SMELL AND TASTE

• Smell and taste are classified as visceral senses, are related to each other, and are both chemoreceptors

  
• Smell receptors


  • Teleceptors
  • No relay in the thalamus
  • No neocortical representation for olfaction

  
• Taste pathways


  • Brainstem to thalamus
  • Postcentral gyrus along with touch & pressure sensibility from mouth
• Olfactory Mucous Membrane


  Where olfactory receptors are located

  
• Olfactory Mucous Membrane


  • Contains supporting cells & progenitor cells
  • 10 - 20 million receptor cells
  • Nervous system is close to the external world

  
• Mitral cells


  In the olfactory bulbs, where 26,000 receptor cells converge on each glomerulus

  
• Olfactory Cortex


  • Piriform cortex bilaterally
  • Activate orbitofrontal cortex only on the right side

  
• Very Old Olfactory System


  • Medial olfactory area
  • Primitive responses to olfaction like licking lips, salivation, feeding responses, primitive emotional drives

  
• Less Old Olfactory System

  • Lateral olfactory area including prepyriform cortex, pyriform cortex, cortical portion of amygdaloid nuclei
  • Pass through the hippocampus for learning to like/dislike foods
  • Most signals feed to the paleocortex in the anteromedial temporal lobe

  
• Newer Olfactory System


  • Passing through the thalamus to the orbitofrontal cortex for conscious analysis of odors

  
• Olfactory thresholds & Discrimination


  • Can recognize 10,000 different odors
  • Poor determination, concentration must change 30%
  • Difference in arrival time of odoriferous molecules
  • High water & lipid solubility are strong odors

  
• Olfactory receptors only respond to odorants that are dissolved in the mucous

  
• Olfactory Membrane & Action Potentials


  • Resting membrane potential of -55mV
  • Can generate continuous action potentials at a very slow rate, once per 20 secs to 2-3 per second
  • Depolarization to -30mV increases frequency to 20 per second
• Olfactory Signal Transduction


  • 1,000 odorant receptors coupled to G proteins, act via adenylyl cyclase & cAMP or phospholipase C
  • One type of receptor in each olfactory receptor neuron, odorants must stimulate more than one type

  
• Odorant-Binding Proteins (OBP)


  Concentrate odorants and transfer them to receptors
• Olfaction is more acute in women, especially during ovulation, and is mediated by the vomeronasal organ
• Olfactory Sniffing


  Eddy currents, semi-reflex
• Olfactory Pain Fibers


  Naked nerve endings in olfactory mucus membrane, from cranial nerve V, respond to peppermint, chlorine, initiating sneezing and lacrimation

  
• Olfactory Adaptation


  • Threshold for other odors are unchanged
  • Desensitization of G protein via high concentration of β-adrenergic receptor kinase 2 & β arrestin-2
  • Inhibitory granule cells in olfactory bulb

  
• Taste Buds


  • Sense organ for taste, 50-70 μm, 4 types of cells including gustatory receptor cells with microvilli projecting to taste pore
  • Each bud innervated by 50 nerve fibers, each fiber receives input from 5 buds
  • Basal cells arise from epithelial cells, half life of 10 days
  • Located in mucosa of epiglottis, palate, pharynx, walls of fungiform & vallate papillae of tongue, 10,000 total
• Taste Pathways


  • Anterior 2/3 of tongue via chorda tympani of cranial nerve VII, posterior 1/3 via cranial nerve IX, other areas via cranial nerve X
  • Unite at medulla, synapse to 2nd neurons and decussate to join the medial lemniscus, foot of postcentral gyrus
• Basic Taste Modalities


  Sweet - tip of tongue, Sour - along edges, Salty - dorsum anteriorly, Bitter - back of tongue
• Substances Evoking Primary Taste Sensations

  Sour - acids, H+ stimulates receptors

  Salty - Na+

  Sweet - organic substances

  Bitter - morphine, nicotine, caffeine, urea, inorganic salts like Mg+, NH4+, Ca++
• Taste thresholds are relatively crude compared to smell, requiring a 30% change in concentration to detect
• Taste Receptor Stimulation


  Chemoreceptors act on exposed microvilli

  Sour - H+ blocks apical K+ channels

  Salty - Na+ enters

  Sweet - G protein & adenylyl cyclase, ↓ K+ conductance

  Bitter - G protein & phospholipase C, trigger Ca++ release from ER

  Ebner's Glands secrete mucus around vallate papillae to concentrate & transport taste-producing molecules