Smell

1. Smell • Olfaction brings both good news and bad news • Pheromones • Smell— a mode of communication as well as of detecting environment • Important signals • Reproductive behavior • Territorial boundaries • Identification • Aggression • Role of human pheromones ; not as prominent as in other animals but still play roles unwittingly!

2. Smell • The Organs of Smell • Olfactory 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)

3. Smell • The 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

4. Smell • Olfactory 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 (evenwhen odorants still be around)

5. Smell • Olfactory 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 (evenwhen odorants still be around)

6. Smell • Olfactory Transduction • Odorant 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

7. Smell • Olfactory Transduction • 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

8. Smell • Central Olfactory Pathways • 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)

9. Smell • Central Olfactory Pathways

10. Smell • Central Olfactory Pathways • 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

11. Smell • 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

13. Smell • 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?

14. Smell • Spatial and Temporal Representations of Olfactory Information • Chemotopy

15. Smell • Spatial and Temporal Representations of Olfactory Information • ORN receptive fields and acuity

16. Smell • 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)

17. Smell • Spatial and Temporal Representations of Olfactory Information • Temporal coding hypothesis