chemical senses olfaction and gustation · 2019. 04. 24. 3 olfactory epithelium • specialized...
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2019. 04. 24.
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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).
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