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Gustation and Olfaction: Non-Topographic Sensory
Modalities
Importance of Taste Perception
• Nice to have • Evolutionary benefits
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Basic Tastes:
• Salty • Sour
• Bitter • Sweet • Umami (amino acids, savory)
• Gustatory afferents synapse on a single taste cell
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• Metabotropic Pathway
• Bitter, Sweet, Umami
GPCRs Used: Bitter: T2R Sweet: T1R2 + T1R3 Umami: T1R1 + T1R3
Taste cells do not coexpress these specific sets of GPCRs
Methods of depolarization
• Ionotropic: Sodium or protons enter the cell
• Metabotropic: GPCRs are activated • Both methods result in vesicle release
onto gustatory afferents
• Gustatory afferents do not receive input from multiple taste cells
Projections of Gustatory Information
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Regions of Taste Specialization
90% of taste cells respond to more than one of the five basic tastes
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Population Coding
• From an ensemble of neurons’ responses, the brain has to determine what the stimulus was
• It cannot simply look at the response of a single cell
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Population Code
• The information from single cells is ambiguous
• If tuning functions are known, the responses of different, broadly tuned cells, can be compared to extract information about the stimulus
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Over 1000 different receptor protein genes
Adaptation occurs after about 1 minute
Signals sent to glomeruli
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Olfactory Bulb
• About 100 2nd order olfactory neurons per glomerulus • Each glomerulus is innervated by about 25,000 ORNs
Receptors Don’t Project Randomly
• ORNs express a few, or one, olfactory receptor proteins • About 2,000 glomeruli per bulb
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Similar distributions of glomeruli in different bulbs, and different animals of the same species
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Olfactory Bulb Circuitry
Temporal Coding
• Gilles Laurent (Caltech) • Lateral inhibition from granule cells is
important for synchronizing responses between Mitral cells (honey bee expt.)
• Timing of spikes in relation to other spikes is important for coding
Central Olfactory Pathways
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Like taste, smell was likely a very important sense in evolution. Humans: about 5 million olfactory receptor neurons Dogs: up to 220 million (and much larger nasal epithelium)
Pheromones • Chemicals released by an animal that
cause physiological or behavioral changes in another animal of the same species
• Mark territory, lay trails, attract mates, promote social cohesion
• Can be used for pest control
Light Brown Apple Moth
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The Vomeronasal System (rodent)
Hypothalamus initiates release of hormones
Published in Science, 2003
Typical response of an AOB neuron
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• AOB neurons show selective responses:
• 3 test strains used: BALBc, B6, and CBA
• Cells were selective for a combination of strain and sex
• Mouse sex detector
Mitral cells of the AOB can also be selective for strain
• Only one cell out of 43 showed broad selectivity
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T-Shirt movie
Do Humans Detect Pheromones?