HNS30 Smell And Taste Flashcards
Flavour
Mixture of:
- Smell (Olfaction)
- Taste (Gustation)
- Tactile sensation (Texture)
Brain:
Network of regions responsible for taste/odour integration + flavour perception
—> Independent presentation of tastant / odorant
—> produces overlapping activation in **Insula, **Operculum, **Orbitofrontal cortex, **Anterior cingulate cortex
—> Insula, Operculum, Orbitofrontal cortex, Anterior cingulate cortex also sensitive to somatosensory stimulation of oral cavity
—> allow integration between 3 senses
Smell (Olfaction)
6 odour qualities (NOT used anymore):
- Floral (roses)
- Ethereal (pears)
- Musky (musk)
- Camphor (eucalyptus)
- Putrid (rotten eggs)
- Pungent (vinegar)
Human:
- can recognise approximately 10,000 different odours
- 1000 olfactory receptor genes —> 390 code for functional receptor protein (others: noncoding pseudogenes)
- decline in functional receptor protein offset by enlarged brains with more capacities for complex processing and analysis of smell —> Flavour perception + Behavioural responses to smell
Olfactory chemoreceptor cells location
- Olfactory epithelium in ***upper part of nasal cavity (Other parts: Squamous mucosa, Respiratory mucosa)
Olfactory chemoreceptor cells
- Neuron on its own (Bipolar neuron)
- Contain ***Cilia
- Each receptor cell expresses ONLY 1 type of odorant receptor
- Each receptor can detect limited number of odorant substances
- Single odorant chemical can be recognised by multiple olfactory receptors
—> one chemical structure trigger firing from a combination of receptors
—> different chemical structures
—> firing from distinct combinations of receptors - Each cell send axons to Olfactory bulb via holes in Cribriform plate
—> Converge on precise region (Olfactory glomerulus) in Olfactory bulb
Olfactory pathway
Odorant molecules diffuse into air
—> pass to Olfactory epithelium
—> dissolve in epithelium mucus
—> bind to specific **odorant receptors on cilia of Olfactory receptor cell
—> several odorant molecules induce **depolarisation of an Olfactory receptor cell
—> neural discharge
—> heterotrimeric **G protein (Golf) activation
—> 2nd messenger generation
—> ↑ Na + Ca conductance —> influx
—> firing of distinct **combination of Olfactory receptor cells (create different smell perception)
—> cells bearing same receptor converge on a precise region in Olfactory bulb (same glomeruli) (一種receptor cell去一種glomeruli)
—> synapse with secondary neurons (Mitral cells)
After synapsing:
—> **particular glomeruli fire
—> **Spatial map (firing of glomeruli at specific positions in Olfactory bulbs)
—> Olfactory tract to Olfactory cortex (have map) (Piriform cortex, Amygdala, Entorhinal cortex)
—> other structures (limbic system, learning, memory, behaviour, emotion)
Patterns of neuron activity: Coding olfactory information
Strength of odorant: Overall amount of afferent neuron activity (frequency of neural discharge)
Olfactory bulb
- Location of secondary neurons
- 5600 glomeruli
- large number of glomeruli + small number of functional receptor genes —> ***more number of glomeruli processing information from each odorant receptor type
(- Tufted cell
- Mitral cell
- Granule cell: modulatory interneuron)
Olfactory signaling
Subject to autonomic modulation
—> BOTH ↑ responses of olfactory receptor
Sympathetic: NA
- ↑ Contrast of odorant (filter out weak response + amplify strong response)
—> ↑ focus on strongest / salient olfactory signals
—> beneficial in times of stress / when focus is required
Parasympathetic: ACh
- Broadly ↑ responses to many odorants
—> ↑ appreciation for richness / complexity of olfactory signals
—> beneficial during feeding
Taste (Gustation)
5 taste qualities:
- Sweet
- identification of energy-rich nutrient - Umami
- sensation elicited by Glutamate: flavour enhancer in MSG
- recognition of L-amino acids - Bitter
- warn against intake of potentially noxious / poisonous chemicals - Salty
- ensures proper dietary electrolyte balance - Sour
- warn against intake of potentially noxious / poisonous chemicals
Taste detectors
Taste receptor cells (TRC)
—> assembled into taste buds
—> located within Papillae (Fungiform, Circumvallate, Foliate) (Filiform NO taste buds)
NOT neurons itself!!! —> need to synapse with afferent neurons
Gustatory pathway models
Labelled-line model:
- each taste quality specified by activity of non-overlapping cells
- TRC tuned to respond to single taste
- TRC innervated by individually tuned nerve fibres
- supported by data
Across-fibre model:
- each TRC tuned to respond to multiple tastes / one taste
OR
- same afferent nerve fibre innervate multiple TRCs —> carrying information for >= 1 tastes
Bitter / Sweet / Umami pathway
Chemicals from food dissolve in saliva
—> contact taste cells through taste pore
—> Bitter / Sweet / Umami tastants activate **G protein through interaction with specific receptors on **microvilli of taste cells
—> 2nd messengers generation
—> depolarisation of taste cells
—> release of NT
—> stimulate afferent neurons connected to taste cells to relay electrical signals
Sweet / Umami
- mediated by small family of 3 GPCR: T1R1, T1R2, T1R3
- GPRC form homodimeric / heterodimeric receptor complexes
- Sweet —> T1R2 + T1R3 heterodimer (記: 甜味5)
- Umami —> T1R1 + T1R3 heterodimer (記: 味精4)
Bitter
- T2R receptors (confirmed) (記: 苦味2)
Binding to T1R / T2R
—> activate heterotrimeric G protein Gustducin / Gαi2
—> release of Gβγ subunits
—> subsequent stimulation of phospholipase C-β2 (PLC-β2)
—> hydrolyse **phosphatidylinositol-4,5-biphosphate
—> produce intracellular messenger **IP3, DAG
—> gating of ***taste-transduction channel (transient receptor potential protein TRPM5)
—> change in membrane potential
Salty / Sour pathway
Chemicals from food dissolve in saliva
—> contact taste cells through taste pore
Compounds that elicit salty / sour are **ions
—> act directly through **ion channels
—> depolarise taste cells
Salty:
- ***ENaC suggested to be low salt receptor
- high salt receptor unknown
Sour:
- Ion channel proteins **PKD2L1 (polycystic kidney disease 2-like 1 protein) + **OTOP1 (otopetrin 1) suggested to be sour taste receptors
- actual identity still unknown
Taste in Brain
- Labelled-line model in periphery
- Each gustatory neuron respond most strong to 1 tastant
—> also generates response to >=1 stimuli with dissimilar taste qualities - Potential existence of “Gustotopic map”
—> distinct clusters of neurons in Gustatory cortex responding to bitter, sweet, salty and umami - Also record other attributes of chemical stimuli e.g. Intensity of taste, Emotional attributes (pleasant / unpleasant / neutral)
(Gustatory cortex: deep to temporal and frontal lobe, along central sulcus —> Area 43 —> deep within Insula + Frontal Operculum)
