BIOL 2052 - Chemical senses

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25 Terms

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general features of sensory systems

  • response to environment which are important in determining mood and behaviour

  • geneally: transmission —> encoding —> processing

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chemosensory receptors

CHEMOSENSORY RECEPTORS (exteroceptors)

  • receptors that generate a signal when they bind to chemicals in the external environment

  • chemosensation one of the oldest senses —> evolutionarily conserved

    • olfaction: info about airborne molecules

    • gustation: info about ingested substances (chemical and physical qualities)

  • lower organisms sensory systems good for seeking and avoiding behaviour

  • in higher organisms like us: chemical senses allow us to:

    • stimulate GI system

    • asses quality of food

    • determine if nutritionally beneficial

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distance chemosensation

  • can sense ethanol at 2mM

  • 2 trans-6-cis nonodenial (found in cucumbers) 0.07nM

  • interpretation of smell can be concentration dependent due to the patterns of neural circuitry (some in mM range, some in nM range) —> indole perfumes putrid at high concs

  • detection of natural odours is detection of a combination of molecules

  • olfaction is a reflection of the pattern of diff cells that are actiavted by the diff molecules interpreted at higher centres of the CNS

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olfaction

  • smell comes in through the nostril —> nasal cavity

  • olfactory epithelium found in the nasal cavity and is a structure which houses olfactory neurons

  • ORN which have cilia which directly engage with the external environment

  • ORN travel through channels in cribiform plate —> olfactory bulb —> brain

  • anosmic = no ability to smell

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olfactory epithelium

  • bipolar neurons

  • projection to olfactory bulb at the surface of the nasal cavity where the cilia lie

  • unmyelinated —> not too far to travel to the brain

  • cilia is embedded in the mucous layer which is produced by Bowmanns gland

  • mucous responsible for concentrating the chemicals and bringing them in contact with cilia

  • these neurons are succeptible to damage

  • replenished by STEM cells roughly every 6-8 weeks

  • STEM cells located in the epithelum

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how are odours transduced

  • response to when the odour is applied to the cilia as opposed to the soma is a much larger response —> cilia have transduction proteins which respond to smells

  • transduction occurs via GPCRs —> evolutionary conserved and encoded for by 3-5% of the genome

  • variation in the AA structure of GPCRs allow detection of diff molecules

  • each neurone will express 1 GPCR

  • selective distribution of GPCRs in diff areas of the nasal epithelium —> 17 GPCRs in one region, M71 GPCR expressed in diff regions

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signalling pathways

  • G protein coupled to GPCR called Golf

  • activated GPCR —> activation of adenylyl cyclase —> increased cAMP

  • increased cAMP acts as a 2nd messenger which activated cyclic nucleotide gated channels

  • calcium and sodium move into the cell which causes depolarisation of the cell

  • some pathwys where calcium can cause chloride to move out of the cell and aid depolarisation

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encoding of olfactory signals

  • individual ORNs are sensitive to a subset of stimuli

  • graohs below show the response of ORNs to smells —> neuron 1 the only one that responds to all 3, neuron 2 the only one that responds to the first one

  • this is how information is encoded

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across fibre pattern coding

  • each neuron has a certain no of molecules required to activate it

  • this will differ for the carbon chain length and for the neuron activated which allow for further encoding

  • coding info across a population of neurons and depending on the output wil determine what it is (across pattern coding)

  • this allows us to encode natural odours to distinct patterns across a number of neurons which allows the scent to be indentified

  • realistically there are lots of compounds in natural odours but we only maximally respond to a few of them

  • E,g: indole —> at low concs it activated a certain pattern of neurons bit activates diff neurons at high concs

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structure of the olfactory bulb

  • glomeruli: spherical structures below the surface of the olfacotory bulb —> regions where synapses made between the receptors and the mitral cells

  • mitral cells - send projections down the glomerulus which forms synapses with ORNs

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convergance and amplification

  • all neurons which converge to the same glomeruli express the same GPCR

  • bilateral pattern

  • single glomerulus can have dendrites from 25 mitral cells and recieve inputs from 25,000 olfactory cells

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molecular endocoding and electrical patterns

  • signals from teh olfactory bulb —> accessory nuclei

  • the relationship between one type of odourant neuron and one glomeruli enables specific regions of the olfactory bulb to respond to diff chemicals

  • diff odours and chemicals will activate a unique spatial pattern in the olfactory bulb depending on their chemical composition

  • chemically distinct single odourants will maximally activate one or only a few glomeruli

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processing olfactory signals

  • signals from the olfactory system travel through cranial nerve 1 —> olfactory bulb where they project to cortical structures

    • piriform cortex (has no clear pattern of organisation like the sensory cortex)

    • olfactory tubercule

    • amygdala

    • entorhinal cortex

  • signals from there —> other structure like the hippocampus

    • this is how smell can impact mood or trigger memory

  • its different to other sensory systems as it doesn’t go straight to the thalamus —> goes via the olfactory bulb targets and olfactory cortex

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gustation

  • is an example of contact chemo sensation —> must come into contact in order to taste something

  • tongue is the main sense organ

  • 4 types of papillae (invaginations) on the tongue with different number of taste buds (house the gustatory receptors)

    • circumvallate located at the back of the tongue —> 250 taste buds

    • foliate at the sides —> organised into parallel ridges with 600 taste buds

    • fungiform at the front —> 3 taste buds

    • filiform in the middle

  • apart from filiform all have associated taste

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circumvalate papillae

  • troughs fed by serous gland which secretes saliva

  • taste buds at the base of the trough

  • saliva breaks down food in the mouth and concentrates down the moleules for taste buds

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5 basic tastes

  • bitter (caffiene, nicotine)

  • sour (actetic acid)

  • sweet (glucose, fructose)

  • salt (NaCl)

  • umami (Msg)

  • all in the millimolar range —> less sensitive than smell

  • sensitive to bitter, can respond at low concentrations

  • less sensitive to glucose —> nutritionally beneficial so drives our sense to consume more of it

  • sensitivity range: bitter>acid> salt> sweet

  • certain regions of the tongue are more sensitive than others —> driven by expression levels of receptors

  • can be advantageous —> bitter at the back —> drives a pathway to spit out the food

  • some selectovity of regions where certain tastes initiate activity

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taste buds

  • a single taste bud can contain up to 50 specialised epithelial cells which convert checmical —> electrical signals

  • the tips of the cells have microvilli which increase the surface area and come together at the taste pore

  • receptors are localised in the microvilli

  • olfactory receptors neurones are bipolar neurons, however, gustatory system receptor cells release neurotransmitters whcih cause signalling in the afferent neurons

  • taste cells are subject to damage - require basal cells to regenerate them

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transduction in taste cells

  • apiccal domain where receptors expressed

  • ion channels and GPCR depending on taste

    • salt/sour - ion channel

    • sweet/bitter/umami - GPCR

  • influx of Na leads to opening of voltage gated Ca channels and influx of Ca which causes the release of serotonin and ATP

  • signals are returned via 3 cranial nerves

    • vagus (10)

    • facial (7)

    • glossopharangeal (9)

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salt/sour sensing

SALT

  • Na ions move down their conc grad through amilloride sensitive ion channels

ACIDS

  • H+ sensitive ion channels

  • H+ ions block K+ channels which prevents K+ from leaving and results in depolarisation which opens voltage gated Ca2+ channels

  • direct depolarisation results in no second messenger

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sweet/umami sensing

  • T1R heterodimers

  • lots of variation in AAs

  • ligands that bind depends on the heterodimer

SWEET

  • T1R2/T1R3 signals through G protein which activates PLCB2 which activates IP3

  • engages with eR receptor and release of Ca

  • activation of TRPM5 Ca ion channel —> depolarisation

  • this is an indirect response to depolarisation

UMAMI

  • diff heterodimer - T1R1/T1R3

  • same pathway as sweet

  • has another signalling pathway of the GluR4 - also an mGluR4 receptor in brain with a longer N terminus

  • inhibits cAMP signalling (diff pathway)

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bitter sensing

  • T2R receptors (GPCRs)

  • 25% of population dont respond to it (phenylthiocarbamide)

  • diff populations of taste cells as T1R

  • shorter N terminal

  • signalling via IP3/Trpm5 pathway

  • specific to G protein alpha gusducin, only expressed in bitter taste cells

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downstream signalling

  • determined by rodent knockouts (of the TrpM5 Ca2+ ion channel)

  • sweet and umami have a positive correlation as conc increases quinine/bitter has a positive response

  • TrpM5 have a phospholipase B important signalling for sweet umami and bitter

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processing

  • 3 cranial nerves return signals from diff parts of the brain

    • 7: tongue pallate —>nucleus of solitary tract (gustatory nucleus)

    • 9: back of the tongue —> medial region of the gustatory nucleus

    • 10: eppiglottis/oesophagus —> caudal region of the gustatory nucleus

  • these are the 1st order neurons

  • medulla —> thalamus and other parts of the brain

  • the nucleus of the solitary tract intergrates visceral info about gut motility (as gut innervated by vagus nerve)

  • drives gustatory visceral reflex arc —> enables you to be sick if you consume something bad tasting

  • from the gustatory nucleus in the medulla the ventral posterior medial nucleus of the thalamus

  • signals from the thalamus go to:

    • amygdala

    • orbitofrontal cortex - 2ndary taste centre

    • insula

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mapping of gustatory cues

  • some mapping that takes place

  • some regions particularly sensitive to NaCl compared to sweet/bitter

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labelled line coding

  • gustatory system uses labelled line coding

  • in mice, experiments undergone where a wild type and a T2R knockout exposed to a taste and response recorded

  • the response in t2R knockout much lower than wild type

  • T2R gene in knockout turned back on to see if wild type response cen be rescued

    • only in mice exposed to glutamate is response restored

    • this means that only cells which express T2R receptors only respond to bitter taste

  • orbitofrontal cortex gives our overall perception of food

    • involved in satiation - as we become satiated, signalling in orbital cortex decreases