The Chemical Senses.doc

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Anatomy and Physiology
Jacqueline Carnegie

The Chemical Senses: Taste and Smell -the receptors for gustation and olfaction are chemoreceptors that respond to chemicals in an aqueous solution -the receptors complement each other and respond to different classes of chemicals -taste receptors are excited by food chemicals dissolved in saliva -smell receptors are excited by airborne chemicals that dissolve in fluids coating the nasal membrane Olfactory Epithelium and Sense of Smell -olfaction detects chemicals / odorants in solution -the olfactory epithelium acts as the organ of smell -made up of pseudostratified epithelium -located in the roof of nasal cavity -air entering the nasal cavity must make a hairpin turn to stimulate olfactory receptors before entering the respiratory passageway below -sniffing draws more air superiorly across the olfactory epithelium and intensifies the sense of smell -the olfactory epithelium covers the superior nasal concha on each side of the nasal septum and contains millions of olfactory receptors -olfactory receptors are surrounded and cushioned by columnar supporting cells that make up the bulk of the epithelial membrane -basal cells lie at the base of the epithelium -olfactory receptor cells are unusual bipolar neurons -each neuron has a thin apical dendrite that terminates in a knob from which several long cilia radiate known as olfactory cilia -olfactory cilia increase the receptive surface area and are covered by mucus produced by the supporting cells -the mucus is a solvent that captures and dissolves airborne odorants -olfactory cilia are non-motile (do not beat) -olfactory receptor cells have a lifespan of 30-60 days and are replaced by differentiation of the basal cells in the olfactory epithelium -there are atleast 1000 smell genes that are active only in the nose, which detect about 10 000 odors; just a few molecules are necessary to activate olfactory neurons Physiology of Smell -in order to smell a particular odorant, it must be volatile (in a gaseous state) and dissolve in the fluid coating the olfactory epithelium 1) Activation of Olfactory Receptors -dissolved odorants stimulate the olfactory receptors by binding to receptor proteins in the olfactory cilium membranes, opening cation channels and generating a receptor potential -when threshold stimulation is reached, an action potential is conducted to the first relay station in the olfactory bulb 2) Smell Transduction -transduction of odorants uses a receptor linked to a G protein -olfactory transduction begins when an odorant binds to a receptor -this activates a G protein (G ) olf -G olfivates enzymes (adenylate cyclases) that synthesize cyclic AMP as a second messenger -cAMP acts directly on a plasma membrane cation channel, causing it to open, allowing Na and Ca 2+ to enter + -an i2+lux in Na leads to depolarization and impulse transmission - Ca causes the transduction process to adapt, decreasing its response to a sustained stimulus; this is known as olfactory adaptation The Olfactory Pathway -axons of the olfactory receptor cells form olfactory nerves (not true nerves; actually olfactory tracts) that synapse in the overlying olfactory bulbs (the distal ends of the olfactory tracts) -filaments of the olfactory nerves synapse with mitral cells to form glomeruli -each glomerulus represents a single aspect of an odor, but each odor activates a unique set of glomeruli and different odors activate different subsets of glomeruli -mitral cells act to refine the signal, amplify it, and then relay it -olfactory bulbs house GABA-releasing cells that inhibit mitral cells so only highly excitatory olfactory impulses are transmitted -when mitral cells are activated, impulses flow from olfactory bulbs via the olfactory tracts (mitral cell ax
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