Smell n Taste

• Smell and taste


  Classified as visceral senses (near intestines), cannot be controlled

  
• Smell and taste are related to each other
• Smell and taste are chemoreceptors, stimulated by chemicals
• Smell receptors


  • Teleceptors, you can smell from afar
  • No relay in the thalamus (in the smell center only)
  • No neocortical representation for olfaction as it is very basic

  
• Taste pathways


  • Brainstem to thalamus
  • Postcentral gyrus (sensory) along with touch and pressure sensibility from mouth
• Olfactory Mucous Membrane


  • Where olfactory receptors are located
  • Microsmatic vs. macrosmatic (easier to smell) species
  • Contains supporting cells and progenitor cells (young versions of receptors that replace the old dying ones)
  • 10-20 million receptor cells in this area
  • Nervous system is close to the external world
• Olfactory pathway

  First order neurons get in touch with olfactory bulb (second order) then will go further up to the brain which is the third order neuron
• Olfactory Bulbs


  • Mitral Cells make up the olfactory glomeruli
  • 26,000 receptor cells converge on each glomerulus
  • Olfactory Cortex - piriform cortex bilaterally, activate orbitofrontal cortex only on the right side, asymmetric cortical representation, mediate olfactory discrimination and conscious perception
  • Anterior Olfactory Nucleus
  • Amygdala - emotional responses
  • Entorhinal Cortex - associated with olfactory memories

  
• Olfactory Centers


  • Very Old Olfactory System - located at the medial olfactory area, primitive responses to olfaction (cannot be controlled)
  • Less Old System - located at the lateral olfactory area, pass thru the hippocampus, learning to like/dislike foods, most signals feed to the paleocortex
  • Newer System - passing through the thalamus, to the orbitofrontal cortex, conscious analysis of odors

  
• Olfactory Thresholds and Discrimination

  • Can recognize 10,000 different odors; poor determination
  • Concentration of a substance must be changed 30%
  • Difference in arrival time of odoriferous molecules
  • High water and lipid solubility are strong odors
  • Respond only that are dissolved in the mucous (those not dissolved in the muscles in the nasal passages would be difficult for us to smell)

  
• Membrane and Action Potentials


  • RMP = -55mV, cells can generate continuous action potentials at a very slow rate (once per 20 secs to 2-3 per second)
  • Depolarization to -30mV, increased frequency of action potentials (20 per sec)
• Signal Transduction


  • 1,000 odorant receptors
  • Coupled to G proteins act via adenylyl cyclase and cAMP, act via phospholipase C
  • One type of receptor in each olfactory receptor neuron
  • Odorants must stimulate more than one type of receptor
• Odorant-Binding Proteins


  Concentrate odorants, transfer odorants to receptors
• Relation to Sex and Memory

  • Mediated by vomeronasal organ (perfume ads)
  • More acute in women during ovulation
• Pheromones


  Hormones found on sweat; love hormones
• Sniffing


  Eddy currents, semi-reflex
• Pain Fibers


  • Naked nerve endings in olfactory mucus membrane
  • Mediated by CN V (Trigeminal Nerve)
  • Peppermint, chlorine; initiating sneezing, lacrimation
• Adaptation


  • Threshold for other odors are unchanged (desensitized)
  • Olfactory dendrites and cilia - high concentration of beta-adrenergic receptor kinase 2 and beta arrestin - 2, desensitization of G protein
  • Inhibitory cells in olfactory bulb, granule cells
• Taste Buds


  • Sense organ for taste, 50-70 micrometer
  • Four types of cells - basal cells, sustentacular cells (Type 1 & 2), gustatory receptor cells (Type 3)
  • Types 1, 2, and 3 have microvilli (project to taste pore)
  • Each is innervated by 50 nerve fibers
  • Each nerve fiber receives input form 5 taste buds
  • Basal cells arise from epithelial cells
  • Half life of 10 days
  • Located in mucosa of epiglottis, palate, and pharynx, walls of fungiform and vallate papillae of tongue
  • Filliform papillae have no taste buds
  • 10,000 taste buds
• Taste Pathways

  • Anterior 2/3 of tongue via chorda tympani of CN VII
  • Posterior 1/3 of tongue via CN IX
  • Other areas via CN X
  • Unite at medulla to enter Nucleus of Tractus Solitarius, synapse to 2 degrees neurons and decussate to join the medial lemniscus, foot of post central gyrus (cutaneous sensation for face)
  • Vagus, Glossopharyngeal, and Facial nerve converge at the Tractus solitarius, nucleus that subserves the taste fibers from the tongue, then will go to the thalamus, then will end at the foot of post-central gyrus
  • Divide at the medial leminiscus, go up to the thalamus, then to the taste area in the opercula of insula region
• Basic Taste Modalities

  • Sweet - tip of tongue
  • Sour - along the edges
  • Salty - dorsum anteriorly
  • Bitter - back of tongue
  • Umami - everywhere
• Substances Evoking Primary Taste

  • Sour - acids, H+ stimulates the receptors
  • Salty - Na+
  • Sweet - organic substances that are present in the food that we eat
  • Bitter - morphine, nicotine, caffeine and urea, inorganic salts: Mg+, NH4+ (Ammonia) and Ca++
• Taste Thresholds


  • Relatively crude as smell
  • A 30% change in concentration
  • The lower the threshold, the more you can easily taste it
• Receptor Stimulation


  • Chemoreceptors, act on exposed microvilli
  • Sour - H+ blocking apical K+ channels
  • Salty - Na+ enters
  • Sweet - G protein and adenylyl cyclase, decreased K+ conductance
  • Bitter - G protein and phospholipase C, trigger Ca++ release from ER
  • Ebner's Glands - secretes mucus, found around vallate papillae, concentrating and transport of taste-producing molecules