Taste and smell

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Taste and smell

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

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