TASTE

• taste buds


  Specialized sense organ for taste
• consists of about 5000 taste buds located primarily on the papillae of the dorsal surface of the tongue
• taste buds are also located in the soft palate, epiglottis, and pharynx
• fungiform papillae


  rounded structures most numerous near the tip of the tongue
• fungiform papillae


  has up to 5 taste buds, mostly located at the top of the papilla
• circumvallate papillae


  prominent structures arranged in a V on the back of the tongue
• circumvallate and foliate papillae


  100 taste buds, mostly located along the sides
• foliate papillae


  posterior edge of the tongue
• Each taste bud contains 50–100 taste receptor cells and numerous basal cells and support cells
• Taste receptor cells are modified epithelial cells that respond to chemical stimuli or tastants.
• saliva


  acts as a solvent for tastants
• Saliva - After dissolving, the chemical diffuses to the taste receptor sites
• saliva


  cleanse the mouth to prepare the taste receptors for a new stimulant.
• Each taste bud is innervated by about 50 nerve fibers, and conversely, each nerve fiber receives input from an average of five taste buds.
• The basal cells arise from the epithelial cells surrounding the taste bud.
• They differentiate into new taste cells as taste cells survive for only about 10 days.
• If the sensory nerve is cut, the taste buds it innervates degenerate and eventually disappear
• Ant 2/3: chorda tympani branch of CN7
• posterior 3rd : CN9
• fibers from areas other than the tongue (pharynx) reach the brain stem via CN10
• On each side, the myelinated but relatively slowly conducting taste fibers in these 3 nerves unite in the gustatory portion of the nucleus of the tractus solitarius (NTS) in the medulla oblongata
• salt - sodium chloride
• sour - hydrochloric acid
• umami - monosodium glutamate
• sweet - sucrose
• bitter – quinine
• Afferent nerves to the NTS contain fibers from all types of taste receptors, without clear localization of types.
• Individual taste receptor cell may respond to more than one type of tastant.
• Salt-sensitive taste - mediated by an epithelial sodium channel (ENaC)
• Sour taste - triggered by protons (H+ ions)
• The H+ ions can also bind to and block a K+ -sensitive channe
• Hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN) and other mechanisms may contribute to sour transduction
• Sweet taste - detected by at least two types of GPCRs, T1R2 and T1R3
• Some bitter compounds (eg, quinine) are membrane permeable and bind to and block K+ - selective channels.
• Many bitter tastants (eg, strychnine) bind to GPCRs (T2R family) that couple to the heterotrimeric G-protein, gustducin.
• Umami tastants activate a receptor comprised of T1R1 and T1R3
• taste threshold


  refers to the minimum concentration at which a substance can be perceived
• hydrochloric acid

  • sour
  • 100
• sodium chloride

  • salt
  • 2000
• strychnine hydrochloride

  • bitter
  • 1.6