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by: Mr. Ana Muller


Mr. Ana Muller
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Class Notes
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This 6 page Class Notes was uploaded by Mr. Ana Muller on Saturday September 12, 2015. The Class Notes belongs to BIOL 3800 at University of Georgia taught by Lauderdale in Fall. Since its upload, it has received 78 views. For similar materials see /class/202300/biol-3800-university-of-georgia in Biology at University of Georgia.




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Date Created: 09/12/15
Overview 0 There are three sensory systems associated with the nose and mouth the olfactory smell gustatory taste and trigeminal chemosensory irritant systems 0 The olfactory system detects airborne molecules called Odorants o Olfactory information thus influences social interactions reproduction defensive responses and feeding behavior 0 The gustatory system detects ingested tastants which are primarily water or fat soluble molecules Tastants provide information about the quality quantity and safety of ingested food 0 Trigeminal chemosensory system provides information about irritating or noxious molecules that come into contact with skin or mucous membranes of the eyes nose and mouth 0 For smell and taste essential aspects of sensory transduction rely on G protein coupled receptors and second messenger mediated signaling 0 The chemical senses particularly olfactionare deemed to be the oldestmost primitive sensory systems The Organization of the Olfactory System 0 Processes information about the identity concentration and quality of a wide range of airborne volatile chemical stimuli called Odorants o Odorants interact with olfactory receptor neurons found in an epithelial sheet the olfactory epithelium that lines the interior of the nose 0 The axons arising from the receptor cells project directly to neurons in the olfactory bulb which in turn sends projections to the pyriform cortex in the temporal lobe o The pathway is known as the olfactory tract 0 This tract DOES NOT INCLUDE A THALAMIC RELAY from primary receptors en route to a neocortical region for processing therefore THALAMUS DOES PLAY AN IMPORTANT ROLE IN OLFACTION THOUGH o Olfactory information from pyriform cortex is relayed to the thalamus where further processing can occur 0 Olfactory receptors9olfactory nerve9olfactory bulb olfactory tract projects to either pyriform cortex then to orbitofrontal cortex amygdale either to orbitofrontal cortex thalamus or hypothalamus or entorhinal cortex then to hippocampal formation 0 Maximal activity is focused on orbitofrontal cortex for pleasant odors or on pyriform cortex olfactory bulbs or amygdale if the odor it intense o The olfactory system projects to the hypothalamus and amygdale sometimes these areas influence motor visceral and emotional reactions to olfactory stimuli o Interactions with stimuli airborne at the periphery are transduced and encoded by receptors into electrical signals which are then relayed to higher order centers Olfactory Perception in Humans o In humans olfaction is considered the least acute of the senses o Other mammals dogs have more olfactory receptor neurons and devote a proportionally larger area of the forebrain to olfaction o The proportional size of the olfactory bulb and related structures dedicated to processing other sensory modalities is quite large in rodents and carnivores 0 Human olfactory system is capable of making perceptual distinctions based on small changes in molecular structure enantiomers o Odor r 0 Threshold concentrations for detecting odorants vary greatly r 39 can be I on 39 of the odorant low vs high concentration 0 Classification of odors pungent floral musky earthy ethereal camphor peppermint ether and putrid o Empiricalclassification 0 These basic properties of olfaction pleaseant or unpleasant are apparently represented in distinct cortical regions 0 Although most people are able to consistently identify a broad range of test odorants some fail to identify some common smells This is called anosmia This is often restricted to a single odorant can be congenital or acquired due to chronic sinus infection or other diseases also aging 0 Human olfactory capability normally decreases with age Physiological and Behavioral Responses to Odorants o Odorants can also elicit a variety of physiological response salivation or gaggingvomiting o Olfaction can also influence reproductive and endocrine functions synchronized menstrual cycles o In other animals speciesspecific odorants called pheromones play important roles in behavior influencing social reproductive and parenting behaviors 0 They are detected by Gproteincoupled receptors located at the base of the nasal cavity in distinct encapsulated chemosensory structures called vomeronasal organs o In humans genes that encode vomeronasal receptor proteins are not expressed so no pheromone perception 0 However exposure to androgen and estrogen like compounds may increase sexual attraction at concentrations below conscious detection Olfactory Epithelium and Olfactory Receptor Neurons o The transduction of olfactory information occurs in the olfactory epithelium the sheet of neurons and supporting cells that lines half the nasal cavity surface 0 The remaining surface is lined by respiratory epithelium that maintains appropriate moisture for inhaled air and provides an immune barrier o The olfactory epithelium includes several cell types 0 Most important of these is the olfactory receptor neurons ORNs I They are bipolar cells and give rise to smalldiameter unmyelinated axons at their basal surface that transmit olfactory information centrally I At the apical surface an ORN gives rise to a knkoblike protrusion from which olfactory cilia extend into a thick layer of mucus 0 Supporting cells detox potentially dangerous chemicals 0 Mucus is produced by Bowman s glands that are distributed throughout the olfactory epithelium I The mucus secreted by Bowman s glands traps and neutralizes potentially harmful agents 0 ORNs are generated continuously from dividing stem cells Generation of receptor potentials in response to odors takes place in the cilia o Odorants presented to the cilia of an olfactory receptor neuron elicit a robust electrical response those presented to the cell body do not Odorant Receptor Proteins Olfactory receptor molecules are homologous to a large family of G protein linked receptors Each receptor is specific to a particular set of odorant molecules The specificity of odorant recognition and signal transduction is the result of this molecular variety of odorant receptor molecules in the nasal epithelium Odorant receptors are the largest known gene family representing 35 of all genes Human genome has 950 odorant receptor genes 1500 in mice 1200 in dogs 0 many of these genes 60 in humans vs 1520 in rodents and dogs are not transcribed Thus the number of functional odorant receptor proteins is estimated around 400 in humans and 1200 in mice and 1000 in dogs Analyses have shown that each olfactory receptor neuron expresses only one or at most a few odorant receptor genes furthermore only one of the two copies of each odorant receptor gene is expressed in any particular olfactory receptor neuron Thus different odors must activate molecularly and spatially distinct olfactory receptor neurons The molecular diversity of the odorant receptors and the accompanying cellular diversity for olfactory receptor neurons most certainly mediates the capacity of most olfactory systems to detect and encode a wide range of odors The Transduction of Olfactory Signals Once an odorant is bound to an odor receptor protein several additional steps are required to generate a receptor potential to convert chemical information to electrical signals to be interpreted by the brain The olfactory receptor neurons express an olfactoryspecific Gprotein G0 which in turn activates adenyl cyclase Iquot an olfactory specific sdenylate cyclase Stimulation of odorant receptor molecules leads to an increase in cyclic AMP which opens cyclic nucleotidegated channels that permit the entry of Na and Ca thus depolarizing the neuron This depolarization amplified by a Caactivated Cl current generates action potentials at the axon hillock via voltage regulated Na channels Inactivation of any one of the major signal transduction elements Golf ACIII or the cyclic nucleotidegated channel results in a loss of receptor potential response to odorants in olfactory receptor neurons 9 no behavior response to odorants Mechanism 0 Odorant molecule binds to receptor protein activates Gprotein Golf which then activates adenyl cyclase III This in turn produces cAMP as the second messenger which opens the cyclic nucleotidegated channel for Na and Ca entry Ca can then activate a Cagated Cl channel to expel Cl out of the cell further depolarizing the cell Individual olfactory receptor neurons are sensitive to subsets of stimuli therefore there is receptor specificity 0 Some olfactory receptor neurons exhibit marked selectivity to a single chemically defined odorant whereas others are activated by a number of odorant molecules 0 There is a response quotsignaturequot to different odorants for ORNs that express individual odorant receptors The different responses seem to reflect chemical differences in the odorants as well as the overall quality of the odorants o Olfactory receptor neurons also exhibit different thresholds for a particular odorant That is neurons that are inactive at a background concentration of a given odorant may be activated when exposed to higher concentrations of that odorant Adaptive capacity of the olfactory system 0 Decreased awareness of smoke smell after being in a room full of smoke The Olfactory Bulb The transduction of odorants in the olfactory cilia and the subsequent changes in electrical activity in the olfactory receptor neuron are only the first steps in olfactory information processing Unlike other primary sensory receptor cells olfactory receptor neurons have axons and these axons relay odorant information directly to the brain via action potentials As the axons leave the olfactory epithelium they coalesce to form a large number of bundles that make up the olfactory nerve Each olfactory nerve projects ipsilaterally to the olfactory bulb o The most distinctive feature of the olfactory bulb is an array of more or less spherical accumulations of neuropil called glumeruli which lie just beneath the surface of the bulb and are the synaptic target of the primary olfactory axons vertebrate ORNs make excitatory glutamatergic synapses with the glomeruli o Periglomerular cells contribute to the glomerulus thought to sharpen sensitivity of glomeruli to specific odorants Within each glomerulus the axons of the receptor neurons contact glutamatergic apical dendrites of mitral cells which are the principal projection neurons of the olfactory bulb o A mitral cell extends its primary dendrite into a single glomerulus where the dendrite gives rise to an elaborate tuft of branches onto which the axons of olfactory receptor neurons synapse 0 Information about odors reaches a variety of forebrain regions allowing olfactory perception to influence cognitive visceral emotional and homeostatic behaviors 0 Increasing the odorant concentration increases the activity of individual glomeruli as well as the number of glomeruli activated 0 Different single odorants maximally activate one glomeruli 0 Complex odorants activate a relatively small number of glomeruli each of which responds selectively to one or two molecules that characterize the complex odor o Mitral cell axons provide the only relay for olfactory information to the rest of the brain 0 The mitral cell axons from each olfactory bulb form a bundle the lateral olfactory tract that projects to the accessory olfactory nuclei the olfactory tubercle the pyriform and entorhinal cortex 0 The major target of the olfactory tract is the threelayered pyriform cortex Neurons in the pyriform cortex responds to odors The Organization of the Taste System 0 Taste system represents the chemical as well as physical qualities of ingested substances 0 Taste reflects the aesthetic and nutritive qualities of food indicating if it is safe to eat 0 Once in the mouth the chemical constituents of food interact with receptor proteins on taste cells located in epithelial specializations called taste buds in the tongue 0 Taste cells transducer these stimuli to encode information about the identity concentration and pleasant unpleasant or potentially harmful qualities of the substance 0 Taste cells are found in taste buds distributed on the dorsal surface of the tongue soft palate pharynx and the upper part of the esophagus 0 Taste cells synapse with primary sensory axons that run in cranial nerves VII IX and X to innervate the taste buds in the tongue palate epiglottis and esophagus o The central axons of these primary sensory neurons in the respective cranial nerve ganglia project to the nucleus of the solitary tract aka gustatory nucleus before synapsing at the ventral posterior medial nucleus of the thalamus o Axons from the gustatory part of the solitary nucleus terminate in the ventral posterior medial nucleus of the thalamus This nucleus projects in turn to several regions of the neocortex 0 Finally reciprocal projections connect the nucleus of the solitary tract via nuclei in the pons to the hypothalamus and amygdale influence affective aspects of appetite satiety and other homeostatic responses Taste Perception in Humans The taste system encodes information about the quantity as well as the identity of stimuli Most taste stimuli are nonvolatile hydrophilic molecules that are soluble in saliva Taste system detects potentially dangerous substances at much lower concentrations Tastants are detected over the full surface of the tongue in receptive specializations called taste papillae 0 Taste buds the sites of taste receptor cells are distributed along the lateral surfaces of the taste papillae 0 There are three types of papillae fungiform 25 of total circumvalate 50 and foliate 25 o Fungiform papillae are found only on the anterior two thirds of the tongue the highest density is at the tip They have a mushroomlike structure 0 There are nine circumvallate papillae arranged at the rear of the tongue 0 Two foliate papillae are present on the posterolateral tongue Each contains the most number of taste buds compared to the others 0 Chemical stimuli on the tongue first stimulate receptors in the fungiform papillae and then in the foliate and circumvallate papillae o The taste system detects five perceptually distinct categories of tastants salt sour sweet bitter and umami savoryamino acids 0 Each tastes activate specific locations on the tongue and insular cortex 0 Sweet and umami toward the tip of the tongue 0 Salty toward the edges of the tongue 0 Sour the sides of the tongue 0 Bitter the back of the tongue Taste Buds Taste Cells Receptor Proteins and Transduction o The initial transduction and encoding of taste information occurs in taste buds 0 Taste buds consist of specialized neuroepithelial receptor cells called taste cells and basal cells 0 Taste cells are clustered around a 1mm opening in the taste bud near the surface of the tongue called a taste pore and solubilized tastants are further concentrated and presented directly to the exposed sensory receptor 0 Taste cells are replaced about every 2 weeks from basal cells 0 Taste cells in individual taste buds have distinct apical and basal domains I Apical domain is responsible for chemosensory transduction I Basal domain is responsible for generating graded receptor potentials via the release of neurotransmitter o Neurotransmitter is thought to be serotonin and ATP I Taste receptor proteins are concentrated on apical surface 0 Salty tastes are likely transduced by AMILORIDESENSITIVE Na CHANNELS o Sour tastes are transduced by a protonpermeant non selective cation TRP channel


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