BISC 421 1st Edition Lecture 27 Outline of Current LectureI.Olfactory System Current LectureOlfactory System•3 diferent types of chemical sensation •1. Olfactory or smell-‐ •2. Taste-‐ detected by tongue and roof of mouth •3. Vomeronasal-‐ sense of pheromones (mice)How can we detect so many odorants ? (estimate is that humans can smell 10,000 diferent odorants)•Smell: some of the things that have smells for us•Most things that are odorants are volatile and can be detected at very low concentrations•How do we detect so many diferent odorants with such diverse structures? How can be distinguish chemicals with very similar structures that are mirror images.•This is CONTRASTED to the visual system – in the visual system you can detect manydiferent wavelengths of light-‐ but this can be explained by the sensitivity to 3 diferent colors (photoreceptors have three diferent pigment types)•Are there odorant primaries? NoOlfactory sensory neurons project an axon to the olfactory bulbAxons pass through the cribriform plate. Odorants are detected on sensory cilia that project into the olfactory mucosa.•This is the human olfactory system•The epithelium is this boney structure that are covered with epithelium so that there is increased surface area for the odorants to bind•At high magnification-‐ see single dendrite to the olfactory surface and this ends in a dendritic knob in which there are cilia (sensory not motile) their whole function is to increase the surface area over which the odorants can be detected•From here they go to the olfactory bulb•Because the axons have to go through this perforated boney structure, what can happen is that if someone has trauma to this. These axons can be sheared off and people lose sense of smell- ‐this can be recovered though•Humans have a lower ability to smell than other animals because:•1. We have fewer receptors than other animals •2. humans have fewer receptor genes Sensory response is initiated in cilia, Sensory response is an inward current. Cell fires action potentials.•Record from a sensory neuron in the olfactory system and pufed on odorants-‐ seean initial inward current (depolarize in response to odorants)•Olfactory neurons are well behaved – depolarize unlike the visual system •Also can see that the sensitivity to odorant is on the cilia and not the cell body1. Receptor binds ligand.2. G protein exchanges GDP for GTP3. Adenylate cyclase makes cAMP from ATP4. cAMP opens a CNG (cyclic nucleotide-gated) ion channel.5. Na+ and Ca2+ comes into the cell – causing depolarization6. Ca2+ opens a Cl- channel, ensuring a response in the absence of Na+•Shows what happens during sensory transduction•1. Odorants bind receptors (standard lock and key)•2. Activates the G protein (in this case G protein will dissociate)•3. Alpha subunit activates adenylate cyclasecAMP from ATP•4. Open Na+ channel and Na+ and Ca2+ come in causing depolarization •5. Ca2+ infux activates the Cl-‐ channel (depolarizes cell) which will ensure this activation in absence of Na+Knock-out mice for olfactory transduction componentsWhat about the receptors?•the receptor is what provides the specificity – because we can respond to many odorants we must have a lot of odorant receptors.Receptors are• 7 TM/ G protein coupled• Encoded in intronless genes• Scattered throughout the genome• Number in the hundreds(Linda Buck and Richard Axel, NobelPrize, 2004)Functional receptorsMouse Human1037 388•Found that the receptors are g protein coupled and have 7 transmembrane domains and are coded in intronless genes (important because the gene is duplicating-‐ hundreds of these genes throughout the genome)•In humans there are about 400 functional receptors (in mice there are 1000)•1. Odorants are being detected by diferent combinations of odorant receptors•2. Each olfactory sensory neuron is making just one type of receptor•3. The olfactory sensory neurons are randomly distributed (no real organization of receptors)-‐smells can be detected anywhere•4. Each neuron as a consequence of only having one type of odorant receptor responds to aSUBSET of odorants•5. The olfactory sensory neurons although they are randomly distributed they converge and find each other in the olfactory bulb•How do we know this?•Method-‐ Transgenic mouse-‐ any particular odorant receptor has a specific locus on the chromosome•Can modify that piece of DNA and introduce GFP-‐ fuorescent protein•Normally when the cell is transcribing a gene, you only make that one gene and then things turn off•Trick: Iris allows the generation of GFP whenever M71 is being transcribed •Therefore makes both proteins – can label the specific odorant receptors•Same thing in higher resolution•Can now see it for 2 other ones •Random but there are these zones and within these zones the receptors are randomly distributed(4) Each neuron responds to a specifc subset of odorants"This is the response profile of a random cell.This is the response profile of a cell that has the M71 odorant receptor.•Can record responses to odorants in individual cells•Recording from a particular cell and this is a response profile from a mixture of random cells(average response)•Can see that you get sensitivity to all of these diferent odorants if record across •Shows that cells respond to subsets of odorants(5) Convergence in the olfactory bulb!Mombaerts et al, 1996•Glomerulus-‐ collection of the axons of the presynaptic cells with a dendrite of the postsynaptic cell•Visualized using GFP – put marker that generates blue color•The olfactory sensory neurons that make the same odorant receptor converge on the same glomerulus in the olfactory bulb•Example of the convergence in the olfactory bulb•Now know why we can detect so many diferent odorants-‐ because there are so many diferent odorant