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Neuroscience > Week 1 - Olfaction and Taste > Flashcards

Week 1 - Olfaction and Taste Flashcards

1
Q

why is taste distorted if one has a cold?

A

distortion of flavor sensation

-taste and mouthfeel is present, but no olfaction b/c retronasal impeded

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2
Q

why are the chemical senses important to lower animals?

A

critical to physiology of lower animals

  • olfactory stimuli regulate reproductive physiology, food intake, and social behavior
  • taste stimuli regulate ingestion or rejection of food and can be altered depending on internal milieu of animal
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3
Q

how is the perception of flavor created?

A

smell, taste, and orosomatosensory (mouthfeel) combine

-olfaction is dominant component in perception

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4
Q

analytical problems faced by olfactory system

A
  1. capacity to discriminate among millions of odorants, including those never experienced, and to identify those on re-exposure
  2. gustatory system is open-ended in capacity to discriminate among compounds, but less so than olfactory
  3. both maintain sensory capacity despite death of old receptor elements and recruitment of new receptor elements
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5
Q

what are the basic peripheral and central components of the olfactory system?

A
  1. receptor cells that transduce odorant stimuli are bona fide neurons
  2. cell bodies located in olfactory epithelium
  3. OE lines posterodorsal part of nasal cavity below cribiform plate (perforated bone where CSF drains out) on nasal septum and lateral nasal wall
  4. OE neurons relay info via axonal projections through cribiform plate to olfactory bulb
  5. OB is the first relay in the sensory chain
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6
Q

what is the initial pathway for olfactory processing?

A
  1. olfactory receptors
  2. olfactory nerve
  3. olfactory bulb (bulbar neurons)
  4. olfactory tract
  5. olfactory bulb targets (within piriform/olfactory cortex at tip of temporal lobe)
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7
Q

what are the olfactory bulb targets? where do they lead?

A

pyriform cortex, olfactory tubercle, amygdala - go to orbitofrontal cortex, thalamus, and hypothalamus

entorhinal cortex - go to hippocampal formation of memories

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8
Q

what does olfactory epithelium contain? what is the structure of its neurons?

A

contains basal cells, neurons, and supporting cells

  • neurons are bipolar with an apical dendrite ending in a knob with immotile cilia at mucus layer
  • -also has thin unmyelinated axon (small, slow conducting C-fibers) that project to bulb
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9
Q

neurogenesis in olfactory epithelium

A

basal cells continually undergo mitotic division in stem cell mode, with life span of 30 days
-basal lamina: globose basal cells –> immature olfactory neurons –> mature OSNs –> dead OSNs

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10
Q

sequence of stimulus transduction in olfactory neurons

A
  1. odorant delivered to epithelium along airstream
  2. odorant diffuses through mucus to reach cilia
  3. odorant interacts with receptor PRO
  4. binding to receptor causes depolarizing receptor potential
  5. increased cAMP (due to GPCR activation of AC), which binds to Ca++ ion channel to open it and depolarize cell
  6. amplification via Ca++ activating Cl- channels
  7. reset via Na+ enter, Ca++ exit antiport
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11
Q

what do odorants bind to?

A

the largest GPCRs

  • there are 1000 of these ORs in humans, so at a molecular level there are 1000 types of olfactory sensory neurons
  • react differently to different odorants
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12
Q

what does a generic olfactory receptor look like?

A

it’s a GPCR with 7 transmembrane domains and AA sequence homology

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13
Q

zonal distribution pattern of ORs

A

neurons expressing a particular OR are limited to one of 4 particular zones

  • any particular neuron expresses only one OR type
  • within a zone, neurons expressing a particular OR can be either homogenously distributed, or have a clustered distribution pattern
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14
Q

how do responses of different olfactory neurons differ?

A
  1. single unit recordings of individual olfactory sensory neurons show that neurons are broadly tuned
  2. diversity of physiologically defined types of olfactory neurons parallels number of ORs, meaning the particular OR that a neuron expresses determines the physiological responsiveness of that neuron
    - thus, any one OR is broadly tuned to respond to a lot of different odorants, which share a common molecular feature
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15
Q

how is the encoding of odorants at the level of the olfactory epithelium?

A

each odorant has a unique pattern of spatial activity

  • there are inherent or intrinsic differences between areas of epithelium in response to odorants
  • inherent differences reflect distribution of ORs
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16
Q

how is the organization and circuitry of the olfactory bulb?

A

laminar organization and circuitry

  1. bulb is most rostral part of CNS
  2. tubular shape, arranged in concentric layers like an onion
  3. the order is: sensory neuron –> cribiform plate –> periglomerular cell –> glomeruli –> external plexiform layer –> mitral layer –> granule cell layer
17
Q

what are mitral and tufted cells of the olfactory bulb?

A

the relay neurons that project to olfactory cortex

18
Q

what are periglomerular and granule cells?

A

inhibitory interneurons that modulate activity of mitral and tufted cells
-periglomerular cells form outermost shell of neurons, and cluster around glomeruli neuropils

19
Q

what is the glomerulus of the olfactory bulb?

A

fundamental unit of anatomical and physiological organization in the bulb
-implied by connectivity, unique responsiveness of individual ORs, and pattern of activation of bulb in response to odorants

20
Q

how is the projection of epithelium into olfactory bulb?

A
  1. quadrant-to-quadrant (defined by boundaries of OR expression zones)
  2. within a quadrant, all neurons expressing a particular OR converge onto same set of glomeruli (past cribiform plate)
    - pattern of projections from epithelium onto bulb is responsible for establishing spatial code for olfactory stimuli across bulb
    - circuitry then acts on this info
21
Q

what determines if ORs respond to an odorant?

A

range of n-CHOs

22
Q

how can the olfactory system to compared to the immune system?

A
  1. Ags can be considered “pattern code” made of all the different types of Abs that recognize the different Ag epitopes
  2. many different ORs can recognize single chemical moiety (odotope), and a single chemical can be made of many odotopes
  3. thus there are as many odotopes as there are OR types
  4. since one glomerulus is unit for recognizing particular odotope, odors are identified and discriminated by overall activation of glomeruli across whole bulb
23
Q

which cranial nerves carry taste information?

A

VII - chorda tympani to anterior 2/3 tongue, greater superficial petrosal to palate
IX - posterior 1/3 tongue
X - epiglottis
each axon innervates from 2-10 taste cells, and gustatory axons innervate NTS

24
Q

central taste projections

A

NTS has projections to VPM (ventral posterior medial nucleus of thalamus), hypothalamus, amygdala, and gustatory cortex
-these then project to insula and frontal cortex, which return to amygdala and NTS

25
Q

what are the 4 primary taste fields?

A

circumvallate (back of tongue), foliate (posterior sides of tongue), fungiform papillae (anterior sides), and soft palate

26
Q

how are taste fields divided?

A

all taste fields respond to all tastands, but each taste field is most sensitive to a particular taste quality

  • bitter: back
  • sour: sides
  • sweet/umami: soft palate, front
  • salty: front circumference
27
Q

organization of trench and taste bud

A
  1. taste buds limited to specialized protrusions (papillae)
  2. microvilli of a taste cell extend through taste pore
    - gustatory afferent axons synapse with receptor cells
  3. stimuli cause depolarization which produce synaptic transmission across receptor cell - axon synapse
28
Q

what is a taste bud?

A

pear-shaped collection of cells surrounded by a basal lamina that are embedded in a stratified squamous epithelium
-each bud has 50-100 cells that include receptor and basal (stem) cells

29
Q

what is the ionic transduction mechanism for salty?

A

amiloride-sensitive Na+ channel

-results in intracellular increase in Ca++ and transmitter release

30
Q

what is the ionic transduction mechanism for sour?

A

H+ sensitive cation channel

-results in intracellular increase in Ca++ and transmitter release

31
Q

how is sweet sensed?

A

receptor-mediated transduction via second messenger

  • heterodimer T1R2 and T1R3 linked to G-PRO
  • increases in cAMP (via PLC) stimulate TRPM5 Ca channel via IP3
32
Q

how is umami sensed?

A

receptor-mediated transduction via second messenger

  • heterodimer T1R1 and T1R3 linked to G-PRO
  • increases in cAMP (via PLC) stimulate TRPM5 Ca channel via IP3
33
Q

how is bitter sensed?

A

receptor-mediated transduction via second messenger

  • homodimer T2R linked to gustaducin G-PRO
  • increases in cAMP (via PLC) stimulate TRPM5 Ca channel via IP3
34
Q

evidence for “labeled line” taste coding

A

specific gene KO and rescue experiments specifically rescue or delete preception of one taste quality without impacting others
-shown with PLC beta deletion, but rescue of bitter T2Rs, causes only recovery for bitter taste

35
Q

what is the trigeminal contribution to taste?

A

conveys oral irritation (mouthfeel)

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