The Chemical Senses Flashcards
What tastes are innately enjoyed and why?
→ Sweet and fatty
→ They provide energy
Why is bitterness disliked?
→ Taste of bitterness is associated with poison
What are the 5 tastes?
→ Sweet → bitter → sour → salt → umami
Where are taste buds present other than the tongue?
→ Palate
→ Epiglottis
What two types of cell are inside a taste bud?
→ Basal cells
→ Taste cells
What are basal cells?
→ Juvenile taste cells
What are taste cells and what do they look like?
→ Mature taste cells which have microvilli that project towards the taste pore
What would be seen if you took a voltage recording of taste cells?
→ Different responses
How are saltiness and sourness transduced?
→ Ion channels
How are bitterness, sweetness and umami transduced?
→ GPCR via T1 and T2
What ion is the major component of salt?
→ Na+
Describe how saltiness is transduced
→ High Na+ concentration when eating salt
→ Na+ passes through Na+ selective channels down its concentration gradient
→ This depolarises the taste cell and activates voltage gated Ca2+ channels
→ Vesicular release of NT (5-HT) is elicited and gustatory afferents are activated
What is the ion component of sourness?
→ H+
Describe how sourness is transduced?
→ H+ can pass through the same Na+ channels that mediate saltiness
→ H+ blocks K+ sensitive channels
→ Increased H+ and blocking K+ depolarise the cell and activate voltage gated Ca2+ channels
→ Vescicular release of neurotransmitters is elicited and gustatory afferents are activated
Describe how bitterness is transduced
→ T2 receptors are Gq coupled
→ PLC converts PIP2 → DAG + IP3
→ IP3 activates a type of Na+ channel and releases Ca2+ from the endoplasmic reticulum
→ Depolarises the taste cell and activates voltage gated Ca2+ channels
→ Release of ATP is elicited and gustatory afferents are activated
What taste receptors is bitterness detected by?
→ T2 receptors
→ 25 types
What are T1/2 receptors coupled with?
→ Gq
What receptor detects sweetness?
→ A dimer receptor formed of T1R2+T1R3
How is sweetness transduced?
→ Same as bittnerness
Why do we not confuse bitter,sweet and umami tastes?
→ Taste cells either express bitter or sweet receptors not both
→ Bitter and sweet taste cells connect to different gustatory axons
What receptor detects umami?
→ Dimer receptor formed of T1R1 + T1R3
How is umami transduced?
→ Same as bitterness and sweetness
What does CN VII transmit information from?
→ Anterior tongue
What does CN IX transmit information from?
→ Posterior tongue
What does CN X transmit information from and what kind of information?
→ Epiglottis
→ Temperature and texture
What is the pathway to the gustatory cortex
→ Gustatory nucleus within the medulla
→ Transmitted to ventral posterior nucleus of the thalamus
→ projects to the gustatory cortex
What are pheromones?
→ Signals for reproductive behaviours, marking territories, indicating aggression or submission
What do we smell with?
→ Olfactory epithelium
What is the olfactory epithelium?
→ Dendrites of olfactory cells that protrude from the olfactory bulbs through holes called the cribiform plate
→ they have cilia and are covered in a mucus layer
What are olfactory receptor cells?
→ Site of transduction and are genuine neurons unlike taste receptor cells
What are supporting cells?
→ They function like glia and help produce mucus
Describe how odorant molecules activate the receptors
→ Odorant molecules bind to odorant receptor proteins on the cilia
→ Olfactory specific Golf G proteins are activated
→ Adenylyl cyclase activation increases cAMP formation
→ cAMP activated channels open allowing Na+ and Ca2+ influx
→ Ca2+ activated chloride channels open enabling Cl- efflux
→ Olfactory cells have a high concentration of Cl- ions so they diffuse out
Describe the changes in action potential frequency from the nerve to the cilia?
→ AP propagate continously along the olfactory axon
→ receptor potential propagates along dendrite and triggers AP within the soma
→ Odorants generate a slow receptor potential in the cilia
Describe what central olfactory pathways are
→ Odor molecules bind to olfactory receptor proteins
→ These transmit to different cells in the olfactory bulbs
→ Olfactory cells expressing the same receptor proteins project to the same glomeruli
→ the glomeruli converge onto common pathways
Where does the olfactory bulb project?
→ Frontal cortex - perception of what smell is
→ hypothalamus and amygdala give you emotional aspect of the smell such as a memory which is coupled to the hippocampus
What do the other senses project through?
→ Thalamus
What channels are used to detect low concentrations of salt?
→ Amiloride sensitive channels
→ insensitive to voltage and stay open
What determines the specificity of sweetness or umami?
→ T1R subunit
What are basal cells?
→ Immature olfactory receptor cells - source of new olfactory receptor cells
Describe an example of population coding for olfaction?
→ When presented with a citrus smell none of the receptor cells can individually distinguish it from other odours
→ the brain can distinguish the citrus smell through combination of responses from all 3 cells
