The chemical senses Flashcards
In addition to smell and taste what can our body detect?
CO2/O2 levels
Chemoreceptors in arteries of the neck measure CO2/O2 levels in our blood
Chemical irritants
Nerve endings in skin/mucous membranes warn us of chemical irritants
Acidity
Sensory nerve endings in muscle respond to acidity –burning feeling that comes with exercise and O2 debt
Is taste innate or learned?
Innate?
• Some of our taste preferences are inborn (or “innate”)
• Humans innately enjoy sweet flavours and avoid bitter flavours – this is evolutionarily ancient (e.g. distinguish food sources, avoidance of toxins)
Learned?
• However, the experience can strongly modify our innate preferences
• Humans can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)
What are the 5 tastes?
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How does the palate, epiglottis and pharynx and nasal cavity play a role in taste?
Palate
• Roof of mouth separating oral and nasal cavities – taste buds present in palate
Epiglottis
Leaf shaped cartilage covering laryngeal inlet – taste buds present in epiglottis
Pharynx and nasal cavity
Odours can pass, via the pharynx, to the nasal cavity to be detected by olfactory receptors
What are the 3 types of papilla the tongue is lined with?
The surface of the tongue contains papillae:
- Ridge-shaped (foliate)
- Pimple-shaped (vallate)
- Mushroom-shaped (fungiform)
What does the papilla of the tongue contain?
The papillae contain taste buds:
• Taste buds contains taste receptor cells
• Taste buds are surrounded by basal cells (precursors of taste cells) and gustatory afferent axons
Describe how taste receptors work
Taste receptors cells express different types of taste receptors – it has been shown that most taste receptor cells respond primarily (or even exclusively) to one of the five basic tastes.
• Three taste receptor cells sequentially exposed to salt, bitter, sour and sweet stimuli – membrane potential recorded
• Taste receptor cells display different sensitivities
• Taste receptor cells form synapses with gustatory afferent axons to transmit this gustatory information
How are saltiness and sourness transduced
Ion channels
What is the chemical composition of salt?
The prototypical salty chemical is table salt (NaCl) - taste of salt is mostly the taste of the cation sodium (Na+).
What is the transduction mechanism for tasting salt?
- Na+ passes through Na+ selective channels, down its concentration gradient
- This depolarises the taste cell, activating voltage-gated Ca2+ channels (VGCCs)
- Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
Special Na+ selective channel (amiloride sensitive) used to detect low concentrations of salt – insensitive to voltage and generally stays open
What causes the taste of sourness
hydrogen ions
What is the transduction mechanism for sourness?
- H+ may affective sensitive taste receptors in several ways – although these processes are not well understood
- However, it is likely that H+ can pass through proton channels and bind to and block K+ selective channels
- This leads to depolarisation of the taste cell, activating VGSC and VGCCs
- Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
What are the taste mechanisms for bitterness, sweetness and Umami?
GPCR mechanisms via T1 and T2 taste receptors
Describe T1 and T2 taste receptors
Transduction mechanisms underlying bitter, sweet and umami tastes rely on two families of related taste receptor proteins – T1Rs and T2Rs.
T1Rs and T2Rs are G-protein coupled receptors (GPCRs) and are Gq coupled – evidence suggests that they form dimers
1. Bitter substances are detected by approximately 25 T2Rs
2. Sweet substances are detected by one receptor – T1R2 and T1R3 proteins
3. Umami substances are detected by one receptor – T1R1 and T1R3 proteins
What is the tranduction mechanism for bitterness?
- Bitter tastants binds to T2R, which is coupled to the G-protein Gq
- This stimulates the enzyme phospholipase C (PLC), leading to the production of inositol triphosphate (IP3)
- IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from intracellular storage sites
- Both these actions depolarise the taste cell – release of ATP is elicited, and gustatory afferents are activated
What is the transduction mechanism for sweetness?
- Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is coupled to the G-protein Gq
- same signal transduction mechanism as bitterness occurs
Why do we not confuse bitter and sweet tastes?
- Taste cells express either bitter or sweet receptors – not both
- In turn, bitter and sweet taste cells connect to different gustatory axons
Why do we not confuse bitter, sweet and umami tastes?
- Taste cells express either bitter, sweet or umami receptors
- In turn, bitter, sweet and umami taste cells connect to different gustatory axons
What is the flow of taste information to the CNS?
- The main flow of taste information is from taste cells to gustatory axons, into the gustatory nucleus (medulla), up to the ventral posterior medial nucleus (thalamus) and to the gustatory cortex
- Three cranial nerves carry gustatory axons and bring taste information to the brain:
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Is smell innate or learned?
Similarly to taste, some of our smell preferences are inborn (or “innate”)
Learned?
However, experience can strongly modify our innate preferences
How is smell used as a mode of communication?
Pheromones are olfactory stimuli used for chemical communication between individuals.
• In some animals, pheromones are important signals for reproductive behaviours, marking territories and indicating aggression or submission
• However, the importance in pheromones in humans is unclear…
What are the layers of the olfactory epithelium?
Olfactory receptor cells
Site of transduction – genuine neurons unlike taste receptor cells
Supporting cells
Function to produce mucus – odorants dissolve in mucus layer before contacting cilia of olfactory receptor cells
Basal cells
Immature olfactory receptor cells able to differentiate into mature olfactory receptor cells – olfactory receptor cells continuously grow, degenerate and regenerate
What is the transduction mechanism for the olfactory region?
- Odorant molecules bind to odorant receptor proteins on the cilia
- Olfactory-specific G-protein (Golf) is activated
- Adenylyl cyclase activation increases cAMP formation
- cAMP-activated channels open, allowing Na+ and Ca2+ influx
- Ca2+ activated chloride channels open enabling Cl- efflux
- Causes membrane depolarisation of the olfactory neuron
Describe the structure of a olfactory receptor neurone
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Where do olfactory receptor cells send information to?
- Olfactory receptor cells send axons into the olfactory bulb
- Olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb
- Signals are relayed in the glomeruli and transmitted to higher regions of the brain
What is the flow of information of smell to the CNS?
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What is population coding?
In population coding, the responses of a large number of broadly tuned neurons are used to specify the properties of a particular stimulus (e.g. taste, smell).
In gustation and olfaction…
• Gustatory and olfactory receptor cells may express only one specific receptor protein
• However, gustatory and olfactory axons and the neurons they activate in the brain respond more broadly
• Only with a large population of neurons, with different response patterns, can the brain distinguish between specific tastes and smells…
Give an example of population coding
An olfactory example…
• Olfactory receptor cells express a single olfactory receptor protein – each can respond to different odours with differing preferences
• When presented with a citrus smell, none of the three receptor cells can individually distinguish it from the other odours
• However, the brain can distinguish the citrus smell through the combination of responses from all three cells
• It is estimated that humans can discriminate at least one trillion differen
• t combinations of odour stimuli
