Smell and Taste (James) Flashcards
What are chemoreceptors?
Receptors that generate a signal when they bind to chemicals in the external environment
What is olfaction and gustation?
Olfaction:
- Information about airbourne molecules (odorants)
Gustation:
- Information about ingested substances (chemical and physical qualities)
What is chemosensation used for?
Lower organisms:
- Avoidance and seeking
Higher organisms:
- Stimulating the gastrointestinal system
- Detecting qualify of food sources
- Detecting nutritional benefit or toxicity (foods)
What is chemosensation threshold?
Threshold for required for olfactory sensation
Threshold varies between different molecules:
- Ethanol 2mM
- 2-trans-6-cis nonadenial 0.07nm
The interpretation of a smell can be concentration dependent
Natural odours = combination of different molecules
How does olfaction actually work?
Olfaction is a reflection of the pattern of different cells that are activated by the different molecules (across-fibre pattern coding) interpreted at higher centres in the CNS
Where are sensory receptors located?
They are located in the olfactory epithelium, which is located in the roof of the nasal cavity
Where do the olfactory afferent fibres project into?
The olfactory afferent fibres project directly to the olfactory bulb in the central nervous system
Processes of the olfactory sensor neurones project through channels within the cribriform plate (bone) to get to the olfactory bulb within the CNS
Where are odorant sensory receptors located on the sensory receptors?
They are located in the cilia, not the soma
What is the olfactory receptor neurones structure?
Olfactory neurons are bipolar
Umyelinated sensory afferent
Has specialised cilia which are embedded within a mucus layer
Mucus concentrates the chemicals and brings them into contact with the cilia
Mucus layer is produced by the Bowman’s gland
Olfactory neurons are prone to damage – last 6-8 weeks
How are GCPRs used in the olfactory system?
GCPRs are embedded in the membrane of the cilia within the Olfactory Sensory Neurones
The GCPRs have regions of conserved amino acids and variable amino acids so that they are able to detect a range of molecules
The role of GPCRs as olfactory receptors is evolutionarily conserved
each ORN (olfactory receptor neuron) expresses 1 receptor gene (will express 1 specific GCPR per neuron)
What happens after odorant binding?
Upon odorant binding, the GPCR undergoes a conformational change, leading to the activation of G proteins (specifically Gα-olf)
The activated G protein then activates adenylyl cyclase III, which generates cyclic AMP (cAMP) from adenosine triphosphate (ATP)
The increase in cAMP levels leads to the opening of cyclic nucleotide-gated (CNG) ion channels in the cell membrane of the olfactory sensory neuron
This results in the influx of cations, particularly calcium (Ca2+) and sodium (Na+), into the cell
The influx of positive ions depolarizes the olfactory sensory neuron, generating an action potential
The Ca2+ also activates a Ca2+ gated Cl- channel which removes chloride from the cell and will further enhance the depolarisation
Do ORNs respond the same to all stimuli?
No, individual ORNs are sensitive a subset of stimuli
What is across fibre pattern coding?
“Across fibre pattern coding” is a mechanism by which the olfactory system encodes and interprets different odours
This coding strategy involves a distributed and combinatorial pattern of activity across multiple olfactory neurons
Each type of odour molecule activates a unique combination of olfactory receptors and neurons, creating a distinct pattern or “fingerprint” that the brain interprets as a specific odour
This allows the olfactory system to distinguish between a vast array of different odours using a relatively limited number of receptors
What does the olfactory bulb look like under microscope?
As pictured, in B, blue spots are present which are glomeruli, which are regions where synaptic connections are being made
In C, mitral cells are present, which are sending their apical dendrites down into the region in which the glomeruli are located
What are the layers of the olfactory bulb?
MITRAL CELL LAYER (olfactory tract)
GLOMERULI LAYER (convergence and amplification)
RECEPTOR LAYER
What is convergence of signals on glomeruli for amplification?
Experiments have shown that olfactory neurons expressing the same odorant receptor send their projections to the same bilateral glomeruli
A single glomerulus can contain dendrites from up to 25 mitral cells and 25,000 olfactory receptor neurones
The axons of the mitral cells project from the olfactory bulb to accessory olfactory nuclei
Electrical patterns in molecular encoding
Relationship between one type of odorant neurone and one glomeruli enables specific regions of the olfactory bulb to respond to different chemicals
Different odours and chemicals will activate a unique spatial pattern in the olfactory bulb depending upon their chemical composition
What is the route of central processing of the olfactory system?
Olfactory receptors –> Olfactory bulb
Olfactory bulb –> Olfactory bulb targets accessory structures
- (Piriform cortex, olfactory tubercle, amygdala, entorhinal cortex)
The accessory targets then relay to peripheral cortex structures such as:
- (Orbitofrontal cortex, thalamus, hypothalamus, hippocampal formation)
In humans the major target of the lateral olfactory tract is the piriform complex
Structure of the tongue
The surface of the tongue has specialised invaginations called taste papillae
There are 4 different types of papillae:
- Filiform papilla
- Circumvallate papilla
- Fungiform papilla
- Foliate papilla
- Each Fungiform has ~3 taste buds on the apical surface
- Each Circumvalate has ~250 taste buds in the trench
- Folliate are organized into parallel ridges with around 600 taste buds
- Filiform have no taste buds
What is the position of taste buds within the circumvallate papilla?
Circumvallate contains deep troughs
At the base of the troughs are where the taste buds are located
Taste buds are structures that contain taste receptors
Located in troughs so food molecules are concentrated down onto the taste buds
Concentration of reactants is in the millimolar range, which is more than required in olfactory (nanomolar), meaning gustation is less sensitive that olfactory
What are the 5 basic tastes?
Bitter
Sour
Sweet
Salty
Umami (meaty)
Taste bud structure
A single taste bud can contain up to 50 specialised epithelial cells (taste cells)
The tips of the cells have microvilli which increases surface area
Microvilli come together at the taste pore - which concentrates chemicals onto the receptor proteins
Taste receptor proteins are located in microvilli
Regeneration occurs via basal cells
Taste cell structure?
Contains 2 key domains:
- Apical (external environment)
- Basolateral (taste bud)
Apical microvilli contains receptor proteins:
- Ion channels: salt & sour/acids
- GPCR: sweet, bitter, umami
Basolateral surface contains ion channels and cellular machinery required to release neurotransmitter
Basolateral neurotransmitter released synapses onto cranial nerves:
- The facial (VII)
- Glossopharyngeal (IX)
- Vagus (X))
Neurotransmitters are Serotonin and ATP
How does signal transduction occur in taste cell?
SALT AND ACIDS:
- Ions move into the cell, causing a depolarisation
- If the depolarisation reaches threshold, voltage gated sodium ion channels are activated
- Influx of sodium activates voltage gated calcium channels
- Influx of calcium leads to release of neurotransmitter
SWEET, BITTER AND UMAMI:
- Signal through second messenger cascade
- Leads to activation of TRPM5 ion channel
- This leads to depolarisation and release of Ca+ from endoplasmic reticulum
What ion channels are used in salt and sour sensing?
SALT:
- Amiloride-sensitive Na+ channels
- Sodium moves down the concentration gradient into the cell
ACIDS:
- H+-sensitive TRP channels
- H+ ions move down the concentration gradient into the cell
- Other method is H+ blocking K+ leak channel, leading to depolarisation of the cell
Both main methods lead to direct depolarisation of the taste cell
Sweet and umami sensors?
Require T1R receptors
T1Rs are GPCR
- Seven transmembrane domain
- N-terminal extracellular
- C-terminal intracellular
- Large N-terminal domain
T1Rs can form heterodimers
The ligands that the T1Rs bind to varies depending on the combination
Different T1R heterodimers encode sweet and umami
Bitter sensors?
Use T2R receptors which is also a GPCR
T2Rs are not present in taste cells that express T1R subtypes
There are many T2R subtypes that recognise different combinations of ligands
Sequence diversity enables the detection of many different bitter compounds
What is the sweet and umami signalling pathway?
Uses heterodimer GPCR T1R receptors
SWEET = T1R2 and T1R3
UMAMI = T1R1 and T1R3
The T1R receptor heterodimer signals to a g protein cascade
Leads to the activation of phospholipase c beta 2 (PLC Beta2)
PLC2B2 then hydrolyses PIP2 into IP3
IP3 binds to its receptors on the endoplasmic reticulum, causing a release of Ca²⁺ ions into the cytoplasm
The increase in cytoplasmic Ca²⁺ activates the TRPM5 channel, causing further influx of Ca2+
The cell then depolarises
Alternative umami signalling pathway?
Uses mGluR4 receptor (m = metabotrophic = GPCR)
- Brain mGluR4 and taste mGluR4 present, taste has much shorter n-terminal
Expressed in mouse taste cells
Produces functional responses to glutamate in CHO cells
Half maximal effective concentration of glutamate is 0.3mmol/L which is consistent with a concentration range for a taste stimulus
Rats respond to the mGluR4 agonist L-AP4 in the same way as glutamate and they taste similar
Inhibits cAMP signaling
Bitter signalling pathway
Same pathway as sweet and umami, however, GPCR is T2Rs
Uses specific G-protein called alpha-gustducin (bitter taste cells only)
What cranial nerves are used in processing of gustation?
Cranial nerve VII: facial nerve and Chorda Tympani (Tongue and palate)
Cranial nerve IX: lingual branch of the glossopharyngeal nerve (Back of tongue)
Cranial nerve X: superior laryngeal branch of the vagus nerve (epiglottis, oesophagus)
What do the cranial nerves do in gustation processing?
Take signals from different regions of the tongue and project into the nucleus of the solitary tracts (gustatory nucleus) which is located in the medulla
There is organisation of the projections, some mapping taking place
Cranial nerves have a topographical organisation in the gustatory nucleus
Cranial nerve VII projects to rostral region
Cranial nerve IX projects to mid region
Cranial nerve X projects to caudal region
The nucleus of the solitary tract facilitates the integration of sensory information about taste and visceral sensory information
Where do projections from the solitary nucleus project to?
Projections from the rostral part of the solitary nucleus project to the thalamus, where they terminate in the medial half of the Ventral Posterior Medial Nucleus
The VPMN projects to the:
- Anterior insula in the temporal lobe (called: insular taste cortex)
- Operculum, which is a secondary taste centre located in the orbitofrontal cortex located in the frontal lobe
Gustation signals go to thalamus first, then to cortical structures (unlike in olfaction, in which cortical structures are before the thalamus)
Orbitofrontal cortex gives us our perception of food, it is also involved in signalling satiety (feeling full)
