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Chemical senses

Olfaction and taste perception

Péter Sántha, 08.04.2019.

Chemical senses

Biological significance:

Feeding: checking the quality and composition of the

food (e.g.: bitter taste – poisonous alkaloids)

Avoidance: protective reflectory reactions (gagging,

regurgitation, vomit, gustofacial rfx.)

Control of secretion and motility in the GI tract:

(gastric-, pancreatic juice secretion – „cephalic phase”)

Sexual behavior, social binding (humane pheromones?)

Emotional reactions, memory functions

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Olfaction – the peripheral receptive field

Mucosal membrane of the upper nasal cavity

Upper and middle turbinates (conches)

Olfactory epithelium ( 10 cm2):

107 olfactory receptor cells (ORC)

Primary sensory cells - neurons

(lifespan is only 4-8 weeks!)

Supporting cells, basal cells (regenerative pool)

Normal breathing - sniffing

Direct connections with the olfactory bulb

Ist cranial nerve.

Other nerves are also involved

(detection of volatile irritant substances):

Vth, IXth, Xth cranial nerves.

Cellular organisation of the olfactory epithelium

Isolated olfactory receptor cell

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Cell types and the neural network in the olfactory bulb

Glomeruli: functional units

Convergence: ~ 1:1000

Periglomerular Cells:

Local interneurons

(lateral inhibition)

Mitral cells: Projection neurons

Granule cells:

inhibitory neurons

(dendrodendritic synapses)

Efferent axons: efferent control of

the sensory processing

periglomerular cells

Efferent axons

granule

cells

mitral

cells

mitral

cells

ORC axons

Similarities between the neural networks of the retina and olfactory bulb

lateral inhibitory connections (lateral inhibition)

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Anatomy of the pathway and cortical (CNS) represntation

TC: Tufted cells VNO:

Vomeronasal Organ

(in humane rudimentary)

AON: anterior olfactory nucl.

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Brain areas involved in the

central processing of the olfactory

Signals

Temporal lobe epilepsy: dysosmia

(cacosmia)

Phylogenetic reduction in the relative size of olfactory system

Odorant qualities – odorant classes

10.000 – 100.000 different odorants can be discriminated

(genetic background, age, experience can influence)

Subjective classification: (John Amoore, 1950)

pungent, floral, musky, earthy, ethereal,

camphor, peppermint, ether, and putrid

Partial anosmia („color blindness” of smelling)

Cross-adaptation

Sensitivity of the olfactory system:

Detection limit: 1-0.001 ppm (pars pro million)

Detection limit ≠ identification limit!

Prevalence of partial anosmias

in the population (%)

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Detection limit of different odorants (human)

Jasmin flower

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Function of the olfactory receptor cells – sensory transduction

(B): Exposure to odorants:

changes in membrane potential

(electrotonic) receptor potential

Axon hillock: – AP generation!

(A) Odorant receptors (ORs):

G-protein coupled 7 transmembrane

domain receptors

Signal amplification:

Second-messenger-cascade

Adenylate-cyclase – cAMP

cAMP sensitive

ionchannels (CNG channels)

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Nobel lecture – R. Axel 2004.

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The Nobel Prize in Physiology or Medicine 2004

"for their discoveries of odorant receptors and the organization of the olfactory system"

Richard Axel Linda B. Buck

See the video of the Nobel lectures at http://nobelprize.org/nobel_prizes/medicine/laureates/2004/

1991

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Nobel lecture – L. Bucks, 2004

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Nobel lecture – L. Bucks, 2004

Nobel lecture – L. Bucks, 2004

MO

LE

CU

LA

R R

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TIV

E R

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One Neuron – One Receptor: development of specific phenotype

and selective sensitivity of the olfactory receptor cells

Localisation of olfactory

receptor cells in nasal

mucosa expressing

different types of odrant

receptors (in mouse)

Galactosidase-odorant receptor construct

Nobel lecture of R. Axel, 2004

Convergence of ORC axons into specific glomeruli of the olfactory bulb

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Specific connections between ORCs and the mitral cells

From the Nobel lecture of R. Axel

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„across fibre pattern” type of coding of the sensory information

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„across fibre pattern” type of coding of the sensory information

-Mucous membrane of the oral cavity - tongue:

Fungiform-, foliate- and circumvallated papillae

Taste buds are mainly associated to the papillae

Topographic distribution:

Fungiform papillae: anterior 2/3 of the tongue (200-400)

Circumvallated papillae: at the rear of the tongue (7-12)

Foliate papillae: posterolateral edge of the tongue (15-20)

Taste perception – peripheral receptors

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Topography of the surface of tongue :

All the basic taste qualities can be

detected in all regions of the tongue!

Localization of taste buds on the papillae

Other localisations are: buccal and gum mucosa, pharynx, larynx,

and upper esophagus.

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Structure of the taste budsIn human: ca. 2000-4000 taste buds

Cell types: taste cells - secondary sensory epithel cells

supporting cells

basal cells

Afferent innervations by ca. 50 axons – peripheral branches of sensory neurons

(type C and Aδ fibres)

α-Gustducin

H-Blutgruppeantigen

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Innervation of taste buds on the surface of the tongue

N. Facialis (VII.) - Chorda tympani

N. Glossopharyngeus (IX.)

(N. Vagus, N. Trigeminus)

N. facialis system – diagnostic importance of the taste sensation

differentiation between proximal and distal types of the nerve lesion

Proximal vs.

Distal

lesions of the

facial nerve:

paralysis

± ageusia

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n. tractus solitarii

(gustatory nucl.)

„Specific system”:

medial lemniscus

(lemniscus trigeminalis)

thalamus (VPM)

anterior insula (temp. lobe)

operculum of the frontal lobe

„Unspecific system”:

n. parabrachialis (pons)

amygdala

hypothalamus

Reflex connections: salivation

motor reactions

Central course of taste pathways

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Detection limits of different tastants (mM)

bitter

sour (acidic)Chloric acid

Citric acid

sweet

salty

Basic taste qualities: above + umami (Na-glutamate)

(other qualities: metallic, pungent/sharp; adstringent etc.)

The taste qualities

Response profiles of taste bud afferents stimulated by different tastants

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Labeld line hypothesis:

The phrase “labeled line” implies that the activity in one neuron type is both

necessary and sufficient to represent a given sensory attribute

- clear situation: primary auditory afferrent fibres – frequency tuning

Ensemble or “across-neuron” hypothesis:

Ensemble or “across neuron” hypothesis proposes that the pattern of responses to

a particular stimulus across all fibers (neurons) is the central feature of coding

Problem of the coding of taste qualities – competing hypotheses

NTS

Thal. VPM

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Specific receptors detect different taste qualities

Different types of taste bud cells – parakrine communication

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Biol Pharm Bull. 2010;33(11):1772-7.

Gustatory signaling in the periphery: detection, transmission, and modulation of

taste information.

Niki M, Yoshida R, Takai S, Ninomiya Y.

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Flavour = smell + taste