8. CHEMICAL SENSES Flashcards
Define gustation
- Gustation refers to taste
Is taste innate or learned?
- Most of our taste preferences are innate such as a liking of sweet & avoidance of bitterness due to evolutionary reasons
- However, we can learn to like tastes such as coffee
What organs are involved in taste?
- Taste is a primary function of the tongue but other organs such as the palate, epiglottis, pharynx & nasal cavity are involved
What is the palate?
- The palate refers to the roof of the mouth which separates the oral cavity from the nasal cavity
- Taste buds can be located in the palate
What is the epiglottis?
- The epiglottis is the leaf shaped structure that covers the laryngeal inlet & prevents food from entering the windpipe when swallowing
What are the three types of papillae found on the tongue?
- FOLLIATE - Ridge shaped
- VALLATE - Pimple shaped
- FUNGIFORM - Mushroom shaped
What are the four components of taste buds?
- TASTE RECEPTOR CELLS
- BASAL CELLS
- TASTE PORE
- GUSTATORY AFFERENTS - transmit information to other areas
What are basal cells of the taste bud?
- Precursor to taste receptor cells
What is the taste pore?
- The taste pore is the chemically sensitive end of the taste receptor cells that interacts with the tastants
- Microvilli protrude from the taste pore to interact with the tastants
What are taste receptor cells?
- Taste receptor cells express different types of receptor, predominantly one
- The taste receptor cells can become depolarised & the electrical signal can be passed on to gustatory afferents
What are the 5 types of taste?
- SALTINESS
- SOURNESS
- BITTERNESS
- UMAMI
- SWEETNESS
What are the two types of transduction mechanisms for taste?
- ION CHANNEL - Saltiness, sourness
2. GPCR MECHANISMS - Bitterness, sweetness, umami
Describe the steps of the transduction mechanism for SALTINESS
- Salt is detected as Na+
1. Na+ enters the taste cell via Na+ selective channels (Amiloride channels) down the concentration gradient
2. Na+ influx causes depolarisation of the taste cell
3. This activates voltage gated Ca2+ & Na+ channels
4. Na+ & Ca2+ causes vesicular release of neurotransmitter serotonin
5. Gustatory afferents are activated & pass on signals to other regions
What are amiloride cahnnels?
- Amiloride channels are Na+ selective channels found on taste receptor cells
- they can detect low levels of Na+ and are insensitive to voltage
Describe the steps for the transduction mechanism of SOURNESS
- Sourness is detected as H+
1. H+ enters the taste cell via proton channels down the concentration gradient
2. H+ binds to & blocks K+ channels
3. The influx of H+ causes membrane depolarisation
4. Depolarisation activates voltage gated Ca2+ & Na+ channels
7. Na+ & Ca2+ influx causes vesicular release of neurotransmitter
8. Gustatory afferent axons are activated
Which three types of taste use GPCR?
- Bitterness
- Sweetness
- Umami
What are the two classes of GPCR taste receptors?
- T1R
- T2R
- These receptors also have sub-types but they are G-protein coupled
- They are coupled to Gq = Phospholipase C
What are the GPCR Taste receptors for bitterness?
- T2R
- Multiple sub-types, could be an evolutionary adaptation to avoid bitter tastes that could potentially be poison
What are the GPCR Taste receptors for umami?
- T1R1-T1R3
- The receptor is a dimer
What are the GPCR taste receptors for sweetness?
- T1R2-T1R3
- The receptor is a dimer
Which taste receptor sub-unit differentiates between sweetness & umami?
- Both sweetness & umami have a T1R3 sub-unit but the T1R1 or T1R2 is what distinguishes them
- T1R = umami
- T1R2 = sweetness
**What is the transduction mechanism for BITTERNESS?
- Bitter tastant binds to T2R which is a G-protein coupled receptor associated with PLC
- Gq activates PLC which converts PIP2 -> IP3 + DAG
- IP3 causes an intracellular increase in Ca2+ AND acts on Na+ channels specific to taste cells
- Influx of Ca2+ & Na+ causes membrane depolarisation which releases ATP. ATP passes through ATP permeable channel
- Gustatory afferents activated
**What is the transduction mechanism for SWEETNESS?
- Sweet tastant binds to T1R1-T1R3 dimer which is a G-protein coupled receptor associated with PLC
- Gq activates PLC which converts PIP2 -> IP3 + DAG
- IP3 causes an intracellular increase in Ca2+ AND acts on Na+ channels specific to taste cells
- Influx of Ca2+ & Na+ causes membrane depolarisation which releases ATP. ATP passes through ATP permeable channel
- Gustatory afferents activated
**What is the transduction mechanism for UMAMI?
- Umami tastant binds to T1R2-T1R3 dimer which is a G-protein coupled receptor associated with PLC
- Gq activates PLC which converts PIP2 -> IP3 + DAG
- IP3 causes an intracellular increase in Ca2+ AND acts on Na+ channels specific to taste cells
- Influx of Ca2+ & Na+ causes membrane depolarisation which releases ATP. ATP passes through ATP permeable channel
- Gustatory afferents activated
Why don’t we confuse bitterness sweetness & umami if they all have the same transduction mechanism?
- Although bitterness, sweetness & umami have the same transduction mechanism they have different receptors
- The tastants would bind to different receptors depending of whether they’re bitter, sweet or umami
- The taste receptor cells also express different receptors
- Different gustatory afferents would also be activated depending on the taste
Which three cranial nerves are involved in gustation?
- CRANIAL NERVE 7,9 & 10 synapse with the gustatory nucleus to detect taste
- Cranial nerve 7 (facial nerve) = detection of taste in the anterior tongue
- Cranial nerve 9 (glossopharyngeal) = detection of taste in the posterior tongue
- Cranial nerve 10 (vagus nerve) = from the epiglottis
**Describe the pathway of taste information from the cranial nerves to the brain?
Cranial nerves -> Gustatory nucleus -> Ventral posterior medial nucleus (thalamus) -> Gustatory cortex
Define olfaction
- Olfaction refers to the sensation of smell
What are pheromones?
- Pheromones are olfactory stimuli used for connection between individuals
What are the three cells of the olfactory epithelium?
- the olfactory epithelium is found within the nasal cavity
- The olfactory epithelium contains cilia which interact with the odorants
- It consists of three cell types:
1. BASAL CELLS
2. SUPPORTING CELLS
3. OLFACTORY RECEPTOR CELLS
What are olfactory receptor cells?
- Olfactory receptor cells are located in the olfactory epithelium of the nasal cavity
- It is the site of transduction for olfaction, and has genuine neurones which project into the CNS
What are the basal cells of the olfactory epithelium?
- The basal cells are the immature olfactory receptor cells
- They are one of the only type of neural cells that can regenerate
What are the supporting cells of the olfactory epithelium?
- The supporting cells of the olfactory epithelium produce mucus containing antibodies & enzymes
- The odorants dissolve in the mucus before contacting the cilia of olfactory receptor cells
Describe the steps for the transduction mechanism for OLFACTION
- Odorants bind to olfactory receptor proteins on the cilia
- This activates the olfactory specific G protein Golf which is coupled to adenylate cyclase
- Adenylate cyclase produces cAMP
- cAMP activated channels open, leading to a Na+ & Ca2+ influx
- The Ca2+ ions open Ca2+ mediated Cl- channels leading to a Cl- efflux
- Membrane depolarisation of olfactory receptor cell
Describe the action potentials of olfactory receptor cells?
- Odorants generate a slow action potential in the cilia
- In the olfactory receptor cells, there’s propagation of the action potentials
Describe the flow of olfactory information to the brain
Oflactory receptor cells -> Olfactory nerve (CNI) -> Olfactory bulb
- The olfactory bulb projects to:
1. Frontal cortex - conscious perception of smell
2. Hypothalamus/Amygdala - motivational & emotional aspect of smell
3. Hippocampus -odour memory - Glomeruli within the olfactory bulb are where axons from olfactory receptor cells converge with those of the olfactory bulb, they tend to express the same receptor
What is population coding?/
- Population coding is when responses of a large group of broadly tuned neurones are tuned to specify the properties of a a particular stimuli
- Gustatory & Olfactory axons & the neurones respond broadly
