Taste and Smell

Taste and SmellTravis Solomon, MD, PhDsolomont@umkc.eduGuyton & Hall, chap. 53, pp 645-652

ObjectivesKnow the steps in detection of odors by the olfactory system and the central pathways involved.Describe the cellular mechanisms that transduce environmental odorants into action potentials in olfactory bipolar cells.Understand the impact of odors on taste and on general behavior (affective responses).Know the organization of the taste system: taste buds, primary afferent neurons, central pathways, and associated affective responses to specific tastes.Describe the classes and mechanisms of signal transduction of taste receptors.Be familiar with the clinical consequences of smell and taste and their disorders.ChemosensationAll chemicals necessary for life enter the body by the nose and mouth.The senses of smell (olfaction) and taste (gustation) monitor those chemicals: determine the flavor, palatability, and toxicity of foods and beverageswarn of dangerous environmental conditions (fire, air pollution, leaking natural gas, and bacteria-laden foodstuffs).Whats That Smell?Odorants enter the nose, dissolve in mucus.6 million bipolar cells (neurons) extend cilia into mucus layer.Each bipolar cell expresses only one type of receptor.About 350 different receptors for odorants.Each receptor is specifically activated by one or more odorants.

OlfactionHumans can distinguish 400,000 different substances by smell.80% 0f these odorants are unpleasant.Olfactory receptors are the largest family of genes known (~350).All olfactory receptors use the same transduction mechanism.The Nobel Prize in Physiology or Medicine 2004 was awarded jointly to Richard Axel and Linda B. Buck "for their discoveries of odorant receptors and the organization of the olfactory system."

Neural Pathways: Nose to Brain

Mucus secretionCNS Pathways for Olfaction

Very sensitive but narrow range.Affective nature:Pleasant vs unpleasantStrong aversionPheromones and sexual drive.Rapid adaptation.Old, less-old, new association areasOdorants Bind to GPCR, Act Through cAMP, Open Ion Channels

Open Ion Channels Depolarize Olfactory Neurons, Initiate Signal to Brain

TasteLess discriminating than smell.Elicits several responses:Pleasurable taste sensationsSalivary secretionGagging, vomitingMultiple contributing factors:Taste receptorsOdor receptors (80% of taste)Mechanoreceptors, thermoreceptors, nociceptorsTaste Buds Contain ModifiedEpithelial Cells

10,000 taste buds,50 150 taste cells,2000-5000 receptorsReceptor cell mayexpress multiple taste receptorsPrimary Taste Qualities and Probable ReceptorsSaltySweetSour (acid)BitterUmami2 sodium receptors2 potassium receptorsChloride receptorAdenosine receptorInosine receptor2 sweet receptors2 bitter receptorsGlutamate receptorHydrogen ion receptorTaste qualities, taste receptors, and examples of natural stimuli.

Chaudhari N , Roper S D J Cell Biol 2010;190:285-296Taste qualities, the taste receptors that detect them, and examples of natural stimuli. Five recognized taste qualitiessweet, sour, bitter, salty, and umamiare detected by taste buds. Bitter taste is thought to protect against ingesting poisons, many of which taste bitter. Sweet taste signals sugars and carbohydrates. Umami taste is elicited by l-amino acids and nucleotides. Salty taste is generated mainly by Na+ and sour taste potently by organic acids. Evidence is mounting that fat may also be detected by taste buds via dedicated receptors. The names of taste receptors and cartoons depicting their transmembrane topology are shown outside the perimeter. Bitter is transduced by G proteincoupled receptors similar to Class I GPCRs (with short extracellular N termini). In contrast, sweet and umami are detected by dimers of Class III GPCRs (with long N termini that form a globular extracellular ligand-binding domain). One of the receptors for Na+ salts is a cation channel composed of three subunits, each with two transmembrane domains. Membrane receptors for sour and fat are as yet uncertain.Relative Taste SensitivityBitter > Sour > Salt = Sweet8 M, 900 M, 10,000 MComplexity of TasteHumans can distinguish taste of up to 10,000 chemicals.Most acids are sour; most salts are salty.Bitter: K+, Mg++, quinine, many alkaloids and drugs.Sweet: sucrose, fructose, monellin (protein), aspartame (aspartyl-phenylalanine methyl ester).

Taste and Smell Interact

Other Oral ReceptorsContribute to TasteTextureTemperaturePainFatPain vs Pleasure

Taste Receptor MechanismsSalty: epithelial Na+ channel (ENAC).Sour: ion channel (TRP).Sweet, bitter, umami: GPCRs.

ALL RESULT IN MEMBRANE DEPOLARIZATION (GRADED/RECEPTOR POTENTIAL)Cellular Transduction ofTaste Receptor Activation

UnknownneurotransmitterConfersspecificityTaste Receptors: SaltMediated by Na+ channels (ENaC) on apical surface of taste bud receptor cells.Receptor cells depolarized.First order taste neurons activated.

Taste Receptors: SourSour (acid) substances act through PKD2L1 receptors.Member of transient receptor protein family.Receptor activation depolarizes taste receptor cell.First order taste neurons activated.

Taste Receptors: Sweet, UmamiFamily of 3 GPCRs: T1R1, T1R2, T1R3.Binding of tastant activates G protein (gustducin), phospholipase C, , TRPM5.TRPM5 is relatively nonselective cation channel, depolarizes cell.First order neurons activated.

Taste Receptors: BitterSeparate family of GPCRs: T2R.About 30 different receptors.Receptors do not distinguish among bitter substances.Receptor activation depolarizes taste receptor cell.First order taste neurons activated.

Taste: Central Pathways

Taste signals can activate salivary secretion, nausea.

Rapid adaptation.Taste preferences.Taste aversion.Taste DisordersGenerally not associated with aging.Often due to olfactory, salivary, or neurologic dysfunction: Alzheimers, diabetes, anticholinergics, antipsychotics, antihypertensives.Oral products and medications can alter taste (& smell): anti-plaque mouthwash, toothpaste, hydrocortisone, lidocaine, tetracycline, captopril, penicillamine.

Smell DisordersFrequently associated with aging.

Upper Respiratory Tract Infections.

Head Trauma.

Iatrogenic: Inhaled corticosteroids, radiation, chemotherapy, streptomycin, codeine.

SignificanceAmong the more important developments in neurology has been the discovery that decreased smell function is perhaps the first sign of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD), signifying their "presymp-tomatic" phase.Diag-nose-is: Things You May SmellHalitosis. Volatile sulfur compounds (VSCs produced by gram-negative anaerobic bacteria) Decreased saliva, inflammationSystemic diseases can alter breath.TB, pneumonia, lung cancer, tonsillitisHiatal hernias, gastroesophageal reflux, gastric ulcerAchalasiaDiabetic ketoacidosisHepatic and renal failure