Slide 1

Smell

Olfaction brings both good news and bad newsPheromones

Smell— a mode of communication as well as of detecting environmentImportant signals

Reproductive behaviorTerritorial boundariesIdentificationAggression

Role of human pheromones ; not as prominent as in other animals but still play roles unwittingly!

Slide 2

Smell

The Organs of SmellOlfactory epithelium

Olfactory receptor cells : 4-8 wks life cycle

Supporting cells : like glia, help producing mucus Basal cells : olfactory neurons source (stem cell)

(Cranial nerve I)

Slide 3

SmellThe Organs of Smell

Mucus layer : water base with mucopolysaccharides antibodies - critical defense en route to brain various enzymes odorant binding proteins - concentrate odorants

Olfactory acuity : The size of olfactory epithelium and the number of olfactory receptors

e.g. dog vs human : 170 cm2 vs 10 cm2, 100 times more receptors per sqarecentimeters

Vomeronasal organ : pheromone sensing organ project to accessory olfactory bulb

Slide 4

SmellOlfactory Receptor Neurons

Olfactory Transduction • Cl- channel opening mediate outflow of chloride ions :Unusually high intracellular Cl- conc. enhance depolarization (in contrast to what happens in other neurons) • Termination of olfactory sensation :

Odorants diffusion Degradation (scavenger enzymes) 2ndary signalling pathway to end the transduction – adaptation (even when odorants still be around)

Slide 5

Smell

• Cl- channel opening mediate outflow of chloride ions :Unusually high intracellular Cl- conc. enhance depolarization (in contrast to what happens in other neurons) • Termination of olfactory sensation :

Odorants diffusion Degradation (scavenger enzymes) 2ndary signalling pathway to end the transduction – adaptation (even when odorants still be around)

Olfactory Receptor NeuronsOlfactory Transduction

Slide 6

Olfactory TransductionOdorant receptors

• More than 1000 genes in rodents (Linda Buck and Richard Axel)

—The largest gene family in mammals

—~350 in human• Genes are scattered (in clusters)

about on the genome• Each Olfactory neurons seem to

express single odorant receptor gene : choice mechanism unknown!!.

• Zonal and random (within a zone) expression

— each zone express a different, nonoverlapping subset of receptor genes

Smell

Slide 7

Olfactory Transduction

Smell

• VNO has its own receptors : — ~300 in mice, 5 in humans— Structurally different from

odorant receptors— Ligands are largely unknown

• Unusual cAMP gated channels are evolutionarily conserved : Used in the visual transduction as well

• Recent finding suggests both olfactory system and vomeronasal system regulate behavioral outcome such as reproduction

• Population coding : both odorant type (which cells) and strength (how active) are used to classify the odors

Slide 8

Central Olfactory Pathways

Smell

• Each Ob has about 2000 glomeruli

• Each glomerulus contains about 25,000 primary olfactory axons and dendrites from about 100 second-order olfactory neurons (Mitral cells)

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Central Olfactory Pathways

Smell

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Central Olfactory Pathways

Smell

Precise mapping :• All axons from neurons that express one particular receptor gene converge onto glomeruli on each bulb• Each glomerulus receives input from only receptor cells of one particular type• Symmetrical positions• Consistent positions of glomeruli from one mouse to another

Intricate circuitry :• Excitatory and inhibitory connections

-Within glomeruli-Among glomeruli-Between obs

• Broad categorization of odorant signals might be done at this level

Slide 11

Central Olfactory Pathways

• Direct projections to cortex (OB-OC):unique to olfactory system• Paleocortex VS. Neocortex• Multiple projections (from OB) to many brain structures• Direct and widespread influence on odor discrimination,

emotion, motivation, memory..• OB-OT-Medial dorsal nucleus of thalamus pathway may be

responsible for the conscious perception of smell, while connections to amygdala and entorhinal cortex may be involved in the affected (associated) components of olfaction such as aversiveness

Smell

Slide 12

Slide 13

Spatial and Temporal Representations of Olfactory Information

Olfactory Population Coding• Combinations increase resolutions

Olfactory Spatial Maps• A sensory map is an orderly arrangement of neurons that

correlates with certain feature of the environment• Reproducible spatial pattern in the system that no where

information is needed- WHY?- Discrimination among different chemicals?- Just for the efficiency? - Easier communications with

neighbors- Systemic relationships between chemicals and map

positions?

Smell

Slide 14

Spatial and Temporal Representations of Olfactory Information

Chemotopy

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Slide 15

Spatial and Temporal Representations of Olfactory Information

ORN receptive fields and acuity

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Slide 16

Spatial and Temporal Representations of Olfactory Information

Temporal coding

Odors are inherently slow stimuli - The timing of odors

is not so important but the quality of odors is

Oscillation between ob and cortex when odors are

present - imply the presence of temporal odor code

ORNs responsive to a particular odor display a wide

range of temporal spiking pattern

In honeybee expts, ablating the synchrony of firing

brought about a loss of bee’s ability to distinguish

between similar odors ( not between broad category of

odors)

Smell

Slide 17

Spatial and Temporal Representations of Olfactory Information

Temporal coding hypothesis

Smell