Sensation and Sensory Processing: Perceiving the world

Module 404

Sean Sweeney

Learning Outcomes:

Understand the purpose of the sensory system to an organism

Differentiate between different sensory ‘modalities’

Understand that the sensory system is organised in a logicalmanner, tuned to life strategy

Understand that sensory stimuli are transduced, encoded andfinally perceived

Understand the basis of light perception in the insect eye

Appreciate the different organisation between the insect andmammalian olfactory and gustatory systems while also appreciating that the systems follow a similar logic forcoding and perception

Sensation involves the ability to transduce, encode andperceive information generated by stimuli arising fromboth the external and internal environment.

Questions:1) How is the stimulus detected (at a molecular level)?

2) How is the stimulus transduced and encoded?

3) How does the CNS perceive the incoming information?

for 2) and 3), how is the wiring diagram organised?

4) How can qualitative and quantitative information aboutthe stimuli be represented?

5) for some modalities, how can location be represented

The stimuli

touchmovement mechanosensationimbalancesoundtemperature thermosensation (Pain!)light photoreceptionpain nociceptiontastesmell chemoceptionmoisture

Monitoring the internal environment for homeostatic regulation

?

internalor external?

The anatomy of detection

touch body surfacemovement body surface, muscle spindles, chordotonal

organsimbalance auditory organsound auditory organtemperature body surface, hypothalamus, gustatory systemlight eye/photoreceptive organpain body surface, internal pain receptorstaste integrated w/ the gustatory system (gut?)smell olfactory organ, body surface?moisture body surface? gustatory system?

Dedicated Organs v Dispersed Receptors

The detection and transduction of the stimuli

The TRP receptor paradigm

Superfamily of channels (found in yeast to humans)

Six transmembrane domains (with varying degrees of homology)

Permeability to cations (varying cation selectivity)

A single channel can be activated by disparate mechanisms

TRP channels play critical roles in responses to all major classesof external stimuli.

TRP channels work as heteromultimers in supramolecularcomplexes

Channel Pca:Pna modulation

TRPC1 nonselective store depletion, stretchconformational coupling

TRPC2 2.7 DAGTRPC3 1.6 store depletion, DAG

conformational couplingexocytosis

TRPV1 3.8(heat) Heat (43oC), vanilloids9.6(vanilloids) anandamide, camphor

piperine, allacin, EtOHproinflammatory cytokinesnicotine, protons, PIP2

TRPV2 3 Heat (53oC), osmotic cellswelling, exocytosis

TRPV3 2.6 PUFAs, menthol, compounds from oregano, cloves, thyme

The capsaicin receptor capsaicin, the active ingredient ofcapiscum or chili peppers

Strength measured in ‘Scoville Units’(Wilbur Scoville, 1912)

Jalapeño, 5000 Scoville units

Habañero, 300,000 Scoville units

Expression cloning of the receptor TRPV1:Caterina et al., (1997) Nature 389: 816-24

In vivo function of the receptor (KO mice):Caterina et al., (2000) Science 288: 306-313

TRPV1 activated by capsaicin, anandamide,heat (>43oC), camphor, piperine, garlic

Mice lacking TRPV1 are deficient for vanilloidellicited pain, thermal sensation, and tissueinjury-induced thermal hyperalgesia

Known sensory modalities mediated by TRP channels:KO organisms, experimental evidence

Chemosensationosm-9, ocr-2 (C. elegans) TRPV response to odorants

(and other modalities)TRPM5 (mammals) TRPM sweet, bitter and a.a. tasteTRPC2 (mouse!!) TRPC pheromone (in VNO!!)

Thermosensation/nociceptionTRPV1 (mouse) TRPV >43oCTRPV2 (mouse) TRPV >52oCTRPV3 (mouse) TRPV >30-39oCTRPV4 (mouse) TRPV ~25-34oCTRPM8 (mouse) TRPM <28oCTRPA1 (mouse) TRPA ??dTRPA1 (Drosophila) TRPA >35-41oC painless (Drosophila) TRPA >39-41oCpyrexia (Drosophila) TRPA >39oC

ThermoTRPs are also required for response to chemical stimuli

MechanosensationTRPV4 (mouse) TRPV hypotonicityosm-9 TRPV osmotic changeocr-2 TRPV osmotic changeTRPY (yeast) hyperosmotic conditionsTRPA1 (mouse) TRPA hearing????TRPML3 (mouse) TRPML hearing?TRPN1 (mouse zebrafish) TRPN hearing?NOMPC (Drosophila) TRPN hearing, mechanosensationNanchung (Drosophila) TRPV hearing, hygrosensationInactive (Drosophila) TRPV hearing, proprioception?TRP-4 (C. elegans) TRPN mechanosensation

water witch (Drosophila) TRPML hygrosensation (moist air)

PhototransductionTRP (Drosophila) TRPC phototransductionTRPL (Drosophila) TRPC phototransductionTRP (Drosophila) TRPC phototransductionTRPC3 (mouse) TRPC phototransduction????

The tuned sensitivity of TRP channels to ranges of temperatures ensuresefficient detection across a range of temperatures for thermosensation/nociception

TRP channels transduce many environmental signals into a physiological response.

Responses may be specific or may be multi-modal depending on the activatoror (possibly) the heteromultimerisation of the channel subunits or the sensory neurons in which the receptors are expressed (?).

The original Transient Receptor Potential: Drosophila phototransductionseven rhabdomeres per ommatidium, some are sensitive to different wavelengths

SMC:submicrovillarcisternae

ROS: Rod outersegments

Rhodopsin, the light sensing molecule (ancient!)

Whole cell patch clamp

1 photon generates 1 ‘quantum bump’

~20ms duration, ~10pA amplitude (in Ca2+) = opening of ~15 TRP channels within one villusShort latency (20-100ms) - time for DAG to accumulate and activate TRP channels

The Drosophila Signalplex

1) Photoisomeration of rhodopsin to meta-rhodopsin activates heterotimeric Gq - releasesGq

2) Gq activates phospholipase C generatingInsP3 and DAG from PIP2. DAG also releasesPUFAs by activation of DAG lipase

3) TRP and TRPL activated by PUFAs (?) and/orDAG. TRPs, PKC, PLC organised in a complexby inaD (5x PDZ domains)

4) SMC (submicrovillar cisternae) Ca2+ stores?Insp3 gated?

5) DAG converted to PA via DAG kinase andCDP-DAG by CD synthase, PI regeneratedAnd transported back to microvillar membraneBy PI transfer protein and converted to PIP2

Things that go ‘bump’: a) 20ms after absorbtion of photonmetarhodopsin activates G-protein, activating PLC generating membrane 2nd messenger (red) - thresholdfor activating one channel is reached

b) Ca2+ influx sensitises other channels - rising phase of bump

c) Ca2+ floods microvillus (>200µM) leading to rapid inactivation and refractory period, Ca2+ returns to resting levels (~150nM) within ~100ms. M, Gq and PLC aredeactivated and PIP2 resynthesised.

The Ca2+/Na+ exchanger Calx extrudes Ca2+

The human eye

Retinaouter segmentof rod

light sensitiveprotein

Rhodopsin catalyses the only light sensitive step in vision. 11-cis-retinal chromophore lies in a pocket of the protein and is isomerised to all-trans retinal when light is absorbed. The isomerisation of retinal leads to a change of the shape of rhodopsin which triggers a cascade of reactions which lead to a nerve impulse which is transmitted to the brain by the optical nerve.

TRPless vision: the mammalian phototransduction cascade

Rods (100 million) detect degree of lightnessbleached by light sensitivity determined by amount of rhodopsin Low sensitivity

Cones (3 million) sensitive to light but retainfunction in high illumination, use pigmentiodopsin

red green blue

Activated rhodopsin binds to transducin (a trimeric G-protein), activated -transducinremoves the inhibitory subunit of phosphodiesterase EPDE hydrolyses cGMP to GMPDark - cGMP high cGMP binds to cyclic nucleotide gated channels (CNG)‘dark current’ flows releasing glutamate to the horizontal and bipolar cells

In light, cGMP is hydrolysed by PDE, the CNG channelcloses, inhibiting glutamaterelease, bipolar cells relaythis to ganglion cells

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Invertebrate photoreception uses the phosphoinositide pathwayBest characterised genetic model of this pathway!

vertebrate rods use the phosphodiesterase pathway.

Both G-protein signal transduction methods employ arrestin to terminatethe signal, also rhodopsin kinase arrests rhodopsin function.

Invertebrates activate TRP channels to activate an electrical response

Vertebrates inactivate CNG channels, inhibiting glutamate release toactivate an appropriate response

Invertebrates employ a highly structured signalling complex

Vertebrates employ a diffusion process

Olfactory and gustatory processing

Single cell prokaryotes can orient towards and move up a gradient towardsnutrients: chemical sensing

Plants can orient towards air-borne chemicals allowing growth to food sources

Smell and taste guide food and mate selection, danger, nutritive value, poison

‘Flavour’ is a fusion of taste and odour

Olfaction: detection of chemicals at a distance

Gustation: requires direct contact with relevant chemical

Sugars are appetitive: important nutrients

bitter or sour elicit rejection: bitter compounds often toxic

Olfaction: optimised for combinatorial detection of vast numbers of odorants

Gustation: organised to categorise tastants into defined non-overlapping modalities (sweet, bitter, sour, salty, umami)

Flies can taste the world with more than their ‘tongue’and ‘nose’.

Drosophila olfactory receptors (OR)and gustatory receptors (GR)

cloned by expression

G-protein coupled????7 transmembraneinverse comformation to mammalianORs

ORs, GRs: one large family

Obligate heterodimer

62 ORs in Drosophila (60 genes)79 ORs in Mosquito157 ORs in Honeybee

68 GRs (60 genes)

Or83b mutant flies are anosmic

two receptors perchemosensory neuron, OR83b + 2nd

CO2!!!!

some ORs v. specific for one chemicalsome broadly tuned for class

Black = posterior, grey intermediate, white posterior

sensilla from similar anatomical regions send projections to closely associated glomeruli

glomeruli are dendrites for the projection neurons(PNs)PNs then project to the Mushroom body calyxand lateral horn

OR67d responds to 11,cis-vaccenyl-acetatethe glomeruli are fruitless positive and sexuallydimorphic

Each OR target a unique and sterotyped glomerulus

Transgenic reporters uncover the odour code

GRs project to the suboesophageal ganglion(somatotopy?)

The mammalian olfactory system:

Closely linked with the respiratory and gustatory apparatusAided by turbulent air eddies

The mammalian olfactory epithelia

lines the nasal cavity - allows direct access to odorant moleculesmucus protects, neurons are turned over

Each olfactory sensory neuron expresses only one type of olfactory receptorORs in human: 950ORs in mouse 1500

Mammalian Odorant receptorsIdentified 1991: Buck and Axel, Cell, 65 175-187G-protein coupled receptors, 7 transmembrane.

1000s of ORs expressed in millions of neurons projecting to 2000 glomeruli

ORs are involved in regulating axon guidance and glomerular targeting

cAMP gates the Ca2+/Na+ channel, depolarisation aided by the Ca2+-gated Cl- channel. rectified by Ca2+/Na+

exchanger

Gustatory Transduction in Mammals:

No Taste map!!

T1R receptors: GPCRs - sweet and umamiT1R1 + T1R3 - umamiT1R2 + T1R3 - sweetT1R3 - common receptor

T2R receptors: GPCRs - bitter

PKD1L3 + PKD2L1: TRP receptors - sour

Two models of taste perception: the ‘labeled line’ and ‘across fibre’ models

Expressing the bitter receptor in the ‘sweet’ cells generates an attractive responseto a bitter tastant : favours the labelled line model

Conclusions:

Insects and vertebrates employ remarkably similar strategies for sensory transductionand coding suggesting ancient origins for sensory systems

Sensory transduction is mediated by ‘molecular sensors’ which detect specific sensory stimuli and transduce this signal to a generate a neuronal code

The neuronal code is processed in secondary and tertiary order neurons

Questions

How are strength, quality and direction of sensory cues transduced and detected?

How are these properties of the sensory cue coded?