cap III_curs 7_8_ Chemical senses_2014_2015_.pdf

I. General principles of sensory physiologyII. The somatosensory System

III. Chemical SensesIV. VisionV. Hearing and Equilibrium

Lect. univ. dr. Loredana - Cristina MEREULaboratory of Biophysics & Med. Physics, Faculty of Physics,

'Alexandru Ioan Cuza' University of Iasi

Three sensory systems associated with the noseand mouth olfaction, taste, and the trigeminal orgeneral chemosensory system are dedicated tothe detection of chemicals in the environment.

All three of these chemosensory systems rely onreceptors in the nasal cavity, mouth, or on the facethat interact with the relevant molecules andgenerate receptor and action potentials, thustransmitting information about chemical stimuli toappropriate regions of the central nervous system.

Mechanical and chemical receptors sense thebodys condition.

Chemical Senses

1. The olfactory system detects airbornemolecules called odorants.

In humans, odors provide information about food,self, other people, animals, plants, and many otheraspects of the environment.

Chemical Senses

Olfactory informationcan influence feedingbehavior, social interactionsand, in many animals,reproduction.

3. The trigeminal chemosensorysystem provides information aboutirritating or noxious molecules thatcome into contact with skin ormucous membranes of the eyes,nose, and mouth.

2. The taste (or gustatory)system detects ingested, primarilywater-soluble molecules calledtastants. Tastants provideinformation about the quality, quantity,and safety of ingested food.

Chemical Senses

Some sensory cells, called chemoreceptors,contain membrane proteins that can bind to particularchemicals in the extracellular fluid.

In response to this chemical interaction, themembrane of the sensory neuron becomes depolarized,leading to the production of action potentials.

Chemical Senses

Chemoreceptors are usedin the senses of taste and smelland are also important inmonitoring the chemicalcomposition of the blood andcerebrospinal fluid.

The most sensitive chemoreceptors are onsensory hairs of the male silkworm which detect sexpheromones.

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Chemical Senses

In terrestrial vertebrates, the sense of smell, orolfaction, involves chemoreceptors located in theupper portion of the nasal passages. Humans detectsmells by means of olfactory neurons located in thelining of the nasal passages. The axons of theseneurons transmit impulses directly to the brain via theolfactory nerve.

Olfactory system - Smell

Eacholfactoryreceptor isspecializedfor 1 odorantmolecule.


Specialized neurons present in the olfactoryepithelium in the nose project cilia into a mucus layer.The cilia are able to bind to odorant molecules thebinding triggers an AP which is transmitted to theolfactory area of the olfactory bulb olfactory cortex.

A terrestrial vertebrate uses its sense of smell inmuch the same way that a fish uses its sense of taste -to sample the chemical environment around it.

Because terrestrial vertebrates are surrounded byair rather than water, their sense of smell has becomespecialized to detect airborne particles (but theseparticles must first dissolve in extracellular fluid beforethey can activate the olfactory receptors).

The sense of smell can be extremely acute inmany mammals, so much so that a single odorantmolecule may be all that is needed to excite a givenreceptor.


Although humans can detect only four(five)modalities of taste, they can discern thousands ofdifferent smells. New research suggests that theremay be as many as a thousand different genes codingfor different receptor proteins for smell.

The particular set of olfactory neurons thatrespond to a given odor might serve as a fingerprintthe brain can use to identify the odor.

From an evolutionary perspective, thechemical senses - particularly olfaction - aredeemed the oldest sensory systems;nevertheless, they remain in many ways theleast understood of the sensory modalities.


The Organization of the Olfactory System

The olfactory system is the most thoroughlystudied component of the chemosensory triad andprocesses information about the identity,concentration, and quality of a wide range of chemicalstimuli that we associate with our sense of smell.

These stimuli, called odorants, interact witholfactory receptor neurons found in an epithelial sheet -the olfactory epithelium - that lines the interior of thenose.

The axons arising from the receptor cells projectdirectly to neurons in the olfactory bulb, which in turnsends projections to the pyriform cortex in the temporallobe as well as other structures in the forebrain.


The pyriform cortexis three layeredarchicortex - consideredto be philogeneticallyolder than the neocortex -and thus represents aspecialized processingcenter dedicated toolfaction.

Projections from the pyriform cortex relayolfactory information via the thalamus to associationareas of the neocortex. The olfactory tract also projectsto a number of other targets in the forebrain, includingthe hypothalamus and amygdala. The further processingthat occurs in these various regions identifies theodorant and initiates appropriate motor, visceral, andemotional reactions to olfactory stimuli.

fMRI images showingfocal activity in theregions of the olfactorybulb, pyriform cortex, andamygdala in a normalhuman being passivelysmelling odors.


Olfactory Perception in Humans

In humans, olfaction is oftenconsidered the least acute of thesenses, and a number of animalsare obviously superior to humansin their olfactory abilities.

This difference may reflect the larger number ofolfactory receptor neurons (and odorant receptormolecules) in the olfactory epithelium in many speciesand the proportionally larger area of the forebraindevoted to olfaction.

In a 70-kg human, the surface area of the olfactoryepithelium is approximately 10 cm2. In contrast, a 3-kgcat has about 20 cm2 of olfactory epithelium.



The most widely used classification wasdeveloped in the 1950s by John Amoore, who dividedodors into categories based on:- their perceived quality,- molecular structure,- and the fact that some people,called anosmics, have difficultysmelling one or another group.

Since the number ofodorants is very large, there havebeen several attempts to classifythem in groups.

Amoore classified odorants as:

Pungent Floral

Musky Earthy

Ethereal Camphor

Peppermint Ether

Putrid



These categories are still used to describe odors,to study the cellular mechanisms of olfactorytransduction, and to discuss the central representationof olfactory information. Nevertheless, this classificationremains entirely empirical.

Chemical structureand human perceptualthreshold for 12common odorants.

Molecules perceived atlow concentrations aremore lipid-soluble,whereas those withhigher thresholds aremore water-soluble.


A further complication in rationalizing theperception of odors is that their quality may changewith concentration.

For example, at lowconcentrations indolehas a floral odor,whereas at higherconcentrations it smellsputrid.

Although most people are able toconsistently identify a broad range of testodorants, others fail to identify one or morecommon smells.

Such chemosensory deficits, called anosmias,are often restricted to a single odorant, suggesting that aspecific element in the olfactory system, either anolfactory receptor gene or genes that control expressionor function of a specific odorant receptor gene, isinactivated.


Anosmias often targetperception of distinct,noxious odorants. About 1person in 1000 is insensitiveto butyl mercaptan, the foul-smelling odorant releasedby skunks.

More serious is the inability to detect hydrogencyanide (1 in 10 people), which can be lethal, or ethylmercaptan, the chemical added to natural gas to aid inthe detection of gas leaks.

A more radically diminished or distorted sense ofsmell can accompany eating disorders, psychoticdisorders (especially schizophrenia), diabetes, takingcertain medications, and Alzheimers disease (all forreasons that remain obscure).


Perceptual thresholds in anosmic and normalsubjects for related organic chemicals. In anosmics,these chemicals are only detected as irritants atrelatively high concentrations (parts per million, ppm); innormal subjects, they are first detected at much lowerconcentrations as odors. The numbers 18 stand for thealiphatic alcohols from methanol to 1-octanol.


Perceptualthresholds for threeadditional commonirritants - phenylethylalcohol (PEA),pyridine (Pyr), andmenthol (Men).

The ability to identify odors normally decreaseswith age. If otherwise healthy subjects are challengedto identify a large battery of common odorants, peoplebetween 20 and 40 years of age can typically identifyabout 5075% of the odors, whereas those between 50and 70 correctly identify only about 3045%.

Normal decline in olfactorysensitivity with age. The abilityto identify 80 common odorantsdeclines markedly between 20and 70 years of age.


The Olfactory Epithelium and Olfactory Receptor Neurons

The transduction of olfactory informationoccurs in the olfactory epithelium, the sheet of neuronsand supporting cells that lines approximately half of thenasal cavities. (The remaining surface is lined byrespiratory epithelium, which lacks neurons and servesprimarily as a protective surface).


Diagram of the olfactory epitheliumshowing the major cell types: olfactory receptor neurons and their cilia, sustentacular (supporting) cells (that detoxify

dangerous chemicals) basal cells.


Basal cells regenerate new olfactory neurons toreplace dead or damaged cells. Olfactory neuronstypically live about one month.

Bowmansglands producemucus. Nervebundles ofunmyelinatedneurons and bloodvessels run in thebasal part of themucosa (called thelamina propria).

The most important of theolfactory epithelium cells is theolfactory receptor neuron, abipolar cell that gives rise to a small-diameter, unmyelinated axon at itsbasal surface that transmits olfactoryinformation centrally.


These proteins haveseven transmembranedomains, plus a variable cellsurface region and acytoplasmic tail thatinteracts with G proteins.


The generic structure of putative olfactory odorant receptors.

G proteins, also known asguanosine nucleotide-binding proteins, are afamily of proteins involvedin transmitting signals froma variety of different stimulioutside a cell into theinside of the cell.

Analysis of the completehuman genome sequencehas idenfied approximately950 odorant receptor genes.

Each gene presumablyencodes an odorant receptorthat detects a particular set ofodorant molecules.


In rodents, genome analysis hasidentified about 1500 different odorantreceptor genes. Thus, in mammals,odorant receptors are the largest knowngene family, representing between 3 and5% of all genes.


At its apical surface, the receptor neuron givesrise to a single dendritic process that expands into aknoblike protrusion from which several microvilli, calledolfactory cilia, extend into a thick layer of mucus.

The mucus that linesthe nasal cavity and controlsthe ionic milieu of theolfactory cilia is produced bysecretory specializations(called Bowmans glands)distributed throughout theepithelium. When the mucuslayer becomes thicker, asduring a cold, olfactory acuitydecreases significantly.


Two other cell classes, basal cells andsustentacular (supporting) cells, are also present inthe olfactory epithelium.

This entire apparatus - mucus layer and epitheliumwith neural and supporting cells - is called the nasalmucosa.

(A) Scanning electronmicrograph of the humanolfactory epithelium,showing cell bodies of theolfactory receptor neurons(O) with their dendrites (D)ending in cilia that form amat within the mucus layeroverlying the epithelium.From the deeper aspect anaxon (arrows) arises,forming bundles (Ax) in thesubmucosa. Red bloodcells (r).


(B) High magnificationview of the distal dendriticknob giving rise toolfactory cilia. Theterminal web is visibleencircling the know(arrows).


The superficial location of the nasal mucosaallows the olfactory receptor neurons direct access toodorant molecules. Another consequence, however, isthat these neurons are exceptionally exposed.

Airborne pollutants, allergens, microorganisms,and other potentially harmful substances subject theolfactory receptor neurons to more or less continualdamage.


Several mechanismshelp maintain the integrity ofthe olfactory epithelium in theface of this trauma.

The Transduction of Olfactory SignalsOdorant transduction begins with odorant

binding to specific receptors on the external surface ofcilia. Binding may occur directly, or by way of proteinsin the mucus (called odorant binding proteins) thatsequester the odorant and are thought to shuttle it tothe receptor. Several additional steps then generate areceptor potential by opening ion channels.

Generation of receptorpotentials in response to odorstakes place in the cilia of receptorneurons.

The Transduction of Olfactory Signals

Thus, odorants evoke alarge inward (depolarizing) currentwhen applied to the cilia (left), butonly a small current when appliedto the cell body (right).

The olfactory receptor neurons express anolfactory-specific G-protein (Golf), which activates anolfactory-specific adenylate cyclase. Both of theseproteins are restricted to the knob and cilia, consistentwith the idea that odor transduction occurs in theseportions of the olfactory receptor neuron. Stimulation ofodorant receptor molecules leads to an increase incyclic AMP (cAMP) which opens channels that permit


Na+ and Ca2+entry (mostlyCa2+), thusdepolarizing theneuron.

This depolarization, amplified by a Ca2+ -activated Cl current, is conducted passively from thecilia to the axon hillock region of the olfactory receptorneuron, where action potentials are generated andtransmitted to the olfactory bulb.


The receptor potential is reduced in magnitudewhen cAMP is broken down by specificphosphodiesterases to reduce its concentration.

At the same time, Ca2+ complexes with calmodulin(Ca2+-CAM) and binds to the channel, reducing itsaffinity for cAMP. Finally, Ca2+ is extruded through theCa2+/Na+ exchange pathway.


Finally, like other sensory receptors, olfactoryneurons adapt in the continued presence of a stimulus.


Adaptation is apparent subjectively as adecreased ability to identify or discriminate odors duringprolonged exposure (e.g., decreased awareness ofbeing in a smoking room at a hotel as more time isspent there).

Physiologically, olfactory receptor neuronsindicate adaptation by a reduced rate of actionpotentials in response to the continued presence of anodorant.


Adaptation occurs because of:(1) increased Ca2+ binding by calmodulin, which

decreases the sensitivity of the channel to cAMP;and(1) the extrusion of Ca2+ through the activation of

Na+/Ca2+ exchange proteins, which reduces thedepolarizing potential from Ca2+ activated Clchannels.


Transduction of stimulus energy into neuralactivity by chemoreceptors and photoreceptors requiresintracellular second messengers.

A1. The olfactory cilia of the olfactory hair cell onthe mucosal surface bind specific odorant moleculesand depolarize the sensory nerve via a second-messenger system. The firing rate signals theconcentration of odorant in the inspired air.


A2. Chemoelectric transduction is produced whenthe appropriate odorant binds to a receptor protein onthe cell membrane, which activates G proteins linked tothe receptor. Channel opening and depolarization inolfactory receptors and certain gustatory receptors aremediated by a second messenger (cAMP) stimulated byG protein activation. A3. Receptor currents evoked bythe appropriate odorant.


The transduction region ingrey - and signaling region in yellow.Transduction occurs in the cilia, whichextend into the mucous layer.

Olfactory sensory cell bioelectrogenesis

A receptor-coupled second-messenger systemresults in the opening of a cation-selective channel in theciliary membrane.


The influx of cations depolarizes the cell membrane fromits resting level near 65 to 45 mV, in a gradedmanner.

This depolarization spreads by passive current flowthrough the dendrite to the soma.

A depolarization that reaches 45 mV is sufficient toactivate voltage-gated Na+ channels and initiateimpulse generation.

This Na+ current along with several varieties of voltage-dependent K+ currents and a small Ca2+ currentproduce one or more action potentials that arepropagated down the axon to the brain.

Olfactory sensory cell bioelectrogenesisThe Transduction of Olfactory Signals

Olfactory Coding

How olfactory receptorneurons represent theidentity and concentrationof a given odorant is acomplex issue that isunlikely to be explainedsolely by the properties ofthe primary receptorneurons.

Nevertheless, neuronswith specific receptors arelocated in particular parts ofthe olfactory epithelium.

Olfactory Coding

As in other sensory systems, the topographicalarrangement of receptor neurons expressing distinctodorant receptor molecules is referred to as spacecoding, although the meaning of this phrase in theolfactory system is much less clear than in vision, wherea topographical map correlates with visual space.

The coding of olfactory information also has atemporal dimension. Sniffing, for instance, is a periodicevent that elicits trains of action potentials andsynchronous activity in populations of neurons.Information conveyed by timing is called temporalcoding and occurs in a variety of species.

Responses of olfactory receptor neurons toselected odorants. Neuron 1 responds similarly tothree different odorants. In contrast, neuron 2 respondsto only one of these odorants. Neuron 3 responds totwo of the three stimuli.

Downwarddeflections representinward currentsmeasured at a holdingpotential of 55 mV.

Olfactory Coding

In vertebratesresponses to an odorcan be measured by anelectro-olfactogram.

Responses of a singleolfactory receptor neuron tochanges in the concentration ofa single odorant, isoamylacetate.

The upper trace in eachpanel (red) indicates the durationof the odorant stimulus; thelower trace the neuronalresponse. The frequency andnumber in each panel of actionpotentials increases as theodorant concentration increases.

Olfactory Coding

Transducing and relaying odorant informationcentrally from olfactory receptor neurons are only thefirst steps in processing olfactory signals.

As the olfactoryreceptor axons leave theolfactory epithelium, theycoalesce (join) to form a largenumber of bundles thattogether make up theolfactory nerve (cranialnerve I).

Olfactory Coding

Each olfactory nerve projects to the olfactory bulbon that side, which lies on the ventral anterior aspect ofthe ipsilateral forebrain.

The olfactory bulb provides the first stage ofsynaptic processing of the sensory information in theolfactory pathway.

Olfactory Coding

The olfactory bulb is comprised of several celllayers:

Glomeruli Mitral cells

Granule cells

Olfactory Coding

which lie just beneath thesurface of the bulb andreceive the primaryolfactory axons.

Within eachglomerulus, the axons ofthe receptor neuronscontact the apical dendritesof mitral cells, which arethe principal projectionneurons of the olfactorybulb.

Olfactory Coding

The most distinctive feature of the olfactory bulb isan array of more or less spherical accumulations ofneuropil 100200 m in diameter called glomeruli,

Finally, granule cells,which constitute the innermostlayer of the vertebrate olfactorybulb, synapse primarily on thebasal dendrites of mitral cellswithin the external plexiformlayer.

These cells lack anidentifiable axon, and insteadmake dendrodendriticsynapses on mitral cells.

Olfactory Coding

The olfactory system enables animals to detectand discriminate between thousands of different odors.

The enormous number of olfactory receptorsallows for the ability to detect a vast array of odors.

Each olfactory receptor recognizes a subset ofchemical cues, with one odor activating more than onereceptor and one receptor recognizing more than oneodor.

Olfactory Coding

Neurons with the same ORare distributed randomly in thenasal epithelium but they all projectto the same glomerulus in theolfactory bulb.

Thus, the activation ofdifferent olfactory receptors leads tothe activation of different glomeruliin the central nervous system.

Different combinations ofactivated glomeruli representdifferent smells.

Olfactory Coding

In the periphery, each olfactory neuronexpresses only one olfactory receptor (OR).

In mammals, one receptor is expressed per celland neurons with the same receptor project to the sameglomerulus, demonstrating that the logic of olfactionhas been maintained through evolutionary time.

In addition to the main olfactory pathway invertebrates, a parallel accessory pathway exists fortransmitting signals from less volatile odorouscompounds called pheromones (the accessoryolfactory system does not function in humans).

Humans do not detect pheromones and haveevolved other strategies to ensure appropriate detectionof mates and attackers.

Olfactory Coding

Higher order processingbegins in the olfactory bulb,where mitral/tufted cellssynapse onto olfactory neuronsand transmit this information tothe olfactory cortex.

Periglomerular cells andgranule cells provideinhibitory connections in thebulb that shape olfactoryresponses.

Olfactory Coding

Information is then relayed to five different brainregions where it is ultimately translated into differentodor percepts and behavior.

Olfactory Coding

Olfactory Coding

Olfactory neurons with the samereceptor project to the sameglomerulus.

Mitral/tufted cells synapse onto asingle glomerulus.

Periglomerulur cells areinhibitory interneurons thatsynapse within and betweenglomeruli.

Granule cells are inhibitoryinterneurons that synapsebetween mitral/tufted cells.

Diagram summarizing the synapticorganization of the glomerular layer.

(a) The olfactory system begins in the peripheral structures of the nasal cavity.(b) The olfactory receptor neurons are within the olfactory epithelium.(c) Axons of the olfactory receptor neurons project through the cribriform plateof the ethmoid bone and synapse with the neurons of the olfactory bulb.

Stem Cells and Olfactory FunctionThere is much current interest in stem cells and

the possibilities they raise for maintaining or repairingbrain function.

The olfactory system is unique in the adult brain inbeing supplied by two sources of stem cells.

First is the olfactory epithelium, where new ORNsarise from basal stem cells during development andthroughout the adult life of the animal. The secondexample is the anterior migratory stream. This processalso occurs throughout adult life.

Further work is thus needed to gaining insightsinto the fundamental problems of stem cell functions inthe brain.

The best definition of the gustatory system is thatit has specialized sensory cells in the periphery andunique regions in the brain dedicated to sensoryprocessing.

The sensory cues detected by the gustatorysystem are soluble chemicals, limiting detection to ashort range by direct contact with a chemical source.

Gustatory system - TASTE

The taste system, acting in concert with theolfactory and trigeminal systems, indicates whetherfood should be ingested.

Once in the mouth, the chemical constituents offood interact with receptors on taste cells located inepithelial specializations called taste buds in thetongue.

The Organization of the Taste System


Taste cells (the peripheral receptors) are foundin taste buds distributed on the dorsal surface of thetongue, soft palate, pharynx, and the upper part of theesophagus.

The tongue is covered with small bumps, calledpapillae, which contain taste buds that are sensitive tochemicals in ingested food or drink. Different types ofpapillae are found in different regions of the tongue.

The taste budscontain specializedgustatory receptorcells that respondto chemical stimulidissolved in thesaliva.


The taste bud is a barrel-shaped structurecontaining approximately 50100 taste cells.

Microvilli of the taste receptor cells project into anopening in the epithelium, the taste pore, where theymake contact with gustatory stimuli.


The taste cells transduce these stimuli and provideadditional information about the identity, concentration,and pleasant or unpleasant quality of the substance.

This information also prepares the gastrointestinalsystem to receive food by causing salivation andswallowing (or gagging and regurgitation if thesubstance is unpleasant).


Of course, food is not simplyeaten for nutritional value; tastealso depends on cultural andpsychological factors. How else canone explain why so many peopleenjoy consuming hot peppers orbitter-tasting liquids such as beer?


Information about the temperature and texture offood is transduced and relayed from the mouth viasomatic sensory receptors from the trigeminal andother sensory cranial nerves to the thalamus andsomatic sensory cortices.

Like the olfactory system, the taste systemincludes both peripheral receptors and a number ofcentral pathways.

Taste cells make synapses with primary sensoryaxons that run in the chorda tympani and greatersuperior petrosal branches of the facial nerve (cranialnerve VII), the lingual branch of the glossopharyngealnerve (cranial nerve IX), and the superior laryngealbranch of the vagus nerve (cranial nerve X),


whose cell bodies liewithin the cranial nerveganglia, to innervate thetaste buds in the tongue,palate, epiglottis, andesophagus, respectively.

The central axons of these primary sensory neurons inthe respective cranial nerve ganglia project to rostral andlateral regions of the nucleus of the solitary tract in themedulla, which is also known as the gustatory nucleus ofthe solitary tract complex.


Neural pathway for taste into the gustatory cortex.

Axons from the rostral (gustatory) part of the solitarynucleus project to the ventral posterior complex of thethalamus, where they terminate in the medial half of theventral posterior medial nucleus.


Neural pathwayTaste impulses travel

through nerves VII, IX andX to a gustatory nucleus inthe medulla oblongata(cross over) thalamus gustatory cortex located inthe parietal lobe in themouth area.

Taste Perception in Humans

Most taste stimuli are nonvolatile, hydrophilicmolecules soluble in saliva.

Examples include salts such as NaCl needed forelectrolyte balance; essential amino acids such asglutamate needed for protein synthesis; sugars such asglucose needed for energy; and acids such as citricacid that indicate the palatability of various foods(oranges, in the case of citrate).

Bitter-tasting molecules, including plant alkaloidslike atropine, quinine, and strychnine, indicate foodsthat may be poisonous. Placing bitter compounds in themouth usually deters ingestion unless one acquires ataste for the substance, as for the quinine in tonicwater.

Mammals are thought to perceiveonly five taste modalities:

sweet

bitter

sour

salty

umami (from the Japanese word for delicious,umami refers to savory tastes, including monosodiumglutamate and other amino acids)


However, there are obvious limitations to thisclassification. People experience a variety of tastesensations in addition to these five, including astringent(cranberries and tea), pungent (hot peppers andginger), fat, starchy, and various metallic tastes, toname only a few.

In addition, mixtures of chemicals may elicitentirely new taste sensations. But even though thetaste code defined by the five primary taste classesis not yet fully understood, these tastes correspond todistinct classes of receptors in subsets of taste cells.

Thus, taste perception is closely linked to themolecular biology of taste transduction.


Diagram of a tongue showing the distribution ofvarious taste bud populations, which are found in thefungiform (F) papillae on the anterior tongue, the vallate(V) and foliate (FO) papillae on the posterior tongue.


Our complexperception of taste isthe result of differentcombinations ofimpulses in the sensoryneurons from these fourkinds of taste buds,together with informationrelated to smell.


The salty taste is produced by the effects ofsodium (Na+) and the sour taste by the effects ofhydrogen (H+).

Organic molecules that produce the sweet andbitter tastes, such as sugars and quinine, respectively,are varied in structure.


Taste buds that respond best to specific tastesare concentrated in specific regions of the tongue: sweet at the tip sour at the sides bitter at the back salty over most of the tongues surface.


The taste system encodes information about thequantity as well as the identity of stimuli. In general,the higher the stimulus concentration, the greater theperceived intensity of taste. Threshold concentrationsfor most ingested tastants are quite high, however.

The effect ofsmell on the sense oftaste can easily bedemonstrated byeating an onion withthe nose open andthen eating it with thenose plugged.

The concentration range for taste detection isbroad and depends on the nature of the chemicalstimulus.

At one extreme, taste cells detect sugars andamino acids at very high concentrations (100millimolar), allowing animals to detect only the mostcaloric foodstuffs instead of food with little nutritionalvalue. At the other extreme, taste cells can also detectminute amounts of noxious substances or toxins,compounds that are harmful at very low concentrations.

Taste buds - collections of chemosensitiveepithelial cells associated with afferent neurons -mediate the sense of taste in vertebrates.


In a fish, the taste buds are scattered over thesurface of the body. These are the most sensitivevertebrate chemoreceptors known.

Taste Perception in fish

They are particularly sensitive toaminoacids; a catfish, for example,can distinguish between two differentamino acids at a concentration of lessthan 100 parts per billion (1 g in10,000 L of water)! The ability to tastethe surrounding water is veryimportant to bottom-feeding fish,enabling them to sense the presenceof food in an often murky environment.

Like vertebrates, many arthropods also have tastechemoreceptors. For example, flies, because of theirmode of searching for food, have taste receptors insensory hairs located on their feet. Each differentchemoreceptor detects a different type of food molecule.

The sensory hairs contain differentchemoreceptors that are able to detect sugars, salts,and other molecules.

Taste Perception in flies

They can detect awide variety of tastes by theintegration of stimuli fromthese chemoreceptors.

Transduction of Taste Signals

The major perceptual categories of taste - salty,sour, sweet, umami, and bitter - are represented by fivedistinct classes of taste receptors, which are found inthe apical microvilli of taste cells.

Salty and sour tastes are primarily elicited byionic stimuli such as the positively charged ions in salts(like Na+ from NaCl), or the H+ in acids (acetic acid, forexample, which gives vinegar its sour taste). These ionsin salty and sour tastants initiate sensory transductionvia specific ion channels: the amiloride-sensitive Na+ channel for salty taste,and an H+-sensitive, cation-selective channel for sour.


Molecular mechanisms oftaste transduction via ionchannels.

The receptor potential generated by the positiveinward current carried either by Na+ for salty or H+ forsour depolarizes the taste cell. This initialdepolarization leads to the activation of voltage gatedNa+ channels in the basolateral aspect of the taste cell.

This additional depolarization activates voltage-gated Ca2+ channels, leading to the release ofneurotransmitter from the basal aspect of the tastecell and the activation of action potentials in ganglioncell axons.


In humans and other mammals, sweet and aminoacid (umami) receptors are heteromeric G-protein-coupled receptors that share a common seventransmembrane receptor subunit called T1R3, which ispaired with the T1R2 seven-transmembrane receptor for

perception of sweet T1R2/T1R3or with the T1R1 receptor for amino acids T1R1/T1R3

The T1R2 and T1R1 receptors are expressed indifferent subsets of taste cells, indicating that there are,respectively, sweet- and amino acid-selective cells in thetaste buds.

Transduction of Taste SignalsUpon binding sugars or other sweet stimuli, the

T1R2/T1R3 receptor initiates a G-protein-mediatedsignal transduction cascade that leads to theactivation of the phospholipase C isoform PLC2,leading in turn to increased concentrations of inositoltriphosphate (IP3) and to the opening of TRPchannels (specifically the TRPM5 channel ~ Transientreceptor potential cation channel subfamily M member5), which depolarizes the taste cell via increasedintracellular Ca2+ .

Transduction of amino acid stimuli via theT1R1/T1R3 receptor also reflects G-protein-coupledintracellular signaling leading to PLC2-mediatedactivation of the TRPM5 channel and depolarization ofthe taste cell.

For sweet tastants, heteromeric complexes ofthe T1R2 and T1R3 receptors transduce stimuli via aPLC2-mediated, IP3(inositol triphosphate)-dependentmechanism that leads to activation of the TRPM5 Ca2+channel.

For amino acids, heteromeric complexes ofT1R1 and T1R3 receptors transduce stimuli via thesame PLC2/IP3/TRPM5-dependent mechanism.


Molecularmechanisms of tastetransduction via ionchannels and G-protein-coupledreceptors.


Another family of G-protein-coupled receptorsknown as the T2R receptors transduce bitter tastes.

Although the transduction of bitter stimuli relies ona similar mechanism to that for sweet and amino acidtastes, a taste cell-specific G-protein, gustducin, isfound primarily in T2R-expressing taste cells andapparently contributes to the transduction of bitter tastes.

The remaining steps in bitter transduction aresimilar to those for sweet and amino acids: PLC2-mediated activation of TRPM5 channels depolarizes thetaste cell, resulting in the release of neurotransmitter atthe synapse between the taste cell and sensory ganglioncell axon.

Bitter tastes are transduced via a distinct set ofG-protein-coupled receptors, the T2R receptor subtypes.The details of T2R receptors are less well established;however, they apparently associate with the taste cellspecific G-protein gustducin, which is not found insweet or amino acid receptor- expressing taste cells.

Nevertheless, stimulus-coupled depolarization forbitter tastes relies upon the same PLC2/IP3/TRPM5-dependent mechanism used for sweet and amino acidtaste transduction.


Molecular mechanisms oftaste transduction via ionchannels and G-protein-coupled receptors.

Taste cells are polarizedepithelial cells with an apical and abasolateral domain separated bytight junctions.

Tastant-transducing channels(salt and sour) and Gprotein-coupled receptors (sweet, aminoacid, and bitter) are limited to theapical domain.

Basic components of sensory transduction in taste cells.

Intracellular signaling components that arecoupled to taste receptor molecules (G-proteins andvarious second messenger-related molecules) are alsoenriched in the apical domain.


Voltage - regulatedNa+, K+, and Ca2+ channelsthat mediate release ofneurotransmitter frompresynaptic specializationsat the base of the cell ontoterminals of peripheralsensory afferents are limitedto the basolateral domain, asis endoplasmic reticulum thatalso modulates intracellularCa2+ concentration andcontributes to the release ofneurotransmitter.

Transduction of Taste SignalsBasic components of sensory transduction in taste cells.

The neurotransmitterserotonin, among others, isfound in taste cells, andserotonin receptors arefound on the sensoryafferents.

Finally, the TRPM5channel, which facilitates G-protein-coupled receptor-mediated depolarization, isexpressed in taste cells. Itslocalization to apical versusbasal domains is not yetknown.

Transduction of Taste SignalsBasic components of sensory transduction in taste cells.

Na+ salt; sweet solutes; acid and sour solutes.A taste receptor cell responding to:



The binding of the receptor to a taste moleculetriggers the entry of calcium in the cell release ofneurotransmitter in a synapse with a neuron.

In the taste system,neural coding refers tothe way that the identity, concentration,and hedonic

(pleasurable oraversive)

value of tastants isrepresented in thepattern of actionpotentials relayed to thebrain.

Neural Coding in the Taste System

Neurons in the taste system (or in any othersensory system) might be specifically tuned torespond with a maximal change in electrical activity to asingle taste stimulus.

Such tuning is thought to rely on specificity at thelevel of the receptor cells, as well as on themaintenance of separate channels for the relay of thisinformation from the periphery to the brain.

This sort of coding scheme is referred to as alabeled line code, since responses in specific cellspresumably correspond to distinct stimuli.


The segregated expression of sweet, amino acid,and bitter receptors in different taste cells is consistentwith labeled line coding.


Specificity in peripheraltaste coding supports thelabeled line hypothesis.(AC) Sweet (A), aminoacid (B), and bitter (C)receptors are expressedin different subsets oftaste cells.


(DE) The gene for the TRPM5 channel can beinactivated, or knocked out, in mice (TRPM5/) andbehavioral responses measured with a taste preferencetest.

The mouse is presented with two drinking spouts, onewith water and the other with a tastant; behavioralresponses are measured as the frequency of licking ofthe two spouts.


For pleasant tastes like sweet (sucrose; D) orumami (glutamate; E) control mice lick the spout withthe tastant more frequently, and higher concentrationsof tastant leads to increased response (blue lines).

In TRPM5/ mice, this behavioral response (i.e., apreference for the tastant versus water) is eliminated atall concentrations (red lines).


(F) For an aversive tastant like bitter quinine,control mice prefer water. This behavioral responsewhich is initially lowis further diminished with higherquinine concentrations (blue line).


Inactivation of TRPM5 also eliminates thisbehavioral response, regardless of tastant concentration(red line).

(GI) When the PLCb2 gene is knocked out,behavioral response to (G) sucrose, (H) glutamate, and(I) quinine are eliminated (red lines).


When PLCb2 is re-expressed only in T2R-expressing taste cells, behavioral responses to sucroseand glutamate are not rescued (dotted green lines in Gand H); however, the behavioral response to quinine isrestored to normal levels (compare the blue and dottedgreen lines in I).

The third of themajor chemosensorysystems, the trigeminalchemosensory system,consists of polymodalnociceptive neurons andtheir axons in thetrigeminal nerve (cranialnerve V) and, to a lesserdegree, nociceptiveneurons whose axons runin the glossopharyngealand vagus nerves (IXand X).

Trigeminal Chemoreception

These neurons and their associated endings aretypically activated by chemicals classified as irritants,including air pollutants (e.g., sulfur dioxide), ammonia(smelling salts), ethanol (liquor), acetic acid (vinegar),carbon dioxide (in soft drinks), menthol (in variousinhalants sensation), and capsaicin (the compound inhot chili peppers that elicits the characteristic burningsensation).


Irritant-sensitive polymodal nociceptors alert theorganism to potentially harmful chemical stimuli thathave been ingested, respired, or come in contact withthe face, and are closely tied to the trigeminal painsystem.


Trigeminal chemosensoryinformation from the face, scalp,cornea, and mucous membranes ofthe oral and nasal cavities isrelayed via the three major sensorybranches of the trigeminal nerve:the ophthalmic, maxillary, andmandibular.


The central targetof these afferent axonsis the spinal componentof the trigeminalnucleus, which relaysthis information to theventral posterior medialnucleus of the thalamusand thence to thesomatic sensorycortex and other corticalareas that processfacial irritation andpain.

Sensory receptors within the body detect avariety of chemical characteristics of the blood or fluidsderived from the blood, including cerebrospinal fluid.

Internal Chemoreceptors

Included among these receptors are theperipheral chemoreceptors of the aortic and carotidbodies and the central chemoreceptors in themedulla oblongata of the brain.


Peripheralchemoreceptors ofthe aortic andcarotid bodies aresensitive primarily toplasma pH.

Centralchemoreceptors inthe medullaoblongata of thebrain are sensitive tothe pH ofcerebrospinal fluid.

When the breathing rate is too low, theconcentration of plasma CO2 increases, producing morecarbonic acid and causing a fall in the blood pH.

The carbon dioxide can also enter thecerebrospinal fluid and cause a lowering of the pH,thereby stimulating the central chemoreceptors. Thischemoreceptor stimulation indirectly affects therespiratory control center of the brain stem, whichincreases the breathing rate.

The aortic bodies can also respond to a loweringof blood oxygen concentrations, but this effect isnormally not significant unless a person goes to a highaltitude.


The chemical senses - olfaction, taste, and thetrigeminal chemosensory system - all contribute tosensing airborne or soluble molecules from a varietyof sources. Humans and other mammals rely on thisinformation for behaviors as diverse as attraction,avoidance, reproduction, feeding, and avoidingpotentially dangerous circumstances.

Chemical Senses - Summary

Each of the approximately10,000 odors that humans recognize(and an undetermined number oftastes and irritant molecules) isevidently encoded by the activity of adistinct population of receptor cellsin the nose, tongue, and oral cavity.

Receptor neurons in the olfactory epithelium transducechemical stimuli into neuronal activity via the stimulation of G-protein-linked receptors; this interaction leads to elevated levelsof second messengers such as cAMP, which in turn open cation-selective channels. These events generate RP in the membraneof the olfactory receptor neuron, and ultimately AP in the afferentaxons of these cells.

Chemical Senses Summary

Taste receptor cells, incontrast, use a variety of mechanismsfor transducing chemical stimuli.These include ion channels that aredirectly activated by salts and aminoacids, and G-protein-linked receptorsthat activate second messengers.

For both smell and taste, the spatial and temporal patternsof action potentials provide information about the identity andintensity of chemical stimuli.

Olfaction, taste, and trigeminal chemosensation allare relayed via specific pathways in the CNS.

Receptor neurons in the olfactory system projectdirectly to the olfactory bulb. In the taste system,information is relayed centrally by cranial sensoryganglion cells to the solitary nucleus in the brainstem. Inthe trigeminal chemosensory system, information isrelayed via trigeminal ganglion cell projections to thespinal trigeminal nucleus in the brainstem.

Each of these structures project in turn to manysites in the brain that process chemosensoryinformation in ways that give rise to some of the mostsublime pleasures that humans experience.

Chemical Senses Summary

http://www.nature.com/nature/journal/v486/n7403_supp/full/486S2a.html

NEUROSCIENCE: Third Edition, Dale Purves et al., 2004 SinauerAssociates, Inc.

Fundamental neuroscience /by Larry Squire et al.3rd ed. 2008, Elsevier Inc. Coding of Sensory Information, Esther P. Gardner John H. Martin;

http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week3/Kandel%20Ch%2021,%2022,%2023.pdf

www.austincc.edu/rfofi/BIO2304/2304LecPPT/2304Sensory.ppt www.mohsenparviz.ir/lesson/L5-%20Sensory%20Receptors.ppt www.med.uottawa.ca/Courses/NSC5104/.../NeuralSystemsSensory1.ppt www.med.muni.cz/biofyz/files/en/HEARING-finx.ppt www.jfmed.uniba.sk/.../Biofysics_of_sensory_p._receptors__vision.ppt faculty.weber.edu/nokazaki/.../PPT%20notes/Sensory%20System.ppt NATURE | VOL 413 | 13 SEPTEMBER 2001 | www.nature.com http://cnx.org/content/m46577/latest/?collection=col11496/latest http://neuroscience.uth.tmc.edu/s2/chapter09.html http://downloadpdfz.com/ppt/what-is-adaptation-of-sensory-receptors

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