The Chemical Senses Flashcards
Main Chemical senses
- Taste
- Smell
- Co2/O2 levels
- Chemical irritants
- Acidity
What do chemoreceptors do?
Chemoreceptors in the arteries of the neck measure Co2 and O2 levels in the blood
How are chemical irritants detected?
Nerve endings in skin/mucous membranes warn us of chemical irritants
How is acidity detected?
Sensory nerve endings in muscle respond to acidity - burning feeling that comes with exercise and O2 debt.
How has taste sense evolved?
Thought that humans innately enjoy sweet flavours and avoid bitter tastes. However, experience can strongly modify our innate preferences
Five basic tastes
- Sweet
- Bitter
- Sour
- Salt
- Umani
What is the umani taste?
The taste of monosodium glutamate
How is ‘flavour’ perceived?
The combination and quantities of the five tastes as well as the texture, temperature and smell of food.
Main organ of taste
Tongue
Other secondary body regions responsible for taste
Palate - roof of mouth containing taste buds that detect flavour
Epiglottis - taste buds present
Pharynx and Nasal Cavity - Odours pass through the pharynx and detected by olfactory receptors in the nasal cavity
Parts of the tongue with specific taste receptors
Front of the tongue - sweet Front sides - Salty Back sides - Acid Back - Bitter All areas of the tongue can detect all tastes just at slightly lower levels.
What are the papillae?
- Fungiform papillae: mushroom shaped
- Folate papillae: ridge shaped
- Vallate papillae: pimple shaped
Structure of the papillae
Contain taste buds embedded into them. In the taste buds is the taste pore that is the chemically sensitive end of the taste bud. Also contains, taste cells which connect and synapse with the gustatory afferent axons that transmit taste information to the brain
Which cells transmit taste information to the brain?
The gustatory afferent axons
What are taste receptors and how were they investigated?
Taste receptors are the cells that respond primarily (or even exclusively) to one of the five basic tastes.
Experiment:
- Each taste cell attached to a microelectrode.
- The taste bud is sequentially exposed to salt (NaCl), bitter, sour and sweet stimuli then the membrane potential recorded.
- The taste receptor cells display different sensitivities.
- Taste receptor cells form synapses with gustatory different axons to transmit this gustatory information. Once the stimulus binds to the taste receptor, there is a firing of action potentials.
Which tastes have ion channel mechanisms?
Saltiness and Sourness both have ion channel mechanisms
What is the taste transduction mechanism for saltiness?
- Na+ passes through the sodium selective channels (amiloride sensitive channels), down its concentration gradient
- Depolarises the taste cells due to the influx of sodium as more positive ions into the cell activating the VGSCs and also activating the VGCCs.
- This causes the release of vesicular neurotransmitters which fuse with the cell membrane and are released by exocytosis. They contain gustatory afferents that transmit the signal to the brain.
What triggers the saltiness transduction mechanism?
It is triggered by the influx of sodium ions into a special Na+ selective channel called the amiloride sensitive channel.
Reason behind the name for amiloride sensitive channels
Amiloride is a diuretic drug that antagonises the receptor and is used to detect low concentrations of salt. It is also insensitive to voltage and generally stays open.
What are the determinants of acidity and sourness?
Protons (H+)
Transduction mechanism of sourness taste
- H+ passes through the same Na+ selective channels that mediate saltiness, down its concentration gradient.
- H+ also binds to and blocks K+ selective channels.
- The action of both of the above depolarises the taste cell, activating VGCCs.
- This causes vesicular release of neurotransmitter and gustatory afferents are activated.
Which tastes work through GPCR mechanisms via T1 and T2 taste receptors?
Bitterness
Sweetness
Umani
What is T1R and T2R?
They are G-protein-coupled receptors (GPCRs) and are Gq-coupled (maybe forming dimers).
Which receptors detect bitter substances?
Detected by approximately 25 T2Rs. There is approximately 25 variants for the bitter receptor.
Which receptors detect sweet substances?
Detected by one receptor - T1R2 and T1R3 proteins.
Which receptors detect umani substances?
Detected by one receptor - T1R1 and T1R3 proteins
defers from a combination of subunits, it varies from the umani taste as the second subunit is different.
Bitterness Transduction Mechanism
- Bitter tastants bind to T2 receptors which is Gq coupled.
- This activates the enzyme phospholipase C which converts PIP2 into IP3 and DAG.
- IP3 activates sodium ion channel and releases calcium from endoplasmic reticulum
- Depolarises the cell
- Causes release of ATP and gustatory afferents activated to transmit to regions of the brain
Sweetness Transduction Mechanism
- Sweet tastants bind to dimer receptor formed by T1R2 and T1R3 which is Gq coupled.
- This activates phopsholipase C which converts PIP2 into IP3 and DAG
- IP3 activates Na+ channel and releases calcium from the channel
- Depolarises the cell
- Causes release of ATP and gustatory afferents activated to transmit to regions of the brain
Umani Transduction mechanism
- Umani tastants bind to dimer receptor formed by T1R1 and T1R3, which is Gq coupled.
- Same signal transduction mechanism as bitterness and sweetness, just a different receptor.
Why do we not confuse bitter, sweet or umani tastes?
- Taste cells express either bitter, sweet or umani receptors. They connect to different gustatory axons not all of them.
What is the flow of taste information to the CNS?
Happens via various cranial nerves from different taste bud regions.
- Anterior tongue (CN VII), posterior tongue (CN IX), epiglottis taste receptors (CN X) work via different cranial nerves.
- These contact the gustatory nucleus in the medulla (brain stem).
- A message is passed to the thalamus, specifically the ventral posterior medial nucleus.
- This is then passed to the gustatory cortex which allows the perception of taste
Another name for smell
Olfaction
Is smell innate or learned?
Some of our smell preferences are innate, however, experiences can be strongly modify our innate preferences
Phermones
Olfactory stimuli used for chemical communication between individuals
Phermones in animals
Important signals for reproductive behaviour, marking territories and indicating aggression or submission. However, the importance in humans is unclear.
What do we smell with?
Small, thin sheet of cells high up in the nasal cavity called the olfactory epithelium
What does the olfactory epithelium contain?
- Olfactory receptor cells
- Supporting cells
- Basal cells
Function of olfactory receptor cells
Site of signal transduction - geniune neurons unlike taste receptor cells.
Function of supporting cells in olfactory epithelium
Function like glia cells and help produce mucus. Where odorants can bind to the cilia of olfactory cells allowing mucus production to happen.
Function of basal cells in olfactory epithelium
Immature olfactory receptor cells so are a source for new cells
Olfactory transduction Mechanisms
- Odorant molecules bind to odorant receptor proteins on the cilia
- Olfactory-specific G-protein (Golf) is activated.
- It activates adenylyl cyclase (as part of the Gs pathway) increasing cAMP formation.
- cAMP activated channels open allowing Na+ and Ca2+ influx
- Ca2+ activated chloride channels open enabling Cl- efflux. Chloride moves out of the cell unlike in other cells where it goes in.
- This causes membrane depolarisation of the cell.
How do olfactory receptor neurons work?
- Odorants bind to the cilia and this generates a slow receptor potential in the cilia.
- The receptor potential propagates along the dendrite and triggers a series of action potentials within the olfactory receptor cell soma.
- The action potentials propagate continuously along the olfactory nerve axon and are transmitted to the brain.
What is the central olfactory pathways?
- The olfactory receptor cells send axons into the olfactory bulb.
- The olfactory receptor cells expressing the same receptor proteins project to the same glomerulus in the olfactory bulb.
- Signals are relayed in the glomeruli and transmitted to higher regions of the brain.
What does the olfactory bulb do with information from the olfactory cells?
- Message is transmitted to the frontal cortex that is used for conscious perception of smell. Also known as the olfactory cortex.
- Hippocampus involved in memory so important for odour memory
- Hypothalamus and amygdala for motivational and emotional aspects of smell
What is population coding for gustation and olfaction?
- The response of a large number of broadly tuned neurons are used to specifiy specific properties of a particular stimulus.
- In gustatory and olfactory receptor cells, they can only express one specific receptor protein.
- The receptor cells connect to axons and the neurons activate areas more broadly than the receptor cells themselves.
- Only a large population of neurons with a different response pattern will allow the brain to distinguish between specific tastes and smells.
Describe the citrus smell
The brain can distinguish the citrus smell only when combined with smell pattern response patterns from three other olfactory receptor cells.
