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Chemical senses – olfaction and gustation

• Both are required to perceive flavour

• Chemically sensitive cells – chemoreceptors

• Both have strong and direct connections to our most basic needs

– Thirst, hunger, emotion, sex, certain forms of memory

• Systems (receptors) are separate and only merge at higher

levels of cortical function

Kékesi Gabriella Pharm.D.

The physiology of olfaction

1. Olfactory sensory neurons

a) Location

b) Structure

c) Afferent pathways

2. Olfactory cilium

3. Olfactory receptors

a) Activation

b) Signal transduction

4. Functional topography in the

olfactory system (epitop map)

5. Olfactory pathway

6. Anosmia, hyposmia, dysosmia

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Olfaction

• High sensitivity or low threshold of detection: 10-8-10-10 g/L – some molecules

• Female vs male

• Not constant: higher sensitivity during ovulation

• Regeneration

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Olfactory epithelium

• specialized epithelial tissue inside the nasal cavity that is involved

in smell

• about 4-5 square centimeters on each side and lies on the roof of

the nasal cavity

• responsible for detecting odors

• consists of four distinct cell types:

– Olfactory (sensory) cells

– Supporting cells

– Basal cells

• Bowman's (olfactory) glands:

– Mucus: to trap and dissolve odiferous substances for the bipolar

neurons.

• Olfactory (sensory) cells

– Bipolar neurons which congregate to form the olfactory nerve

– Dendritic process: non-motile cilia (6-8/cell) with olfactory receptors

– Central axons – synapse at the glomerulus of olfactory bulb

• Supporting cells

– Metabolic and physical support for the olfactory cells

• Basal cells

– Stem cells capable of division and differentiation

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http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/illpres/7_species.html

Species differences in the area of olfactory epithelium

dog:human ~ 40:1

Nose hair vs Olfactory cilia

• The visible NOSE HAIR is just hair,

not cilia

• helps to trap particulate matter,

preventing harmful materials from

entering the nasal passages and

defending the body from potential

sources of infection

• Tiny hair-like CILIA protrude from

the olfactory receptor cell's dendrite

into the mucus covering the surface of

the olfactory epithelium

• express olfactory receptors – binding

odour molecules

• transmit the olfactory information to

the olfactory bulb

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Signal transduction mechanism

Each olfactory sensory cell expresses only one type of olfactory receptor, but many separate sensory cells express olfactory receptors, which bind the same set of odors.

Epitope: specific functional group or feature of the odorant that

could be recognized by the receptor

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http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/illpres/6_codes.html

Combinatorial receptor codes

Different odorants are detected by different combinations of receptorsThese codes are translated by the brain into diverse odour perceptionsPotential receptor combinations - ability to distinguish more than 10,000 different odorants

http://www.nobelprize.org/nobel_prizes/medicine/laureates/2004/illpres/2_olfactory.html

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Olfactory bulb• 1st neuron - Olfactory sensory neuronsproject axons to the brain within theolfactory nerve

• Pass to the olfactory bulb throughperforations in the cribriform plate

• Glomeruli in the olfactory bulb– synapsewith the second neuron (large degree of convergence)– The axons of olfactory receptor cells which

express the same receptor converge to form

glomeruli in the olfactory bulb.

– Each glomerulus recieves signals from multiplereceptors that detect similar odorant features

– Multiple receptor types are activated due to thedifferent chemical features of the odorant, multiple gomeruli will be activated as well

EPITOPE MAP

• 2nd neuron (relay cells and interneurons)

– Mitral cells

– Tufted cells

– Periglomerular cells – lateral inhibition (GABA)

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Olfactory bulb projections

1. Mitral cells leave the olfactory bulb in the lateral olfactory tract

A. medial dorsal thalamus

B. orbitofrontal cortex → conscious perception of the odour

C. limbic system:

– Amygdala – hippocampus, hypothalamus → emotions, behavior, learning

and memory related to odors

Short-term adaptation of the odor response of an intact olfactory sensory neuron. It was

stimulated with two identical 100 ms pulses of cineole (300 μM) with varying interpulse

intervals, separated by 40 s rest periods.

Frank Zufall, and Trese Leinders-Zufall Chem. Senses

2000;25:473-481

Oxford University Press

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Schematic diagram of olfactory signal transduction and its Ca2+-dependent feedback

regulation during long-term adaptation and desensitization.

Frank Zufall, and Trese Leinders-Zufall Chem. Senses

2000;25:473-481

Oxford University Press

Disorders of olfaction

• Hyposmia: decreased ability to smell,• Anosmia: permanent inability to smell,• Hyperosmia: abnormally acute sense of smell,• Cacosmia: things smell like feces• Dysosmia: things smell different than they should• Olfactory Reference Syndrome: psychological disorder which causes the patient to imagine he or she has strong body odor• Parosmia: things smell worse than they should• Phantosmia: "hallucinated smell," often unpleasant in nature

Quantifying the intensity of odors

• Olfactometer can be utilized to determine the magnitude of an odor

• The basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard.

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Vomeronasal organ – accessory olfactory system?

• The vomeronasal organ is mainly used to detect pheromones, chemical messengers that carry information between individuals of the same species.

• Animals: the axons from these neurons project to the accessory olfactory bulb, which targets the amygdala and bed nucleus of the stria terminalis, which in turn project to the hypothalamus.

• Human: Androstenon, androstendion

The physiology of taste sensation

1. Taste sensory cells

a) location

b) structure

c) afferentation

2. Taste buds

3. Basic taste sensations, gustatory

modalities

4. Taste receptors

a) activation

b) transduction mechanisms

5. Cranial nerves and central taste

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Gustation/Taste

• Quality control

• Basic tastes: sweet, sour, salty, bitter and umami

• Taste map myth: different sections of the tongue specialized in

different tastes – misconception

• Complex flavors:

– Combination of basic tastes

– Other sensory modalities may contribute to a unique food-tasting experience

• Hunger

• Aversive reactions

All taste sensations come from all

regions of the tongue

BUT there are regions most sensitive

to a given taste

Taste sensitivity

Highest sensitivity to bitter (no or minimaladaptation)

Influencing factors:

– temperature:

• higher: bitter⇓, sweet⇑

– circadian rhytm: better in afternoon

– sex: female vs male

– concentration

– age (infants, over 40)

– genetic factors

– colour

– odor (zamat)

– texture (macaroni – spaghetti)

• Concentration-dependent affectivecomponent

– Sweet and umami

– Sour and bitter

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Gustatory papillae of tongue have one to several

hundred taste buds:

types: circumvallate, foliate, fungiform, filiate

Taste bud has 50-150 taste cells:

location: top of the tongue, epiglottis, palate, pharynx,

larynx, proximal esophagus

• Portion of taste buds:

• supporting cells

• gustatory/taste cells (1% of the tongue epithelium)

• Not neurons

• microvilli – sensory receptors, ionchannels

• Synapses with gustatory afferent axons near

the bottom of the tase bud

• taste pore at the top of the taste buds

• Activated only substances dissolved in the saliva.

Gustatory/taste organ

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Taste receptors and transductions

ATP

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http://www.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2121/images/gustatory.jpg

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Pattern theory vs „labeled lines”

• One gustatory cell is sensitive to more types of tasting molecules - one cell

makes connection to several neurons – central analysis of the taste pattern

from gustatory cells

vs

• Monospecificity of gustatory cells

• Interaction at the level of central processing

• Year 2000 – bitter sensing receptor (T2R)

• Year 2001 – sweet sensing receptor

• Year 2002 – umami receptor

• Year 2005 – H+ channels – acid sensation - sour

• Hypovitaminosis: B12, A

• Endocrine disorders – hypo- or hyperthyreoidism, diabetes mellitus

• Smoking

• Chemotherapy in cancer, radiation

• Ageing (decreased number of taste buds)

• Psychological and neurological disorders

• Infection, catch cold, influenza

Dysgeusia: distortion of the sense of taste

Hypogeusia: decreased taste sensitivity

Ageusia: complete lack of tase

Parageusia: bad taste in the mouth

Cacogeusia: unpleasant taste

Reasons of decreased taste sensitivity

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• Smelly fruit

• The durian fruit smells horrible. Some people cannot bear to eat it because it smells so foul. But it is called the "King of Fruits" and tastes delicious. It is very large (can be the size of a football) and comes from South East Asia.

Miraculin(Synsepalum dulcificum)

• Is a glycoprotein extracted from the fruit of Synsepalum dulcificum.

• Miraculin itself is not sweet but has a unique ability to sweeten sour tastes: after the taste buds

are exposed to miraculin (which binds to sweet receptors on the tongue), acidic foods which

are ordinarily sour (such as citrus) are perceived as sweet.

• the first (and is still recognized as the largest) known macromolecule able to elicit a taste

sensation

• Although speculative mechanisms have been proposed in the literature, what is known is that

miraculin binds tightly to the lingual epithelium’s plasma membrane microvilli of the sweet-

taste receptors (hT1R2-hT1R3) without activating them and is consequently experienced

without flavor (Asakura et al., 2011; Cagan, 1973; Misaka, 2013; Montmayeur and

Mantsunami, 2002). It does not activate these receptors until subjected to an acidic pH,

generally between pH 3.0 and 6.0 (Kurihara, 1992; Wong and Kern, 2011; Paladino et al.,

2010).