Olfactory System PDF

Olfactory system Asghar Ghasemi  Definitions Olfactory sense is a chemical sense because it is due to interaction of chemicals with olfactory receptors (chemoreceptors). Olfactory sense detects chemical volatile molecular subst...

Olfactory system Asghar Ghasemi  Definitions Olfactory sense is a chemical sense because it is due to interaction of chemicals with olfactory receptors (chemoreceptors). Olfactory sense detects chemical volatile molecular substances called odorants Smell/odor is the cognitive perception of odorant molecules transduced through the olfactory system  Functions of olfactory system 1. Protective avoidance behavior and food selection Only 20% of all recognizable odors are pleasant and the rest are unpleasant and represent potentially dangerous substances, thus supporting the assumption that the major role of the olfactory system is to provide a trigger for protective avoidance behavior. 2. Elicits physiological responses involved in the digestion and utilization of foods 3. Allows animals to recognize the proximity of other animals, the identification of food, predators and for mates. Whole picture  Humans Can detect ~10000 odors Can detect ~1000 odorants With training, individuals such as perfumers can increase this number tenfold Have ~100 million olfactory receptor cells (olfactory bipolar neurons) Have 300-500 genes coding olfactory receptor proteins Have 1000 types of odorant receptor proteins  Odorants Mostly are organic Most odorants are organic molecules although some well known odorants like ammonia are inorganic Are small molecules Since molecules that are too heavy cannot easily move through air after evaporation, the maximum weight of an odorant molecule is around 300 dalton. Odorants are fairly small molecules with at most around 20 carbon atoms.  Olfactory membranes 1. Olfactory mucosa 2. VMN Upper part of the nasal cavity Nasal septum Olfaction Pheromones  Vomeronasal organ (VMN) Another patch of olfactory membrane is present along the nasal septum, called VMN organ VMN organ detects pheromones Pheromones are volatile chemicals that are produced by some species and used to transmit messages, generally to other individuals of the same species. Pheromone: from the Greek words “ pherein” (to transport) and “ hormon” (to stimulate). Olfactory mucosa in humans Cribriform plate of ethmoid bone Location: Upper part of the nasal cavity Total surface area: 5 cm2 Innervation: Cranial nerve I (olfactory nerve) Bowman glands: Secret mucus Has 3 cell types: 1. Odorant receptor cells (olfactory bipolar cells) 2. Supporting cells → provide physical support = sieve; = in the shape of 3. Basal cells (stem cells) → produce new  Mucus secreted by Bowman glands Mucus is composed primarily of a mucopolysaccharide solution containing enzymes, antibodies, salts, and odorant-binding proteins (OBP) The total mucus content is replaced every 10 minutes and Mucus 1. Acts as a physical protective barrier over the olfactory epithelium 2. Has immune function because it contains Ab 3. Increases the sensitivity of olfactory system by increasing the concentration of odorants in the vicinity of odorant receptor proteins  Water soluble vs. lipid soluble odorants The water-soluble odorants dissolve in the mucus overlying the olfactory epithelium to reach the cilia of the olfactory receptors. The lipid-soluble odorants bind to an Odorant-Binding protein (OBP) to get transported in the hydrophilic mucus and reach the receptors. Olfactory bipolar neurons Olfactory receptor cells Olfactory chemoreceptors Olfactory sensory neurons Olfactory bipolar neurons Number: ~100 million Life span: 60 days Unmyelinated Makes cranial nerve I (olfactory nerve) Cell body  Olfactory trasnduction Olfactory receptor proteins are GPCRs coupled with Golf (metabotropic receptors) Olfactory pathways  (Axonless) Olfactory Olfactory bulb mucosa  Olfactory bulb Convergence of first order neurons on second order neurons: 100 to 1→ ↑ olfactory sensitivity Olfactory tract=Axons of mitral and tufted cells Olfactory pathway  Rapid adaptation of olfactory sensations The olfactory receptors adapt about 50% in the first second or so after stimulation. Thereafter, they adapt very little and very slowly. Smell sensations adapt almost to extinction within a minute or so after entering a strongly odorous atmosphere. This adaptation occurs in: 1. Receptors 2. CNS  Olfactory threshold Olfaction has a low threshold, is extremely sensitive, and can detect even the lightest of smells The substance methylmercaptan can be smelled when only one 25 trillionth of a gram (4 × 10-10 mg/mL) 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 pipeline.  Why olfaction is so sensitive? 1. Presence of cilia on dendrites of olfactory bipolar neurons that increase surface area for interaction of odorants and protein receptors 2. Presence of OBP in olfactory mucus that concentrates odorants 3. High convergence ratio of first-to-second order neurons 4. Using Golf-ACIII-cAMP pathway for olfactory transduction Gradations of Smell Intensities Although the threshold concentrations of substances that evoke smell are extremely slight, for many (if not most) odorants, concentrations only 10 to 50 times above the threshold evoke maximum intensity of smell. Ranges of intensity discrimination are: Eye: 500,000 to 1 Ears: 1 trillion to 1 Smell is concerned more with detecting the presence or absence of odors rather than with quantitative detection of their intensities. Thank you