Lecture9_taste_smell - Chemical senses Olfaction and Taste...

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Chemical senses: Olfaction and Taste
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Transduction by sensory systems Mechanical senses transduce mechanical energy into nerve impulses Somatosensation Vestibular Hearing Chemical senses transduce energy from chemical reactions into nerve impulses Nociception Taste Olfaction Photosenses transduce electromagnetic energy into nerve impulses Vision
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Chemical senses The function of taste and olfactory senses is to evaluate the qualities of substances we ingest into our bodies, namely, the food we eat (or drink) and the air we breathe The sense of taste arises from sensory receptors on the tongue called taste buds that directly contact the food and drink we ingest in our mouths Olfaction (sense of smell) arises from sensory receptors in the nasal cavity that detect aerosolized molecules in the air that we breathe into our lungs
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Olfaction As we breath in and out, aerosolized molecules are drawn into the nasal cavity The air passes across the olfactory epithelium , which is the sensory organ for smell Odor molecules bind to receptors on the hair-like cilia which extend from the olfactory epithelium, and are in fact the “dendrites” of bipolar olfactory sensory neurons Orthonasal olfaction is the sensing of odor molecules from the outside air; this occurs during inspiratory breathing Retronasal olfaction is the sensing of odor molecules from the mouth; this occurs during expiratory breathing
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Olfactory chemotransduction is mediated by g-protein receptors in the cilia of olfactory sensory neurons These g-protein receptors are embedded in the membrane of the olfactory neuron cilia Olfactory sensory neurons are sometimes referred to as “olfactory receptors” for short, but don’t confuse the sensory neurons with the g-proteins. Both can be referred to as “olfactory receptors.”
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Olfactory Receptor Gene Family There are about 1000 unique olfactory receptor genes, which is 3% of all our genes! But only about 400 of these genes are functional in humans. Each gene codes for a unique receptor, and each receptor binds a variety of different odor molecules (also most odor molecules bind to more than one receptor) All of the known olfactory receptors are g-protein coupled receptors Richard Axel Linda Buck Winners of the 2004 Nobel Prize for Physiology & Medicine
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Olfactory Sensory Neurons The olfactory epithelium is coated with olfactory sensory neurons Humans have about 40 million olfactory sensory neurons, some dog breeds have as many as 220 million Each olfactory sensory neuron expresses exactly ONE type of olfactory receptor Olfactory sensory neurons that express the same receptors tend to cluster near on another in the olfactory epithelium, as shown below for 4 receptor genes in the mouse
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Olfactory Neurons send their axons to the glomeruli of the olfactory bulb, where they synapse on mitral cells .
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