neurons in the gustatory nucleus receive input from many axons → becoming even less selective.

  • Problem with labeled lines

    • If we used only highly specific cells, we’d need thousands of receptor types (chocolate, strawberry, mango, beef, etc.).

    • That would still fail for new or unusual tastes we haven’t evolved receptors for.

  • Population Coding (the actual strategy used)

    • The brain interprets taste by looking at the pattern of activity across many neurons.

    • Each food excites a unique combination of neurons:

      • Some neurons fire strongly, some weakly, some not at all, some may even be inhibited.

    • The overall pattern tells the brain “this is chocolate ice cream” vs. “this is chocolate cake.”

    • This method allows for huge variety and flexibility in taste recognition.

  • Other sensory contributions

    • Taste perception isn’t just about taste receptor cells.

    • Smell (olfaction), temperature, and texture also contribute.

    • Example: chocolate ice cream vs. chocolate cake → both taste like chocolate, but coldness and creaminess vs. warmth and fluffiness help the brain tell them apart.

  • Big picture

    • Taste coding is not just one-to-one “wires” for each flavor.

    • It’s a population code: the brain compares patterns across many neurons to identify and distinguish foods.

    • This scheme is similar to how the brain processes other senses (like smell, vision, and movement).