Chapter 14 - PSYB51

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Mathias Niemier

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Chapter 14 – Taste  Olfaction and gestation are grouped together as chemical senses, and in terms of physiology, two sensory systems are in some ways quite similar  Taste serves most specific function of any of senses – discerning which chemicals we need to ingest because they are nutritious and which we need to spit out because they may be poisonous Taste Vs. Flavor  Molecules we taste are dissolved in saliva and passed over taste receptors on taste buds  When we chew and swallow foods, molecules are released into air inside mouths and forced up behind palate into nasal cavity  Retronasal olfactory sensations: sensation of odor that is perceived when chewing and swallowing force an odorant in mouth up behind palate into nose. Odor sensations are perceived as originating from mouth, even though actual contact of odorant and receptor occurs at olfactory mucosa  Flavor: combination of true taste and retronasal olfaction  Foods perceived by somatosensory system via touch, temperature, and pain receptors in tongue and mouth  Somatosensations provide info about nature of foods and beverages  Localizing Flavor Sensations o Because you taste and feel food only in mouth, brain concludes that sensations must have arisen entirely from mouth o Chorda tympani: branch of cranial nerve VIII (facial nerve) that carries taste info from anterior mobile tongue (part that can be stuck out). Chorda tympani nerve leaves tongue with lingual branch of trigeminal nerve (cranial nerve V) and passes through middle ear on its way to brain o Cranial nerves: 12 pairs of nerves (one from each pair for each side of body) that originate in brain stem and reach sense organs and muscles through openings in skull o In both patient and experimental subjects, pathway from mouth and nasal cavity was completely intact  Why were retronasal sensations blocked? - Brain processes odors differently, depending on whether they come from mouth or through nostrils. Distinction makes good sense functionally because significance of odors in mouth is different from that of odors sniffed from outside world Anatomy and Physiology of Gustatory System  Taste perception consists of sequence of events – chewing breaks down food substances into molecules which are dissolved in saliva o Saliva borne food molecules flow into taste pore that leads to taste buds embedded in papillae o Taste buds: globular clusters of cells that have function of creating neural signals conveyed to brain by taste nerves. Some of cells in taste bud have specialized sites on apical projections that interact with taste stimuli. Some of cells form synapses with taste nerve fibers o Papillae: structures that give tongue bumpy appearance. From smallest to largest, papillae types that contain taste buds are fungiform, foliate and circumvallate; filiform papillae, which don’t contain taste buds, are smallest and most numerous  Taste receptor cells: cells within taste bud that contain sites on apical projections that can interact with taste stimuli. Sites fall into 2 major categories – interacting with charged particles and interacting with specific chemical structures  Each taste receptor cell responds to limited number of molecule types; when one of its preferred molecules makes contact with it, produces action potentials that send info along one of cranial nerves to brain  Taste Buds and Taste Receptor Cells o Each taste bud is a cluster of elongated cells, organized much like segments of an orange o Microvilli: slender projections on tips of some taste bud cells that extend into taste pore o Taste nerve fibers once believed to be connected to receptors on one end and brain at other end o Considerably more complex series of events is now beginning to emerge o In fungiform papillae, taste nerve fibers that enter taste buds branch so that individual cell can be innervated by more than one taste fiber and an individual taste fiber can innervate more than one cell o Taste receptors have limited life span – 10 days and then replaced o Tastant: any stimulus that can be tasted. Contacts its microvilli and divided into 2 large categories  One class of tastants made up of small, charged microvilli membranes allow some types of charged particles to enter cells but bar others  In second class, produce sensations that we label as sweet or biter, perceived via a mechanism similar to that in olfactory system, using G protein-coupled receptors (GPCRs). The GPCRs wind back and forth across microvillus membranes and when particular tastant molecule “key” is fitted into “lock” portion of a GPCR on outside of membrane  Taste Processing in CNS o After leaving taste buds through cranial nerves, gustatory info travels through way stations in medulla and thalamus before reaching cortex o Primary cortical processing area for taste - part of cortex that first receives taste info – insular cortex
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