Olfaction

1. Olfaction Smell aka Olfaction (Latin olfacere)

2. Sense of Smell Smell is the least understood of our senses. one of the chemical senses This results partly from the fact that the sense of smell is a subjective phenomenon that cannot be studied with ease in lower animals. Another complicating problem is that the sense of smell is poorly developed in human beings (Microsmatic) in comparison with the sense of smell in many lower animals (Macrosmatic). Important for pleasure and for enjoying the taste of food. Chemical detection without the danger of ingesting poison alert us to potential dangers, e.g. smoke

3. In humans • Strong link to sense of taste • Strong link to emotional states • Exploited by commercials/marketing • Used for identification of gender • Dangers and hazards • Pheromones - signal sexual arousal or a readiness for mating Where does smell figure?

4. Olfactory Membrane lies in the superior part of each nostril Located 7 cm inside the nasal cavity In each nostril, the olfactory membrane has a surface area of about 2.4 square centimeters The human olfactory epithelium covers an area of roughly 1 cm2 on each side Olfactory Cells - The receptor cells for the smell sensation - bipolar nerve cells derived originally from the CNS about 10 million of these cells in the olfactory epithelium interspersed among sustentacularcells--rabbit have approximately 50 million olfactory axons, whereas humans  have only 6 million bilaterally Olfactory mucous membrane is the place in body where NS is closest to external world

5. Olfactory Membrane The mucosal end of the olfactory cell forms a knob from which 4 to 25 olfactory hairs (also called olfactory cilia),project into the mucusthat coats the inner surface of the nasal cavity cilia react to odors in the air and stimulate the olfactory cells Spaced among the olfactory cells - Bowman’s glands that secrete mucus onto the surface of the olfactory membrane. Olfactory cells are constantly being replaced with a half-time of a few weeks

8. The olfactory mucosa

9. Olfactory Receptor Cells have cilia Cilia are bathed in mucous Mucous contains Odorant binding proteins (OBP) OBP transmits the odorant molecules from the air to receptor sites on the cilia -Each neuron has a thick dendrite with an expanded end called olfactory knob -From rods cilia project to the mucous surface -Each receptor neuron has 10-20 cilia -Axons of olfactory receptor neurons pierce cribriform plate of ethmoid bone and enter olfactory bulbs -Olfactory neurons have half-time of few weeks- Live for 5- 8 weeks & then die

13. Excitation of the Olfactory Cells The portion of each olfactory cell that responds to the olfactory chemical stimuli is the olfactory cilia. The odorant substance, on coming in contact with the olfactory membrane surface, first diffuses into the mucus that covers the cilia. Then it binds with receptor proteins in the membrane of each cilium- OBP Receptor protein – G protein – cAMP pathway – opening of Na channels – action potential – exciting the olfactory neuron - olfactory nerve – CNS

14. Excitation of the Olfactory Cells Only volatile substances that can be sniffed into the nostrils can be smelled Substance must be at least slightly water soluble so that it can pass through the mucus to reach the olfactory cilia. substance to be at least slightly lipid soluble, presumably because lipid constituents of the cilium itself are a weak barrier to non-lipid-soluble odorants. Sniffing is a semi-reflex response that usually occurs when a new odor attracts attention.

15. Ever wonder why we sniff? • Because only 5% of our nostril is covered with the olfactory epithelium, we sniff to maximize the coverage of the air intake over this portion of our nose as seen in the diagram to the left. • Cool huh?

16. Odor pathways ➜ OrthonasalPathway ➜ Retronasalpathway

17. AdaptationOlfactory fatigue The olfactory receptors adapt about 50 per cent in the first second or so after stimulation. Thereafter, they adapt very little and very slowly. our own experience that smell sensations adapt almost to extinction within a minute or so after entering a strongly odorous atmosphere.

18. Adaptation Because this psychological adaptation is far greater than the degree of adaptation of the receptors themselves most of the additional adaptation occurs within the CNS Large numbers of nerve fibers pass from the olfactory regions of the brainbackward along the olfactory tract and terminate on special inhibitory cells in the olfactory bulb, the granule cells. cross-adaptation

19. Primary Smell Sensations 1. Camphoraceous 2. Musky 3. Floral 4. Pepperminty 5. Ethereal 6. Pungent 7. Putrid 100 primary sensations of smell

20. Affective Nature of Smell affective quality of either pleasantness or unpleasantness. smell is probably even more important than taste for the selection of food a person who has previously eaten food that disagreed with him or her is often nauseated by the smell of that same food on a second occasion

21. Very little known • No odorotopic map found. • All cells fire for all odors but form different patterns different odors => Population code. • Other complications: Change in intensity leads to increase in the firing rate of neurons but triggers changes in the perceived quality of the odorant. • Rats are 8 to 50 times more sensitive to odors than humans • Dogs are 300 to 10,000 times more sensitive • The difference lies in the number of receptors they each have • Humans have 10 million and dogs have 1 billion olfactory receptors Neural Representations

22. Threshold for Smell the minute quantity of stimulating agent in the air can elicit a smell sensation. the substance methylmercaptan can be smelled when only one 25 trillionth of a gram is present in each milliliter of air. Because of this very low threshold, this substance is mixed with natural gas to give the gas an odor that can be detected when even small amounts of gas leak from a cylinder.

23. Odor (as well as taste) are much more phenomenological • than vision or audition. • Odor Detection • Very good: Mercaptan 1 part per 50 billion parts of air. • Varies greatly from odor to odor (molecular properties) • Varies with a variety of other factors • Time of day, Age and Gender • Smoking is injurious to odor detection! • Odor Identification • Much worse • Shows context dependence • Varies once again with age and gender • Humans can discriminate among 100,000 odors but they cannot label them accurately • This appears to be caused by an inability to retrieve the name from memory, from a lack of sensitivity Odor Perception

24. Olfactory thresholds and discrimination Olfactory receptors respond only to substances in contact with olfactory epithelium and need to be dissolved in mucus Methyl marcaptan one of the substances in garlic can be smelled at very low concentration showing the remarkable sensitivity of olfactory receptors Humans can recognize more than 10.000 different odors However determination of intensity of odor is poor

25. Determined by Gender • Women vastly superior • Can be improved with practice • - benefits both genders • Stimulus Saliency • Infants and mothers can recognize each other by smell alone • Infants hedonic reactions to smell develops with age • Odor Familiarity - Seen in elderly people Odor Identification Smoking is injurious to odor identification!

26. Odor Constancy • Natural sniffs lead to constant odor perception but artificial blowing of air does not • Common Chemical Sense • Odors judged pleasant at moderate concentrations are judged unpleasant at high concentrations • Related to the action of free nerve endings – stimulated in an indiscriminate manner • Warning system for the organism • Adaptation - Adapt to a particular odor • Cross Adaptation: Adapt to a different odor • Could have led to odor categorization but turns out that cross-adaptation is not symmetric! Some more amazing facts

27. Researchers have found it difficult to map perceptual experience onto physical attributes of odorants. • Linking chemical structure to types of smells • Some molecules with similar shapes had very different smells • Some similar smells came from molecules with different shapes • 1,000 different receptor types - about 1% of your DNA codes for olfactory receptors making it the largest single gene family

28. We can distinguish between about 10,000 different smellsDifferent threshold levels for different smells Two thresholds for each smell -low threshold for the existence of a chemical, somewhat higher threshold to discriminate one smell from another Perception of Smell

29. Smell Pathway Olfactory bulb - olfactory tract – olfactory nerve – 1st cranial nerve both the tract and the bulb are an anterior outgrowth of braintissue from the base of the brain olfactory bulb lies over the cribriform plate, separating the brain cavity from the upper nasal cavity The cribriform plate has multiple small perforations through which an equal number of small nerves pass upward from the olfactory membrane in the nasal cavityto enter the olfactory bulb in the cranial cavity

31. Olfactory bulb - organ which houses all the nerves which receive inputs from the olfactory receptors -Axons of receptors contact the primary dendrites of mitral cells and tufted cells. -Forming complex globular synapses called olfactory glomeruli. -Periglomerular cells are inhibitory neurons connecting one glomerulus to another Olfactory Bulbs

32. Olfactory Bulb

33. There are approximately 20 olfactory foramina on either side of the nose in the cribiform plate of the ethmoid bone passes 40 or so bundles of axons form right and left olfactory nerves • They terminate in the olfactory bulbs – below the frontal lobes of the cerebrum • Axons of the olfactory bulbs form the olfactory tract which projects to the primary olfactory area of the cerebral cortex. • Some project into the limbic system and hypothalamus (emotional and memory evoked responses). • Olfactory sensations are the only sensations that reach the cerebral cortex without first synapsing in the thalamus. • The primary olfactory area has axons that extend to the orbit frontal area (frontal lobe) – region for odor identification. Olfactory pathway

34. Olfactory pathway

36. -Granule cells have no axons and make reciprocal synapses with lateral dendrites of tufted and mitral cells -Mitral and Tufted – excite granule releasing glutamate and granule cell in turn inhibits both by releasing GABA Olfactory Bulbs

37. Stimulation of Olfactory cells

39. Smell Pathway short axons from the olfactory cells terminating in multiple globular structures within the olfactory bulb called glomeruli Each glomerulus is the terminus for dendrites from about 25 large mitral cells and about 60 smaller tufted cells, the cell bodies of which lie in the olfactory bulb superior to the glomeruli – granule cells - Periglomerular cells mitral and tufted cells sendaxons through the olfactory tract to transmit olfactory signals to higher levels in the CNS Mucus – cilia - Axons of olfactory cells – glomeruli in bulb – dendrites of mitral, tufted cells in bulb – axons of mitral, tufted cells in tract - CNS

41. Smell CNS Pathway Olfactory tract divides into medially into the medial olfactory area (stria) of the brain stem – very old olfactory system other passing laterally into the lateral olfactory area (stria) - a newer & less old system The Medial Olfactory Area (very old) – septal nuclei – hypothalamus – limbic system – removal – not much effect The Less Old Lateral Olfactory Area - prepyriform and pyriform cortex plus portion of the amygdaloid nuclei – limbic system (hippocampus) – learning & aversion

42. Smell CNS Pathway lateral olfactory area - anteromedial portion of the temporal lobe (cerebral cortex) *This is the only area of the entire cerebral cortex where sensory signals pass directly to the cortex without passing first through the thalamus The Newer Pathway - passes through the thalamus, passing to the dorsomedial thalamic nucleus - orbitofrontal cortex conscious analysis of odor Granule cells, Periglomerular cells – lateral inhibition

43. Mitral cell axons form the lateral olfactory tract. Projects to accessory olfactory nuclei, olfactory tubercule, entorhinal cortex, amygdala, pyriformcortex Pyriform cortex axons project to thalamus, hippothalamus, hippocampus, amygdala. Mitral cell projections

44. Lateral olfactory tract projects directly to the piriform cortex (= primary olfactory cortex = paleocortex) adjacent to lateral olfactory tract in temporal lobe. This is only sense that does not have relay in thalamus on way from receptors to cerebral cortex. From piriform cortex there are projections to hypothalamus, the thalamus, amygdala, entorhinal cortex. From thalamus there is a projection to orbitofrontal cortex where odor perception and discrimination takes place. Electrical stimulation of piriform cortex causes olfactory sensations. People with lesions of orbitofrontal cortex are unable to discriminate odors. Pathways through amygdala and hypothalamus mediate emotional, motivational and many physiological effects of odors. Central olfactory pathways

45. Transmission of smell signals to CNS

46. Structure of the Olfactory System • Signals are sent to • Primary olfactory (piriform) cortex in the temporal lobe • Secondary olfactory (orbitofrontal) cortex in the frontal lobe • Amygdala deep in the cortex ch 15

50. Central Olfactory Pathways Anthony J Greene