Chemical Senses Flashcards
What is gustation
Taste
What is olfaction
Smell
What do chemoreceptors respond to
Chemicals in aqueous solution
Taste receptors respond to what
Substances dissolved in saliva
What do smell receptors respond to
Substances dissolved in fluids of nasal membranes
What does the taste system evaluate
Potential food for nutritional and poisonous chemical features
What does flavor of food depend on
A combination of taste, smell, texture and temperature
Sense of gustation depends on what
Chemical stimuli and tastants present in food
Where do you find taste buds
On the dorsal surface of the tongue in papillae
What papillae do you find on the anterior dorsal surface of the tongue
Fungiform
What papillae do you find on the posterior and lateral dorsal surface of the tongue
Foliate
What papillae do you find on the base of the dorsal surface of the tongue
Circumvallate
Where can taste buds also occur
-soft palate
-pharynx
-epiglottis
What cells make up taste buds
-chemoreceptors ( about 50-150 taste receptor cells)
-supporting cells (sustentacular cell)
-basal cells
How many taste buds do adults have
3000- 10 000
How many taste buds does each papillae have
1- several hundred
What happens to taste buds with age
They degenerate
The apex of taste chemoreceptors have what
Microvilli
Chemoreceptors (taste cells) synapse with what
Afferent neurons
Taste chemoreceptors are continually replaced by what
Mitotic division of the surrounding epithelial cells
What do taste chemoreceptors detect
Stimulatory molecules that diffuse into the taste pore from the overlaying fluid layer
What are the basic tastes
-sour
-salty
-sweet
-bitter
-umami
What causes the sour taste
Acids I.e. the H+ conc
What causes salty taste
Ionized salts ,mainly the Na+ conc
The sweet taste isn’t caused by what
A single class of chemicals
What causes the sweet taste
Sugars, glycols, aldehydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids and inorganic salts of lead and beryllium
The bitter taste isn’t caused by what
Any single type of chemical agent
What causes the bitter taste
Long chain organic substances that contain nitrogen and alkaloids e.g. quinine, caffeine, nicotine and strychnine
Some substances that are first sweet have what
Have a bitter aftertaste d.g. Saccharine
What is the umami taste
-It’s delicious…it has a pleasant taste sensation that is qualitatively different from the other tastes
-it’s the dominant taste of food containing L-glutamate e.g. meaty extracts
What’s the chemoelectrical transduction for salty taste
-Na+ flow into taste cells through the apical amiloride sensitive Na+ channels
-this depolarizes the taste cell
-voltage-dependant Ca2+ channels open
- there is an influx in Ca2+
-Ca2+ causes serotonin vesicles to release their neurotransmitter which then stimulates the afferent taste axon
What’s the chemoelectrical transduction for sour taste
-H+ enter the cell through proton selective ion channels
-the H+ depolarize the taste cell
-this opens voltage-dependent Ca2+ channels
-this causes an influx of Ca2+
- Ca2+ causes the release of the neurotransmitter serotonin, which stimulates the afferent taste axons
-the H+ also blocks K+ channels, which causes an increase of K+ and that adds to the chemoreceptor depolarization
What else can elicit sour sensation
Potassium channel blockers eg tetraethylammonium
What’s the chemical transduction for bitter, sweet and umami taste
- compounds from these tastes activate taste receptor cells via the G protein coupled receptors
- when they bind to the G protein receptor this activates phospholipase C into IP3
-IP3 causes the release of Ca2+ from the ER
-Ca2+ release then causes the neurotransmitter ATP to be released
Why is Ca2+ influx not needed in the chemoelectrical transduction of bitter/sweet/umami
Because ATP isn’t released from vesicles
Taste buds are innervated by what
-facial nerve (chorda tympani) (anterior 2/3 of tongue)
-glossopharyngeal nerve (posterior 1/3 of tongue)
-vagus nerve (palate, pharynx and epiglottis)
What is the central pathway for taste
- the taste fibers ( 1st order neurons/ afferent gustatory neurons ) synapse in the gustatory division of the solitary nucleus in the medulla
-the 2nd order neurons then synapse in the ventral posterior medial nucleus of the thalamus
-the 3rd order neuron carries signals to the primary gustatory cortex
-from the gustatory nucleus in the brain stem taste pathways diverge to:
1. Cerebral cortex i.e. somatosensory and frontal cortex for conscious perception of taste
2. Brain stem regions involving swallowing, salivation, gagging, vomiting, digestion and respiration
3. Limbic system i.e. amygdala and hypothalamus for food palatability and motivation to eat
4. Hippocampus for food memory
Taste disorders are the result of what
Medical conditions
Pharmacological or surgical interventions
Environmental exposure to toxic chemicals
Head injury
Poor oral hygiene
Advanced age
Whats hypogeusia
A diminished sense of taste
Whats ageusia
A complete loss of taste
What’s dysgeusia
An alteration/ distortion in the perception of taste
What’s phantogeusia
It’s a phantom taste which is often a bitter or metallic taste when no gustatory stimuli is present
People with what conditions experience phantogeusia
-schizophrenia
-seizure aura for focal epilepsy
olfaction serves as an early what
Warning system for detection of fire
Dangerous fumes
Polluted environments
Olfaction largely determines what
The flavor and palatability of food and beverages
Where is the olfactory epithelium located
Olfactory nasal mucosa
What increases the surface area of the olfactory epithelium
Conchae
What is the olfactory bulb
It’s the part of the brain that receives direct input from the primary olfactory neurons and is an outgrowth of brain tissue from the base of the brain
Olfactory chemoreceptors are what
Bipolar neurons
What does the apical surface of the olfactory chemoreceptors have
4-25 immobile cilia
What is the base of the olfactory chemoreceptor like
It’s a neuron
Olfactory receptors are interspaced between what
Sustentacular cells
What do sustentacular cells in olfaction do
They provide physical support, nourishment, electrical insulation and help detoxify chemicals
What cells of olfaction does SARS-coV-2 infect
Sustentacular cells and not chemoreceptors/olfactory bulb neurons
To infect a cell what does SARS-CoV-2 do
It must bind to the angiotensin converting enzyme 2 (ACE2) receptor in the cell membrane
ACE2 is expressed by what
Sustentacular cells in the human olfactory epithelium
ACE2 isn’t expressed by what
The olfactory sensory neurons
What’s the life span for olfactory chemoreceptors
60 days
What must the axon of each olfactory cell do
It must make its way to the olfactory bulb to make the proper synaptic connection
What can harmful substances in the olfactory epithelium do
They can invade the sensory neurons and be transported into the CNS via the olfactory nerve
What replaces damaged neurons in olfaction
Neurogenesis
What are the 6 odor qualities
Floral e.g. roses
Ethereal e.g. pears
Musky e.g. perfume, aftershave
Camphor e.g. eucalyptus
Putrid e.g. rotten eggs
Pungent e.g. vinegar
Humans recognize how many separate odors
10 000 ( 20% pleasant, 80% bad)
What are of odorless gases
Nitrogen
Hydrogen
Ethane
Carbon monoxide
Radon
Helium
How are molecules introduced for olfaction
By sniffing
By air
By diffusion from the mouth
How does olfactory transduction work
-odorants bind to membrane odorant receptor proteins
-this then stimulates the G protein (Golf protein)
-the G protein then activates adenylyl cyclase
-adenylyl cyclase then forms cAMP
-cAMP then binds to a cyclic nucleotide gated cation channels
-the binding opens the channel and there is an influx of Na+ and Ca2+
-the Ca2+ then opens Ca2+ activated Cl- channels and Cl- leave the cell
-this then causes the membrane to depolarize
what type of cells does the olfactory bulb contain
-mitral cells
-tufted cells
-interneurons (granule and periglomerular cells)
what are mitral and tufted cells
they are the output neurons of the olfactory bulb
what are the granules and periglomerular cells
they are inhibitory interneurons
nerve fibers leading backwards from the olfactory bulb form what
the olfactory nerve
what do the glomeruli in the olfactory bulb contain
axonal ends of the olfactory receptors and apical dendrites of the 2nd order afferent fibres
whats the only sensation thats relayed to the cerebral cortex without passing through the thalamus
olfaction
how do axons of the mitral and tufted cells move
they pass posteriorly through the intermediate olfactory stria and lateral olfactory stria to the olfactory cortex
where do other fibers from the olfactory bulb project to
- the entorhinal cortex, for olfactory memories
-the frontal lobe i.e. the intermediate and medial olfactory areas , where the medial olfactory area is responsible for visceral and emotional reactions to odors
-limbic system i.e. amygdala
what is anosmia
absence of smell
what is hyposmia
diminished sensitivity of smell
what is dysosmia
distortion of normal smell
what is phantosmia
its when there is a perception of odor in absence of an odor stimulus
what is parosmia
its when there is distortion of odor perception when an odor is present
what are causes for disorders of olfaction
-aging
-respiratory infections
-smoking
-growths in the nasal cavities
-head injury
-hormonal disturbances
-dental problems
-exposure to certain chemicals
-medications e.g. antibiotics and antihistamines
