smell & taste 2

describe the olfactory system of mice

has 4 olfactory organs: main olfactory epithelium, vomeronasal organ, grueneberg ganglion, septal organ of Masera

MOE, GG and SO project to main olfactory bulb, VNO project to the accessory olfactory bulb

describe the olfactory system of drosophila

2 olfactory organs: the 3rd antennal segments & maxillary palps

the olfactory info relayed to antennal lobe (processing then takes place at lateral horn of protocerebrum & kenyon cells in the mushroom body)

how does the human olfactory system differ?

larger number of glomeruli and smaller number of functional odor receptor genes,

lack the accessory olfactory system (detects low volatility odorants in liquid phase),

larger and more intricate orbitofrontal cortex (for processing)

how many ORs in humans vs in mice

1100 in mouse, 350 in human (alot were pseudogenized)

how much of human ORs are pseudogenized

15-78% or Ors & all V2Rs

what are the 2 classes of vertebrate ORs

teleost fish class 1 - tuned to water-soluble odorants

terrestrial vertebrates class 2 - tuned to hydrophobic odours (amphibians have both classes, in dolphins class II have been pseudogenized)

how could polymorphisms in ORs underlie variation in human odour perception?

-can cause anosmic for certain odours

-altered detection threshold

-altered perceptual quality

what is the buck and axel method?

dissected out olfactory epithelium -> extracted RNA -> RT to cDNA -> pairs of degenerate primers used for GPCRs to amplify further candidate members of the GPCR superfamily (in olfactory epithelium)

how did Buck and Axel analyse their results?

if PCR products =a mixture of DNA sequences, then= amplification of members of a multigenefamily

if an individual PCR product =several different DNA sequences (consistent with the amplification of members of a multigene family) -> restriction endonuclease digestion

should generate a large array of fragments whose molecular weights sum > than the molecular weight of the original PCR product

what are the features of ORNs?

-8-20 non-motile cilia

-cilia contain receptors

-other end has unmyelinated axon

10-100 axons from into a bundle, becomes a primary olfactory nerve fibre. where does it go?

passes through cribriform plate, first synapse at olfactory bulb

what do odorant binding proteins (OBP) do?

help odorants bind receptors on cilia, clear cilia of odorants

how does clearance work (aided by OBPs ofc)

olfactory epithelium & nasal & Bowmans glands secrete enzymes e.g. cytochrome P450, dehydrogenates, oxidases, reductases and esterases

what are the steps of olfactory transduction?

OBP binds odour to receptor

G protein binds adenyl cyclase so it converts ATP to cAMP

cAMP binds CNG carion channel

depolarisation means Ca activates Cl channel

more depoarisation

how can adenylyl cyclase cause smell dysfunction?

no/ reduced cAMP -> completely anosmic (US offer PDE inhibitor treatment)

what is an example of a ciliopathy causing smell dysfunction?

Bardet-Biedl syndrome- mutation in BBS4 gene -codes for a protein in the basal body of cilia (causes anosmia 60% cases)

differences between mouse and fly glomerular

mouse- tufted cells (glu), mitral/tufted cells (glu), granule cells (GABA)

fly- projection neurons (ach), local neurons (GABA) (ORNs use ach not glu)

what has Mapping The Glomeruli with 2DG (labeled glucose analogue) revealed?

if more similar chemicals then lights up same areas

why do we crave salt?

sodium and chloride needed for maintenance of osmotic balance, membrane potential, nutrient absorption and transport, blood volume and pressure

the 5 basic tastes?

salt, sour, sweet, bitter, umami

what do taste buds look like? what are they composed of?

50-150 taste receptor cells

what do circumvallate papillae look like? where are they found?

are at back of the tongue (have hundreds-thousands taste buds)

what do foliate papillae look like? where are they found?

posterior lateral edge (dozen -hundreds taste buds)

what do fungiform papillae look like? where are they found?

in anterior 2/3 of tongue (containone/ a few taste buds)

what were the 3 proposed models for how taste buds encode smell?

labelled line- receptor cells tuned to single taste modalities (innervated by individually tuned nerve fibres) (most evidence)

across-fibre pattern (1)- individual TRCs are tuned to multiple taste qualities

across-fibre pattern (2)- receptor cells tuned to single taste modalities, same afferent fibre carries info from more than one taste modality

what is the structure of sweet receptors?

heteromeric, made of T1R2 & T1R3

what is the structure of umami receptors?

heteromeric, made of T1R1 & T1R3

what does an insect sensillum do?

olfaction, taste, chemoreception, hygroreception

activation of one neuron inhibits its neighbour.

can be seen via optogenetics- use light to stimulate one neuron

or can deliver background odour, deliver a second odour to a separate neuron to activate (sustained train of APs from neuron A, when 2nd odour imposed, neuron B fires & theres a marked reduction in neuron a firing)

what channels can be used in optogenetic control, (ionic vs g protein)

ionic- ChR2, VChR1, NpHR

G- synthetic rhodopsins (intracellular loops of bovine rhodopsin replaced w loops of GPCRs- light activated) OptoXR

how does ephaptic inhibition work in maunther cells?

hair cell excites VIII nerves

this excites ipsilateral Mauthner cells & inhibitory interneurons

the inhibitiory interneurons act on both Mauthner cells (go at the axon cap (a structure of high resistivity), influx at the heminode increases extracellular potential, hyperpolarises the zone where impulse initiation occurs)

how does ephaptic inhibition work in drosophila olfactory receptor neurons?

odour molecules enter via pores

dendrites of 2 neurons in hair lumen

both share an extracellular space

opening in channels in neuron 1 draws current from this space (so when channels open in neuron 2 there is less to draw from)