Special Senses - Taste & Olfaction .docx

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Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Kem Rogers

Special Senses: Taste & Olfaction • Receptors for general senses are widely distributed • Special sensory receptors are localized & confined to head region • Receptors for special senses are not free endings of sensory neurons but distinct receptor cells • Neuron-like epithelial cells or small peripheral neurons that transfer information • Special sensory receptors are housed either in complex sensory organs (eye or ear) or in distinctive epithelial structures (taste buds or olfactory epithelium) • Sensory information travels to brain via cranial nerve The Chemical Senses: Taste & Smell • Receptors for taste (gustation) & smell (olfaction) are classified as chemoreceptors Taste (Gustation) Taste Buds • Location of taste receptors in mucosa of the mouth & pharynx • Majority of the 10,000 or so taste buds are on surface of the tongue • Few others occur on posterior region of palate (roof of mouth), on inner surface of cheeks & on posterior wall of the pharynx and on the epiglottis (leaf-shaped flap behind tongue) • Most taste buds occur in peglike projections of the tongue mucosa called papillae • Taste buds occur within epithelium that covers papillae • In small fungiform papillae scattered over entire surface of tongue –buds are on apical surface • In small filiform papillae scarred over entire surface of tongue • In large (circum)vallate papillae arranged in inverted V near back of tongue –buds are in side walls • In foliate papillae on posterolateral surface of tongue – taste buds are in the side walls • Each taste bud - globular collection of 50-100 epithelial cells that resembles bud on tree or closed tulip • Each contains two major cell types: o Gustatory epithelial cells o Basal epithelial cells • Long microvilli: gustatory hairs project from gustatory epithelial cells & extend through taste pore to surface of the epithelium • Microvilli then are bathed in saliva containing dissolved molecules that stimulate taste • Molecules bind to plasma membrane, inducing gustatory epithelial cells to generate impulses in nerve • Cells in taste buds are replenished every 7-10 days by division of basal epithelial cells, replacing ones that are scraped & burned off during eating • Entire bud is destroyed – new one will form after nerve ending grows back into regenerating epithelium Taste Sensations & Gustatory Pathway • Taste sensation can be described in terms of 5 basic qualities: o o Sweet o Sour o Bitter o Salty o o Umami (deliciousness)  Elicited by substance: glutamate that is found naturally in meat, aged cheeses & tomatoes • Combination of activated cells + pattern of activity = specific tastes • Exception: bitter receptors found on many cells • Maps that assign specific taste sensations to specific areas of the tongue are common • Mapped areas are dubious & all modalities of taste can be elicited from all areas that contain taste buds • Taste information reaches brain stem & cerebral cortex through gustatory pathway • Sensory fibres carrying taste information occurs in 3 nerves: o Facial (VII) transmits impulses from taste receptors from anterior 2/3 of tongue o Glossopharyngeal (IX) carries sensations from tongue’s posterior 1/3 & few taste buds in pharynx o Vagus (X) carries taste impulses from few taste buds on epiglottis & lower pharynx o Overlap – facial & glossopharyngeal • Thalamic relay: ventral posteromedial nucleus of thalamus • All sensory neurons that carry taste information synapse in medullary nucleus: solitary nucleus • Impulses are then transmitte
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