Sensation of Smell

2. Sensation of Smell Professor A.M.A Abdel Gader MD, PhD, FRCP (Lond., Edin), FRSH (London) Professor of Physiology, College of Medicine & King Khalid University Hospital Riyadh, Saudi Arabia

4. The Physiology of Smell(Olfaction) Smell & Taste: Similar: Chemical Senses Determine food flavor (intake) Dissimiliar: Smell receptors… telereceptors- sense projected the environment Taste is confined to mouth Smell pathway does not relay in the thalamus does not reach sensory cortex Taste pathway finishes in the sensory cortex (PCG)

5. Taste & Smell – cont. • Chemical Senses: • vital for survival • Determine appetite (Retreat from noxious stimuli)

6. Chemical Senses • “Gatekeepers”:molecule detectors • identify what the body needs for survival • identify what is dangerous and should be rejected • Neurogenesis: constant renewal of receptors • Olfactory receptors = 30-60 days • Taste cells = 10 days • Affective component: emotions aid in discrimination of molecules (good vs. bad)

7. Chemical Senses - cont • Taste plays a vital role in food selection, • sweet (and umami) are associated with nutritious food • Bitter tastes are associated with the possible presence of toxins and are usually avoided. • Taste and smell are closely linked even though they involve different receptors and receptive processes. (??overlap in central processing). • In many species olfactory stimuli play an important role in reproduction although not well developed in humans (?).

8. Sense of Smell –species difference: Macrosmatic animals… Dogs: highly developed sense of smell & reliance on olfactory system Microsmatic… Man: weakly developed sense of smell & less reliance on olfactory system The Physiology of Smell(Olfaction)

9. Species differences Dog: Has40 sq inches of epithelium compared to 1 sq inch in humans

10. Dogs have about 40 times the area for olfaction that humans do

11. Olfactory Receptor Neurons (ORN) • Olf. Signal Transducers • 1000 different types • Each type found in only 1 zone of mucosa • Vision: • 3 cone types, 1 type of rod • 6 million cones, 120 million rods

12. Olfactory Receptors • 1000 different types of receptors each has odorant-binding protein • Only one protein type per ORN • These membrane bound proteins • Located in cilia on tips of ORN’s • ORN: olfactory receptor neuron

13. Olfactory receptors: (a) Location of receptors in the roof of the nasal cavity. (b) Closeup of olfactory cells.

14. Olfactory epith. at roof • of nasal cavity • Warm nasal epithelium • Conviction currents • ( bysniffing)

15. Olfactory Receptors: Bipolar neurons with cilia Receptors are genuine neurons (unlike photoreceptors and hair cells) Unlike other neurons, receptors are continually regenerated The Physiology of Smell(Olfaction)

16. Olfactory Epithelium • 5 cm2 of membrane • 10-100 million receptors

17. Cells of the Olfactory Membrane • Olfactory epithelium made up of three types of cells: • Olfactory receptors • bipolar neurons with olfactory hairs • Supporting cells • columnar epithelium • Basal stem cells • replace receptors monthly • Olfactory (Bowman’s) glands • produce mucus • Dissolves odorants

18. Olfactory Epithelium • Receptors have four parts cilia,olfactory knob,olfactory rodand the axon • Olfactory nerve - the axons of the olfactory receptors form bands which travel to the olfactory bulb

19. BIPOLAR OLFACTORY NEURONS IN THE NOSE. RECEPTORS FOR ODOR MOLECULES ARE IN THE CILIA.

21. Olfactory System

22. Olfactory System Each sensory cell type connects to a particular pair of glomeruli Each sensory cell expresses one receptor type (indicated by color) Different colors represent different sensory cells

23. Olfactory Receptors: Bipolar neurons with cilia Mitose throughout life (only part of CNS that is known to regenerate) Their axons project to the olfactory bulb The Physiology of Smell - (Olfaction) – cont.

25. Mechanism of Olfaction

26. Physiology of Olfaction – Mechanism Molecules must dissolve in mucus  Combine with receptors on the cilia  Stimulation of a G-Protein  Activation of Adenyl Cyclase  Increase I.C. cAMP  Opening of Na Channels  Na influx  Depolarization (Receptor Potential)

28. Physiology of Olfaction – cont. Threshold to Different Substances

29. Discrimination of Intensity • Poor • Requires 30% increase of intensity • Strong smell highly water and lipid soluble • Man can distinguish 2000-4000 different odors

30. Physiology of Olfaction – cont. Adaptation to Smell • Peripheral • Central

31. Olfactory Pathway • Axons from olfactory receptors synapse in the olfactory bulb • Second-order neurons within the olfactory bulb form the olfactory tract • synapses on primary olfactory area of temporal lobe • conscious awareness of smell begins • Other pathways lead to the frontal lobe where identification of the odor occurs

32. Early Olfactory Pathway

33. Olfactory Mucosa & Olfactory Pathway

34. Early Olfactory Pathway

35. Central Olfactory Pathways

36. Central Olfactory Pathways

37. Central Olefactory Pathways

38. Central Olefactory Pathways

39. Central Olefactory Pathways

40. Central Olefactory Pathways

41. Physiology of Olfaction – cont. Clinical Considerations: Abnormalities of the sense of olfaction: • Anosmia: Absence of the sense of smell • Dysosmia; Disturbed “ “ “ “ “ • Hyposomia: Reduced “ “ “ “ “ Vit. A deficiency and hypogonadism • Hyperosmia: Increased sense of smell (Adrenal insufficiency)

43. Advances in Olfactory Physiology& Noble Prize

44. How does the sense of smell work? • Discovered fairly recently (1991) • Nobel prize in 2004, to Richard Axel and Linda Buck • Discovery in part due to (& thanks to) molecular biology and genetic engineering

45. How did they do it? • There are three visual receptors (to distinguish different colors) • Buck got the idea that maybe smell receptors might be similar • She used a genetic technique called the Polymerase Chain Reaction (PCR) to find them • There are about 1000 different smell receptors in humans!

46. http://nobelprize.org/medicine/laureates/2004/illpres/4_gene.htmlhttp://nobelprize.org/medicine/laureates/2004/illpres/4_gene.htmlhttp://nobelprize.org/medicine/laureates/2004/illpres/4_gene.htmlhttp://nobelprize.org/medicine/laureates/2004/illpres/4_gene.html