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Taste and smell


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  • 1. PHYSIOLOGY OF TASTE AND SMELL Prof. Vajira Weerasinghe Dept of Physiology
  • 2. Introduction
    • Smell and taste are chemical sensations
    • Taste is the gustatory sensation
    • Smell is the olfactory sensation
    • Taste drives appetite and protects us from poisons
  • 3. Taste buds
    • Contains taste receptor cells called
    • aggregations of 30-100 individual elongated "neuroepithelial" cells
      • 50-60 microns in height
      • 30-70 microns in width
    • embedded in specializations of surrounding epithelium, termed papillae
      • Types of papillae: Fungiform, foliate, circumvallate and the non-gustatory filiform
  • 4.  
  • 5. taste buds
    • below the taste bud, taste cells are joined by tight junctional complexes
    • At the base of the taste bud, afferent taste nerve axons invade the bud and ramify extensively, each fibre typically synapsing with multiple receptor cells within the taste bud
    • located throughout the oral cavity, in the pharynx, the laryngeal epiglottis and at the entrance of the oesophagus
    • number of taste buds declines with age
  • 6.  
  • 7. Salt taste
    • Na+ ions enter the receptor cells via Na-channels
    • These are amiloride-sensitive Na+ channel (as distinguished from TTX-sensitive Na+ channels of nerve and muscle)
    • entry of Na+ causes a depolarization
    • Ca2+ enters through voltage-sensitive Ca2+ channels
    • neurotransmitter release occurs and results in increased firing in the primary afferent nerve
  • 8. Sour taste
    • Sour is acidic
    • H+ ions block K+ channels
    • Blocking of these channels causes a depolarization
    • Ca2+ entry, transmitter release and increased firing in the primary afferent nerve
  • 9. Sweet taste
    • glucose binding to receptors activates adenylyl cyclase, thereby elevating cAMP
    • This causes a PKA-mediated phosphorylation of K+ channels, inhibiting them
    • Depolarization occurs
    • Ca2+ enters the cell through depolarization-activated Ca2+ channels
    • transmitter is released increasing firing in the primary afferent nerve
  • 10. Bitter taste
    • Bitter substances cause the second messenger (IP3) mediated release of Ca2+ from internal stores (external Ca2+ is not required)
    • The elevated Ca2+ causes transmitter release
    • this increases the firing of the primary afferent nerve
  • 11. New taste - Umami taste
    • Umami is the taste of certain amino acids (e.g. glutamate, aspartate and related compounds)
    • It was first identified by Kikunae Ikeda at the Imperial University of Tokyo in 1909
  • 12. Umami taste
    • Recently it has been shown that the metabotropic glutamate receptor (mGluR4) mediates umami taste
    • Binding to the receptor activates a G-protein
    • this may elevate intracellular Ca2+
  • 13. Umami taste
    • Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of Soy sauce), may stimulate the umami receptors
    • But, in addition, there are ionotropic glutamate receptors (linked to ion channels), i.e. the NMDA-receptor, on the tongue.
    • When activated by these umami compounds or soy sauce, non-selective cation channels open, thereby depolarizing the cell. Calcium enters, causing transmitter release and increased firing in the primary afferent nerve
  • 14. Modifying taste
    • Taste exhibits almost complete adaptation to a stimulus
        • perception of a substance fades to almost nothing in seconds
    • Taste can be suppressed by local anaesthetics applied to the tongue
    • Amiloride, a blocker of epithelial Na channels, reduces salt taste in humans
    • AMP may block the bitterness of several bitter tasting agents
  • 15. Taste map
    • the classical "taste map" is an over simplification
    • Sensitivity to all tastes is distributed across the whole tongue and to other regions of the mouth where there are taste buds (epiglottis, soft palate)
    • some areas are indeed more responsive to certain tastes than others
  • 16. Innervation
    • Taste receptor cells do not have an axon
    • Information is relayed onto terminals of sensory fibres by neurotransmitter
    • These fibres arise from the ganglion cells of the cranial nerves
      • Vll (facial - a branch called the chorda tympani)
      • lX (glossopharyngeal)
  • 17.  
  • 18. Central pathways
    • Primary gustatory fibres synapse centrally in the medulla (in a thin line of cells called the nucleus of the solitary tract)
    • From there the information is relayed
      • 1 to the somatosensory cortex for the conscious perception of taste
      • 2 to the hypothalamus, amygdala and insula, giving the so-called "affective" component of taste
        • This is responsible for the behavioural response, e.g. aversion, gastric secretion, feeding behaviour.
  • 19. Flavour
    • Flavour is a combination of
      • Taste
      • Smell
      • texture (touch sensation)
      • and other physical features (eg. temperature)
  • 21. Introduction
    • sample our environment for information
      • Air we beathe
      • presence of food or another individual
      • Gives a warning
      • Recognition function
      • odour molecules must be small enough to be volatile (less than 300-400 relative molecular mass) so that they can vapourise, reach the nose and then dissolve in the mucus
  • 22. Importance
    • we can distinguish around 10,000 different smells
    • why we smell and the impact of smell on our everyday life are poorly understood
    • anosmic (someone who has lost some or all of their sense of smell)
      • some anosmics suffer from depression and their quality of life is severely affected
  • 23.  
  • 24. Olfactory epithelium
    • Contains
      • sensory cells
      • Bowman's glands producing the secretion that bathes the surface of the receptors
        • This is an aqueous secretion containing mucopolysaccharides, immunoglobulins, proteins (e.g. lysozyme) and various enzymes (e.g. peptidases)
      • pigmented-type of epithelial cell
  • 25. Odorant Binding Proteins
    • facilitate the transfer of lipophilic ligands (odorants) across the mucus layer to the receptors
    • increase the concentration of the odorants in the layer, relative to air
    • as a transporter, in which they would bind to a receptor with the ligand and accompany it across the membrane
    • as a terminator, causing "used" odorants to be taken away for degradation, allowing another molecule to interact with the receptor
    • as a protector for the receptor, preventing excessive amounts of odorant from reaching the receptor.
  • 26. Odorant receptor neurons
    • bipolar neurons in the nasal epithelium
    • they are capable of regenerating
    • Contain cilia which project into the mucus (these contain the receptor proteins)
    • axons that project to the olfactory bulb. 10-100 axons form up into bundles that penetrate the ethmoidal cribriform plate and terminate in the olfactory bulb
  • 27.  
  • 28. Mitral cells
    • the principal neurons in the olfactory bulb
    • There are about 50,000 of these cells in each bulb
    • They have a primary apical dendrite which extends into a spherical bundle of neuropil called a glomerulus
    • which receives the input from the olfactory receptor neurons
    • Their axons merge together to form the lateral olfactory tract
    • They possess colaterals, involved in negative feedback and positive feed-forward
  • 29. Other cells
    • Periglomerular cells
      • - are involved in lateral inhibition at the level of the glomeruli
    • Granule cells
      • -inhibitory interneurones
    • Olfactory ensheathing cells
      • – like glial cells
  • 30. Neurotransmitters
    • Glutamate
    • Noradrenalin
    • Dopamine
    • GABA - inhibitory
  • 31. Central connections
    • Neurons from the lateral olfactory tract project to
      • Areas of the limbic system (amygdala, septal nuclei, entorhinal cortex and hippocampus)
        • The septal nuclei and amygdala contain regions known as the "pleasure centres“
        • The hippocampus is concerned with motivational memory (the association of certain stimuli with food)
      • Projections are also sent to the thalamus and thence to the frontal cortex for recognition
      • There are many forward and backward connections between each of these brain centres.
  • 32.