Chemical senses Flashcards
How does our body detect CO2/O2 levels?
chemoreceptors in arteries of the neck measure CO2/O2 levels in blood
How does our body warn us of chemical irritants?
nerve endings in skin/mucous membranes warn us of chemical irritants
How does our body detect acidity?
sensory nerve endings in muscle respond to acidity, giving us that burning feeling that comes with exercise and O2 debt
What are the different tastes?
Sweet Sour Salt Bitter Umami (monosodium glutamate taste)
How do we perceive flavour?
Taste (combination of all 5) of food
Touch, texture and temperature of food
Smell of food
What is the main organ of taste?
Tongue
What are the Taste organs other than the tongue?
Palate
-contains taste buds which detect flavour
Epiglottis
-contains taste buds which detect flavour
Pharynx and Nasal Cavity
-odours can pass via the pharynx to the nasal cavity to be detected by olfactory receptors
Describe the Concentrations of taste receptors.
Whilst there are particular concentrations of taste receptors in different regions, all areas of tongue can detect all tastes at slightly lower levels
What are Papillae?
structures on the tongue containing taste buds
What are the Types of papillae?
- Fungiform papillae: mushroom shaped
- Foliate papillae: ridge shaped
- Vallate papillae: pimple shaped
Describe the Cellular structure of a papillae.
- Papillae contain taste buds
- Taste buds contain taste pore (chemically sensitive end of taste bud)
- Taste pores contain taste cells (connect and synapse with gustatory afferent axons which transmit information to the brain)
What are the Mechanisms of taste transduction?
Saltiness& Sourness
-ion channel mechanisms
Bitterness, Sweetness & Umami
-GPCR mechanisms
Run through the Saltiness transduction pathway.
Salt (Na+):
1) Na+ passes through Na+ selective channels down its concentration gradient
2) This depolarises the taste cell, activating voltage-gated Ca2+ channels (VGCCs)
3) Vesicular release of neurotransmitter and neurotransmitter exits membrane via exocytosis
4) Gustatory afferents are activated and transmit the signal to the brain
Run through the Sourness transduction pathway.
Sour (H+):
1) H+ can pass through the same Na+ selective channels that mediate saltiness, down its concentration gradient
2) Once H+ inside the cell, H+ can also bind to and block K+ selective channels, preventing efflux of K+
3) Influx of H+ and prevention of K+ efflux cause depolarisation of the taste cell, activating VGCCs
4) Vesicular release of neurotransmitter and neurotransmitter exits membrane via exocytosis
5) Gustatory afferents are activated and transmit the signal to the brain
What do transduction processes underlying bitter, sweet & umami rely on?
G-protein coupled receptors (Gq)
-T1Rs & T2Rs
*evidence suggests they form dimers
What are bitter substances detected by?
~25 T2Rs
What are sweet substances detected by?
one receptor made of T1R2 & T1R3 proteins
What are umami substances detected by?
one receptor made of T1R1 & T1R3 proteins
Run through the Bitterness transduction pathway.
1) Bitter tastants bind to T2 receptor (~25 of these), which is Gq coupled
2) Stimulate enzyme phospholipase C (PLC), which converts PIP2 to IP3 (and DAG)
3) IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from the endoplasmic reticulum
4) Activation of Na+ ion channel and release of Ca2+ from ER depolarises the taste cell
5) ATP released through an ATP-permeable channel
6) ATP binds to the gustatory afferent and activated it, transmitting the signal to the brain
Run through the Sweetness transduction pathway.
1) Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is Gq coupled
2) Stimulate enzyme phospholipase C (PLC), which converts PIP2 to IP3 (and DAG)
3) IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from the endoplasmic reticulum
4) Activation of Na+ ion channel and release of Ca2+ from ER depolarises the taste cell
5) ATP released through an ATP-permeable channel
6) ATP binds to the gustatory afferent and activated it, transmitting the signal to the brain
Run through the Umami transduction pathway.
1) Umami tastants bind to dimer receptor formed from T1R1 and T1R3, which is Gq coupled
2) Stimulate enzyme phospholipase C (PLC), which converts PIP2 to IP3 (and DAG)
3) IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from the endoplasmic reticulum
4) Activation of Na+ ion channel and release of Ca2+ from ER depolarises the taste cell
5) ATP released through an ATP-permeable channel
6) ATP binds to the gustatory afferent and activated it, transmitting the signal to the brain
Why do we not confuse bitter, sweet and unami tastes?
Taste cells express either bitter, sweet or unami receptors.
In turn bitter, swett and unami taste cells connect to differe glustatory axons.
Describe the Central Gustatory Pathways.
Taste signal transmission to brain via cranial nerves from different regions:
Anterior tongue taste receptors
-CN VII
Posterior tongue taste receptors
-CN IX
Epiglottis taste receptors
-CN X
These cranial nerves contact the gustatory nucleus in the medulla. A message is then passed on to the ventral posterior medial nucleus in the thalamus. Message is then passed to a region in the cortex known as the gustatory cortex, which allows us to perceive our taste.
What is Olfaction?
sense of smell
What are Pheromones?
olfactory stimuli used for chemical communication between individuals
What is the Main organ of olfaction?
Olfactory epithelium, comprised of:
· Olfactory Receptor Cells
> Site of signal transduction- genuine neurones unlike taste receptor cells
· Supporting Cells
> Function like glial cells and help produce mucus
> Mucus layer allows odourants to bind to cilia of olfactory cells
· Basal Cells
> Immature olfactory receptor cells- source of new olfactory receptor cells
Where is the olfactory epithelium located?
roof of nasal cavity
Describe the Olfactory transduction pathway.
1) Odourant molecules bind to odourant receptor proteins (G-protein) on the cilia
2) Olfactory-specific G-protein (Golf) is activated
3) Adenylyl cyclase activation increases cAMP formation
4) cAMP-activated channels open, allowing Na+ and Ca2+ influx
5) Ca2+ activated chloride channels open, enabling Cl- efflux
6) Causes membrane depolarisation of the cell
What happens after depolarisation of olfactory receptor cell?
slow receptor potential in cilia generated which propagates along the dendrite and triggers a series of action potentials within the olfactory cell soma
action potentials then propagate continuously along the olfactory nerve axon and transmitted to the brain
Describe the Central Olfactory Pathways.
Olfactory receptor cells send axons (CN I) into glomeruli in the olfactory bulb
-glomerulus is a collection of olfactory axons in the bulb
once signals have reached the glomerulus, they are delayed in the glomeruli and transmitted to higher regions of the brain to detect smell:
- Frontal (Olfactory) Cortex: used for the conscious perception of smell
- Hippocampus: important in odour memory
- Hypothalamus and Amygdala: used for motivational and emotional aspects of smell
What is Population coding?
neural representation of a stimulus by the pattern of firing of a large number of neurones:
In gustation and olfaction:
· Gustatory and olfactory receptor cells may express only one specific receptor protein
· However, those receptor cells then connect to gustatory or olfactory axons, and these neurones in the brain respond more broadly than the receptor cells themselves
· Only with a large population of neurones, with different response patterns to particular stimuli, can the brain then distinguish between specific tastes and smells
