Lecture 6: Olfaction and Gustation Flashcards
Where are the odorant receptors (OR) located and how many do humans express?
- On the cilia of olfactory epithelium
- Humans express 350 different odorant receptors
What type of receptor is the odorant receptor and what is the cascade upon binding of an odorant?
- GOlf stimulates adenylyl cyclase, which increases cAMP, which open cyclic nucleotide gated channnels (CNGC)
- Leads to the influx of cations (Na+ and Ca2+) and depolarization of the membrane
What occurs that allows us to “get used to the smell” of an odorant after stimulation persists for some time?
- Sensitivity of the CNGC to cAMP decreases, reducing cation influx
- A smaller generator potential results, reducing the probability of an AP in the axon
What are the odorant receptors like on one olfactory neuron and where do they project to?
- One olfactory neuron expresses the same odorant receptor on ALL of its cilia
- The projections of similar odorant receptors are collected into one glomerulus
How are different odorant receptors distributed in the olfactory epithelium?
- Localized to different part of the olfactory epithelium
- But are not equally between these areas
- Each zone (area) has a different receptor type that predominates
What is the advantage to the distributed localization of odorant receptors within the olfactory epithelium?
- If a part of the olfactory epithelium is damaged or blocked, you will likely be able to still detect an odorant.
- A receptor for a certain odorant is distributed in certain areas throughout the epithelium
Each odorant acts on how many recpetors and is the significance of the concentration of an odorant?
- Each odorant activates different combination of odorant receptors to produce its characteristic odor
- Odorant concentration changes the perceived smell
Through what mechanism is a higher odorant concentration able to change the perceived smell?
- At higher concentrations, the odorant starts to bind to receptors with a lower affinity for the odorant in addition to the receptors with a higher affinity
- At low concentrations, indole smells floral, but at high concentrations, it smells putrid!
What are the smaller, trace-amine associated receptors (TAARs) for odorants?
- Produce physiologic/endocrine responses to pheromones
- Many of the chemicals that activate TAARs are found in the greatest concentration in urine from the male of the species being studied
The olfactory bulb consists of how many layers and contains what cells?
- Consists of 5 layers
- Mitral cells
- Periglomerular cells
- Tufted cells
- Granule cells
Within the olfactory bulb are the glomeruli where the axon terminals from olfactory neurons synapse and release what NT to what cells?
- Release EAA (glutamate) on the dendritic trees of the post-synaptic neurons:
- Mitral cells
- Tufted cells
- Periglomerular cells
Which 2 cells of the olfactory bulb have axons that will go to the olfactory cortex?
1) Mitral cells
2) Tufted cells
Which 2 cells of the olfactory bulb are the local interneurons that release GABA?
1) Granular cells
2) Periglomerular cells
All the axons from the olfactory neurons expressing the same odorant receptor converge on how many glomeruli, located where?
- Converge on two glomeruli (one medial, one lateral)
- Located in the ipsilateral olfactory bulb
What is the function of periglomerular cells releasing GABA at their synaptic contacts with other glomeruli?
- Inhibits the activity from those glomeruli of related odorants
- This means that the odorant producing the strongest stimulation will suppress the input from other glomeruli that weren’t as strongly activated
- Improves the signal specificty in the brain
What is the function of granular cells releasing GABA at their synaptic contacts with mitral and tufted cells?
They also work to increase the specificity of the message going to the olfactory cortex
Where is the first possible synapse within the olfactory cortex; where is info relayed from here?
- Anterior olfactory nucleus
- These synapses relay the info to the contralateral side
- Axons also continue on to the ipsilateral side
The olfactory cortex is composed of 5 areas of interest?
1) Anterior olfactory nucleus
2) Piriform cortex
3) Anterior cortical amygdaloid nucleus
4) Periamygdaloid cortex
5) Lateral entorhinal cortex
What is the importance of the olfactory output from the entorhinal cortex and where does it project?
- Projects to the hippocampus, which is important in memory formation
- Olfactory input facilitates both memory formation AND recall
What is the importance of the olfactory output from the piriform cortex and where does it project?
- Projects to the lateral hypothalamus
- Important in the control of appetite
- Olfactory inputs influence hunger and appetite
The piriform cortex also relays olfactory input to the thalamus and then medial orbitofrontal cortex which plays a role in?
- Integration of taste, sight, and smell
- Appreciation of the FLAVOR of food
Why does the olfactory cortex also send input BACK TO the olfactory bulb/olfactory epithelium?
Modifies receptor responses to odorants
What is the importance of the anterior cortical amygdalaoid nucleus processing olfactory input?
Emotional learning; olfactory fear conditioning
Explain the mechanism for the transduction of Sour tastes (i.e., ions involved and NT released).
- Evoked by H+ ions entering cell
- Once inside causes a proton sensitive K+ channel to shut down
- Accumulation of positive charges = depolarization
- Serotonin (5-HT) release
Explain the mechanism for the transduction of Salty tastes (i.e., ions involved and NT released).
- Epithelial Na+ channel (ENaC)
- Increases the [Na+] in saliva and on the tongue
- Na+ and/or Ca2+ diffuses down gradient into cell and depolarizes membrane
- Serotonin (5-HT) release
Explain the mechanism for the transduction of sweet, bitter, and umami (savory) tastes through the Type II cell (i.e., receptors, ions, and NT).
- Tastant binds to receptor (GPCR) and Phospholipase Cbeta2 is activated
- DAG and IP3 released; IP3 causes release of Ca2+ from the ER
- Ca2+ activates a TRPM5 receptor, allowing Na+ into the cell
- Depolarization produced by released Ca2+ and Na+ open a gap junction subunit and ATP leaves the cell throough Panx1 channels
Where are the secondary sensory neuron cell bodies located in the ascending taste pathway; where do neurons ascend to and finally terminate?
- Nucleus tractus solitarius
- Ascend to synapse on tertiary sensory neuron cell bodies in: VPM of the Thalamus
- Finally ascend through posterior limb of the internal capsule and terminate in the postcentral gyrus, frontal operculum, and insular cortex
What is the pathway for integrating visual, somatosensory, olfaction and gustatory stimuli?
VPM of thalamus —> Gustatory Cortex —> Orbitofrontal Cortex
Perception of flavor requires what 3 inputs?
- Gustatory input from the gustatory cortex
- Olfactory input from the olfactory cortex (especially piriform cortex)
- Somatosensory information from the mouth
*Remember the piriform cortex takes olfactory inputs and projects to the thalamus and then medial orbitofrontal cortex to provide info used to identify the flavor of foods!
Neurons from all three areas of the cortex (gustatory, olfactory, somatosensory) project where to give us the sensation of flavor and appreciation of food?
Lateral posterior orbitofrontal cortex
Odorants arising from food in the mouth activate which neurons and how does this relate to what we taste?
- Activate olfactory neurons in the olfactory epithelium, but will be perceived as being part of the taste sensation
- A large part of what we ‘taste’ was really detected in our noses.
- Anything that interferes with the sensation of smell will alter the tastes/flavors we perceive
Adult neurogenesis is thought to occur in which 2 places in the brain?
1) Olfactory bulb
2) Dentate gyrus of the hippocampus
