L50. Taste and Smell Flashcards
(25 cards)
What can gustation and olfaction help us with?
- Sample the content of food/drink for nutrient content, palatability, toxicity
- Generate sensory perceptions in response to ingested and inhaled chemicals
- Guide appetite, trigger processes for absorbing nutrients
- Avoid poisons and chemical hazards
- Avoid predators
- Find a mate
What are our chemical senses stimulated by?
Chemicals either in the air we inhale (odour) or in food/drink (tastes)
What are the 5 taste receptors?
- Sweet
- Sour
- Salt
- Bitter
- Umami (meaty/savoury/MSG)
How many basic odorant receptor types do we have and what amount are perceived as “pleasant”?
1000 receptor types, with 20% perceived as “pleasant”
How many taste buds do we have?
2000-5000
Where are our taste buds located?
Mostly on the tongue, but also some in palate, pharynx, and epiglottis
- Taste buds are clustered in papillae
What is the type of cell found in taste buds?
Taste cell: detect tastants; type II and type III cells
- Tastants dissolve in saliva and act on taste receptors at the taste pore
- Taste cells are turned over approximately every 20-30 days
What is the taste cell mechanism?
- Usually 1 stimulus type per cell
Type II taste cells
Modality: have GPCR chemoreceptors for sugar, amino acids (umami), or bitter stimuli
Type III taste cells
Modality: Na+ or H+ sensitive cation channels
Vesicle and atypical non-vesicular release of neurotransmitters onto gustatory afferent nerve
- Vesicle release: type III taste cells
- Non-vesicular release: type II taste cells
What is the taste transduction process of Type II taste cells?
Sweet, umami, or bitter tastants:
- Stimulated by sugars and sugar-like molecules
- Activation of T1R receptor complex (sweet and umami) or T2R (bitter)
- Depolarisation and activation of afferent and gustatory nerve via atypical non-vesicular ATP release via CALHM1/3 channels
- Action potential initiated at afferent nerve
What is the taste transduction process of Type III taste cells for acid (sour)?
- H+ enter taste cell via cation channel (Otop1)
- Via electrochemical gradient
- Change in intracellular pH blocks K+ leak
- Combined effect of positive charge influx and blocked of K+ outflow depolarise cell
- Activation of voltage-gated Ca2+ channels
- Vesicle release of serotonin (5-HT) onto afferent gustatory nerve fibre to initiate action potential
What is the taste transduction process of Type III taste cells for salt?
- Na+ enter taste cell via cation channel (ENaC)
- Via electrochemical gradient
- Depolarise cell and initiate action potential
- Activation of voltage-gated Ca2+ channels
- Vesicle release onto afferent gustatory nerve fibre
What is the taste pathway?
–> Taste receptors activate cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus)
–> Synapse onto secondary afferent neurons in the nucleus of solitary tract of medulla
–> Primarily ipsilateral pathway, however, cross-inhibition can occur for sensory contrast and enhancement
–> Synapse onto tertiary afferent neurons in the thalamus
–> Thalamus neurons project to gustatory cortex for taste sensation and perception
–> Chemotopic organisation
What is spiciness sensed by and why?
Spiciness is sensed by heat and pain nerve fibres
This is because spiciness (along with several other qualities of foods) are not true taste sensations because the stimulating chemicals do not interact with specific oral taste receptors
What receptors do capsaicin (in chillis) and piperine (in pepper) activate?
TRPV1
What receptor does menthol (minty/fresh) activate?
TRPM8
What are odorants?
Odorants trigger the sense of smell
- They are inhaled and swirled around in nasal conchae
- They dissolve in watery mucus in roof of nasal cavity
- They are detected by cilia projecting from olfactory sensory neurons
What is olfactory signal transduction?
–> Odorant molecules bind to specific GPCRs on cilia of olfactory sensory neuron
–> Activation of cAMP and opening of ion channels
–> Na+ and Ca2+ influx leads to depolarisation (graded potential)
–> Upon threshold, action potential initiated at axon hillock
How many odorant receptor types do we have?
We have about 1000 different receptor types, but can detect > 10,000 odours
Many odorants activate more than one receptor type (combinatorial logic)
What is the olfactory pathway (olfactory bulb)?
–> Glomeruli receives signals from olfactory receptor neurons and sends them to the mitral and tufted cells
–> Mitral and tufted cells then transmit the information to the olfactory cortex
Activated by input from odour sensory neuron; bypasses the thalamus and projects directly to “olfactory cortex”
- The olfactory pathway is considered to be phylogenetically older than the neocortex and hence never needed the thalamus
What are glomeruli?
Spherical clusters of synaptic input and dendritic architecture, encapsulated by glia
What are mitral and tufted cells?
Encode odour identify and intensity (second afferent neurons)
What is the structure of the olfactory cortex?
Olfactory cortex is not a single structure but multiple brain regions that contribute to odour identification, emotional response, and memory association
What is flavour perceived in the brain as a sum of?
- Input from taste cells; combination of taste receptors generate complex and unique sensory profiles
- Input from odour receptors
- Temperature, texture, spiciness
- Also influenced by appearance
The tympanic membrane separates an air-filled chamber from a fluid-filled chamber
True or false?
False
