The Special SensesPart A

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ObjectivesDescribe the location, structure, and afferent pathways of taste and smell receptors, and explain how these receptors are activated.

Cranial Nerves and Functions

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Chemical Senses5 SensesTouch, Taste, Smell, Vision and HearingTouch Excluded as special senseVarious aspects of touch (Pain, heat, pressure) Somatic senseChemical senses gustation (taste) and olfaction (smell) The receptors for taste (gustation) and smell (olfaction) are Chemoreceptors.Chemoreceptors respond to chemicals in aqueous solutionTaste to substances dissolved in salivaSmell to substances dissolved in fluids coating nasal membranes

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Taste Buds and the Sense of TasteThe word taste comes from the Latin taxare, meaning to touch, estimate, or judge. It is considered by many to be the most pleasurable of the special senses.Taste Bud: Sensory receptor organs for taste10,000 or so taste buds Most of the them are found on the tongue(cheeks, pharynx)Taste buds are found in papillae of the tongue mucosaPapillae Peglike projections of the tongue mucosa give slightly abrasive feelFiliform papillaeThin, long papillae "V"-shaped cones DDon't contain taste buds but are the most numerous. Mechanical and not involved in gustation.

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Types of PapillaeFungiform papillaemostly at the apex (tip) of the tongue, as well as at the sides. Innervated by facial nerve. Contains most taste budsFoliate papillaeRidges and grooves towards the posterior part of the tongue found on lateral margins. Facial nerve (anterior papillae)glossopharyngeal nerve (posterior papillae). Circumvallate papillae Only about 3-14 of these papillae on most people, Present at the back of the oral part of the tongue. Form an inverted V at the back of the tongue.Innervated by the glossopharyngeal nerve.

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Anatomy of a Taste BudEach Funnel-shaped taste bud consists of 50 to 100 three major cell types

Supporting cells insulate the receptor Basal cells dynamic stem cells (dividing and differentiating into new gustatory cells)Gustatory cells taste cellsLong microvilli called gustatory hairs project from the tips of all gustatory cells and extend through a taste pore to the surface of the epithelium, where they are bathed by saliva.The gustatory hairs are the sensitive portions (receptor membranes) of the gustatory cells.

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Taste Buds

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Taste SensationsThere are five basic taste sensationsSweet sugars, saccharin, alcohol, and some amino acidsSalt metal ionsSour hydrogen ionsBitter alkaloids such as quinine and nicotineUmami (Savoury) elicited by the amino acid glutamateSweet, Bitter, and Umami work with a signal through a G protein-coupled receptor. Salty and Sour, which work with ion channels.

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Physiology of TasteIn order to be tasted, a chemical:Must be dissolved in salivaMust contact gustatory hairsActivation of Taste Receptors:Binding of the food chemical to receptorsDepolarization the taste cell membrane, Releasing neurotransmitterInitiating an action potentialThreshold of activationBitter receptors detect substances present in minute amountsOther receptors are less sensitive Adaptation is rapid, with partial adaptation in 35 seconds and complete adaptation in 15 minutes.

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Taste TransductionPartly knownEach taste quality has its own special mechanism. All of these mechanisms ultimately cause an increase in intracellular Ca2+, which increases neurotransmitter releaseThe stimulus energy is converted into a nerve impulse by:Na+ influx in salty tastesH+ influx in sour tastes by one of three waysDirectly entering the cell, Opening cation channels,Blockade of K+ channelsG-protein mediated (Gustducin)Bitter causes Ca2+ release from intracellular storesSweet causes closure of K+ channels,( depolarizing the cell )

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Gustatory PathwayCranial Nerves VII(Facial), (Anterior two-thirds of the tongue)IX(glossopharyngeal) (posterior third and the pharynx just behind)X (vagus) (epiglottis and the lower pharynx )carry impulses from taste buds to the solitary nucleus of the medullaThese impulses then travel to the thalamus, and from there fibers branch to the:Gustatory cortex (taste)Hypothalamus and limbic system (appreciation of taste)

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Gustatory Pathway

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Influence of Other Sensations on TasteTaste is 80% smellWhen olfactory receptors in the nasal cavity are blocked by nasal congestion (or pinching your nostrils), food is blandThermoreceptors, mechanoreceptors, nociceptors also influence tastesTemperature and texture enhance or detract from tasteHot foods such as chili peppers actually bring about their pleasurable effects by exciting pain receptors in the mouth

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Sense of SmellOlfact (To Smell)The organ of smell is called the olfactory epitheliumYellow-tinged patch (about 5cm2) Pseudostratified epithelium Located in the roof of the nasal cavity Olfactory receptor cells are bipolar neurons with radiating olfactory ciliaOlfactory receptors are surrounded and cushioned by supporting cellsBasal cells lie at the base of the epithelium

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Sense of Smell

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Olfactory PathwayOlfactory receptor cells synapse with mitral cellsGlomerular mitral cells process odor signalsMitral cells send impulses to:The olfactory cortex The hypothalamus, amygdala, and limbic system

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Physiology of SmellHumans can distinguish10,000 or so odoursOlfactory receptors respond to several different odour causing chemicalsWhen bound to ligand these receptors initiate a G protein mechanism, which uses cAMP as a second messengercAMP opens Na+ and Ca2+ channels, causing depolarization of the receptor membrane that then triggers an action potential

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Olfactory Transduction Process

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