Olfaction What is the function of olfaction Approach Avoidance Additional Social Role pheromones Dogs have more olfactory receptor neurons that humans and rats have more types of olfactory receptors than humans What are the functional consequences of these differences 20 mil olfactory sensory neurons in humans over 10x that in dogs SO Dogs have a very good detection of smell Speaking of dogs 5 of dog brain dedicated to olfaction 0 1 of human brain dedicated to olfaction 350 different olfactory receptors in humans 1K in mice SO Mice are better at differentiating between smells What makes something an odorant Are there compounds that should be odorants but we cannot smell Small 5 8x10 22 g Volatile can float thru air Hydrophobic We cannot smell Not all molecules that should smell to humans according to above criteria do smell to us they meet the requirements but we cant smell them Natural gas methane CO bi product of methane Why don t we smell them Because our olfactory system isn t made to detect them We don t have olfactory receptors that detect those We don t have those receptors because there was not an evolutionary pressure to be able to detect those Anatomy of the olfactory system What are turbinates What is the olfactory epithelium what types of cells does it contain Olfactory bulb Cribriform plate Glomeruli Olfactory nerve Turbinates Ridges that add turbulence to incoming air to send some thru the olfactory cleft and onto the olfactory epithelium Olfactory epithelium 2 75 inches up from nostril 1 2 sq inches A plate of bone ventral to the brain 3 cell types receptor cells for smell Supporting cells Basal cells Stem cells that mature into new olfactory receptor neurons Olfactory sensory neurons Neurons with axons Metabotropic Receptor proteins are on the cilia Die are replaced every month or so 20 million Small neurons w cilia Project to glomeruli clusters of neurons in olfactory bulb Olfactory bulb Contains the olfactory receptor cells that synapse on the clusters glomeruli of dendrites of mitral cells and tufted cells Cribriform plate bony plate Glomeruli clusters of neurons in the olfactory bulb Olfactory nerve carries info to the brain Understand the anatomy of the olfactory receptor cells in olfactory epithelium cilia dendrites extend into mucosa and contain receptors axon travels thru cribriform plate as part of olfactory nerve and ends in olfactory bulb Axons go thru cribriform plate bony plate and end in olfactory bulb All of these axons together from all OSNs create the olfactory nerve Ipsilateral projection all the way to cortex CN I How is the olfactory information organized in the olfactory bulb especially w regard to glomeruli What is the projection neuron within the glomerulus that receives info from the olfactory nerve and sends info to the cortex What parts of cortex get olfactory info Where else in the brain does olfactory information go and what are the roles of each of those regions Organization very well organized Axons from olfactory receptor cells synapse on clusters glomeruli Glomerulus projection neuron tufted cells all w in olfactory bulb is composed of dendrites of mitral cells main 20 million OSNs 350 different types of receptors Each cell only has 1 kind of receptor different receptors scattered throughout olfactory epithelium but all converge on same glomerulus Brain knows from what glomerulus axons have traveled which helps brain determine smell like labeled line right But an odorant activates a pattern of OSNs so pattern of glomeruli so really cross fiber patterning Glomeruli pattern of activation differs based on experience preference for the odorant Projection neuron Mitral Cells Cortex Olfactory Bulb mitral cells projects directly to Entorhinal cortex Piriform cortex Orbitofrontal cortex Assigning value to things Flavor Amygdala and other places not exactly limbic Emotional memory Approach avoidance behavior Amygdala Hypothalamus Hormones Reproductive behavior Feeding What is shape pattern theory The shape of the odorant determines what receptors it binds to which causes a pattern of activity Lock receptor and key odorant if the key fits into the lock it activates that receptor Odorant molecules have different shapes Olfactory receptors have different shapes Different scents as a function of odorant shape or OR shape fit activate different arrays of olfactory receptors in the epithelium Various arrays produce specific firing patterns of neurons in OB a pattern is generated across the glomeruli which determines what scent we perceive Do we use labeled line or cross fiber patterning to identify odors Understand why We use cross fiber patterning We understand what orders are based on a pattern of activity across a lot of different types of receptors and each type of receptor converges on a different part of the olfactory bulb Staircase method Triangle test Similar to method of limits in audition Odor presented in increasing concentration increments then decreasing multiple times To determine if a person can discriminate between odorants One of these things is not like the other What is anosmia and what can cause it What is specific anosmia and what is its most common cause Anosmia inability to smell 50 of people at or older than 85 are anosmic Causes Whip lash type accident sheer axons of olfactory receptor cells grow back If cribriform plate is fractured by blow to head it could scar and axons cannot regrow Other causes of anosmia Sinus infection Sinonasal disease polyps Medications for high blood pressure or cholesterol also antibiotics and others also some illicit drugs Congenital rare Kallmann Syndrome see notes section of slide if interested Connection w mental health Lose sense of smell more likely to go into clinical depression Depressed report altered sense of smell Specific anosmia Common cause USUALLY GENETIC What are some differences between men and women in olfaction Women detect odors more readily than men Women detect odors at lower thresholds than men especially when ovulating Brain responses to odors are stronger in women Occurs in all age groups and cultures tested Women of child bearing age sensitize to odors and they smell them in very faint amounts Prepubescent women post menopausal women and men don t Odor more important to women than men in choosing partners Smell is largely synthetic sense like color mixing don t perceive all of the molecules or wavelengths as for color separately perceive a synthetic whole bacon for
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