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Chemical Senses

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Chemical Senses

  1. 1. Chemical Senses Taste and Smell
  2. 2. Chemoreceptors in the Human Body <ul><li>Several types of chemoreceptors are present: </li></ul><ul><li>Taste buds - receptors of taste (gustatation) </li></ul><ul><li>Olfactory receptors- receptors of smell (olfaction) </li></ul><ul><li>Cutaneous nociceptors -Irritating chemicals on skin </li></ul><ul><li>Muscle sensors - burning during heavy exercise, acidity receptors </li></ul><ul><li>Circulatory sensors - oxygen & CO 2 receptors </li></ul><ul><li>Digestive tract sensors - receptors for various ingested substances </li></ul>
  3. 3. Taste & Smell <ul><li>We will consider only those dealing with taste and smell </li></ul><ul><li>These two senses are mediated quite differently, but are perceived together </li></ul><ul><li>Other receptors in the mouth also participate: </li></ul><ul><ul><li>Texture </li></ul></ul><ul><ul><li>Temperature </li></ul></ul>
  4. 4. Gustation <ul><li>Gustation = taste </li></ul><ul><li>Basic tastes: </li></ul><ul><ul><li>Sweetness - outer tip of tongue </li></ul></ul><ul><ul><li>Saltiness - lateral edge of tongue behind tip </li></ul></ul><ul><ul><li>Sourness - lateral edge of tongue behind saltiness region </li></ul></ul><ul><ul><li>Bitterness - back edge of tongue toward throat </li></ul></ul><ul><ul><li>Umami - taste of glutamate - not localized </li></ul></ul><ul><li>Tastes triggers are not unique </li></ul><ul><ul><li>many chemicals can trigger each receptor type </li></ul></ul><ul><li>Loss of taste is ageusia </li></ul>
  5. 5. Organs of Taste <ul><li>The tongue is the major organ of gustation </li></ul><ul><li>There are also some tastebuds on the palate, pharynx, and epiglottis </li></ul><ul><li>Papillae - contain taste buds </li></ul><ul><ul><li>Fungiform papillae (mushrooms) - top surface of tongue </li></ul></ul><ul><ul><li>Foliate papillae (ridges) - rear, lateral margin of tongue </li></ul></ul><ul><ul><li>Vallate papillae (pimples) - rear, medial margin of tongue </li></ul></ul>
  6. 6. Mapping Taste
  7. 7. Taste Buds <ul><li>Only 1% of the epithelial cells on tongue’s surface are taste buds </li></ul><ul><li>Contain 50 to 75 sensitive cells each </li></ul><ul><li>Taste-sensitive cells have a limited lifetime, and are constantly being replaced. </li></ul><ul><li>Taste pore permits substances to enter </li></ul><ul><li>Three cell types in each taste bud: </li></ul><ul><ul><li>Taste receptors – chemoreceptors </li></ul></ul><ul><ul><li>Basal cells - source of new taste receptors </li></ul></ul><ul><ul><li>Gustatory afferent fibers - conduct action potentials to brain </li></ul></ul>
  8. 8. Taste Cells - Taste Receptors <ul><li>Come from the division and differentiation of the basal cells </li></ul><ul><li>Not neurons in the strict sense, but have synapses with gustatory afferent fibers </li></ul><ul><li>Arrival of chemical molecules on surface leads to change in membrane potential - receptor potential </li></ul><ul><li>Most taste receptors respond to at least two of the basic tastes </li></ul><ul><li>Sometimes taste receptors generate action potentials </li></ul><ul><li>Transduction differs for each type of taste receptor </li></ul>
  9. 9. Taste Transduction - Salty <ul><li>Sodium in food goes through a sodium channel (amiloride sensitive sodium channel) which always sits open </li></ul><ul><li>Sodium influx causes membrane depolarization (receptor potential) </li></ul><ul><li>Voltage-gated calcium channel opens </li></ul><ul><li>Inward calcium current causes synaptic release onto gustatory afferent at taste cell synapse </li></ul>
  10. 10. Taste Transduction - Sour <ul><li>Hydrogen ions in sour food goes through the amiloride sensitive sodium channel and closes an open potassium channel </li></ul><ul><li>Membrane depolarization (receptor potential) </li></ul><ul><li>Voltage-gated calcium channel opens </li></ul><ul><li>Inward calcium current causes synaptic release </li></ul>
  11. 11. Taste Transduction - Sweet <ul><li>The sweet molecule in food binds to G-protein coupled receptor </li></ul><ul><li>Effector protein is adenylyl cyclase </li></ul><ul><li>cAMP activated </li></ul><ul><li>Protein Kinase A activated </li></ul><ul><li>PKA closes a potassium channel </li></ul><ul><li>Membrane depolarization (receptor potential) </li></ul><ul><li>Voltage-gated calcium channel opens </li></ul><ul><li>Inward calcium current causes synaptic release </li></ul>
  12. 12. Taste Transduction - Bitter <ul><li>Bitter molecule in food blocks potassium channel </li></ul><ul><li>Membrane depolarization (receptor potential) </li></ul><ul><li>Voltage-gated calcium channel opens </li></ul><ul><li>Inward calcium current causes synaptic release </li></ul>
  13. 13. Taste Transduction – Bitter II <ul><li>Bitter molecule in food binds G-protein coupled receptor </li></ul><ul><li>Effector protein is phospholipase C </li></ul><ul><li>IP3 produced </li></ul><ul><li>Calcium released from internal stores </li></ul><ul><li>Calcium causes synaptic release </li></ul><ul><li>No change in membrane potential (no receptor potential) </li></ul>
  14. 14. Taste Transduction – Umami <ul><li>Glutamate in food binds to transmitter-gated sodium channel </li></ul><ul><li>Membrane depolarization (receptor potential) </li></ul><ul><li>Voltage-gated calcium channel opens </li></ul><ul><li>Inward calcium current triggers synaptic release </li></ul>
  15. 15. Neural Pathways for Taste <ul><li>Afferents </li></ul><ul><ul><li>Anterior 2/3 of tongue - VII (facial nerve) </li></ul></ul><ul><ul><li>Posterior 1/3 of tongue - IX (glossopharyngeal nerve) </li></ul></ul><ul><ul><li>glottis, epiglottis, pharynx, palate - X (vagus nerve) </li></ul></ul><ul><li>All afferent fibers end in gustatory nucleus in the medulla </li></ul>
  16. 16. Pathways to the Brain <ul><li>Taste fibers proceed along several pathways to the medulla oblongata or brain stem , then to the thalamus , and finally to the taste area on the anterior cortex. </li></ul><ul><li>For taste sensation, gustatory nucleus neurons send fibers to the ventral posterior medial nucleus (VPM) of thalamus </li></ul><ul><li>VPM sends fibers to the primary gustatory cortex (Brodmann's area 43 in ventral parietal lobe </li></ul><ul><li>For autonomic functions, gustatory nucleus sends fibers to the many brainstem regions involved in swallowing, salivation, gagging, vomiting, digestion, respiration and to the hypothalamus and amygdala </li></ul><ul><ul><li>appetite and food preferences </li></ul></ul>
  17. 17. Neural Coding for Taste <ul><li>Labeled line hypothesis - if each receptor only responds to a specific flavor, then each axon would represent a particular taste </li></ul><ul><li>Population coding - if each receptor responds to many flavors, but differentially, then the population of activity across all the fibers would represent a particular taste </li></ul><ul><li>Gustation uses a population coding scheme </li></ul><ul><li>Other inputs participate in the overall perception of taste as mentioned above </li></ul>
  18. 18. Common Ideas from Other Senses <ul><li>Taste perception is a result of differences in neural stimulation </li></ul><ul><li>Different perceptions can arise from the same cells. </li></ul><ul><li>There are synaptic connections between neighboring cells, as in the case of vision and hearing. </li></ul><ul><li>The taste sense exhibits adaptation and masking, like the other senses. </li></ul>
  19. 19. Olfaction - Smell <ul><li>Inside each side of the nose is an air chamber, the nasal cavity. </li></ul><ul><li>Air comes in through the nostril and flows down, around the rear of the roof of the mouth, into the throat. </li></ul><ul><li>When you sniff, air swirls up into the top of the cavity. </li></ul><ul><li>A small patch of about 10 million specialized olfactory (smelling) cells are in the cavity </li></ul><ul><li>Loss of smell is anosmia </li></ul>
  20. 20. Organs of Smell <ul><li>Olfactory epithelium in roof of nasal cavity </li></ul><ul><li>Olfactory receptors – chemoreceptors </li></ul><ul><li>Supporting cells - secrete mucous </li></ul><ul><li>Basal cells - generate new olfactory receptors </li></ul><ul><li>Size of the olfactory epithelium is a measure of keenness of smell </li></ul><ul><ul><li>humans have about 10 sq. cm </li></ul></ul><ul><ul><li>dogs have 170 sq. cm and dogs may also have 100 times the olfactory receptor density </li></ul></ul>
  21. 21. Visualizing Smell
  22. 22. Olfactory Receptor Cells <ul><li>The olfactory sensors are located on each side of the inner nose. </li></ul><ul><li>True neurons: </li></ul><ul><li>Dendrite ends in knob with multiple cilia (containing receptors) at surface of epithelium </li></ul><ul><li>Odor particles in the air stick to the cilia </li></ul><ul><li>Unmyelinated axon leaves base of epithelium thru cribiform plate and ends in olfactory bulb </li></ul><ul><li>Born, live, and die with a 4-8 week cycle - only neurons regularly replaced throughout life </li></ul><ul><li>The olfactory cells produce nerve signals, which travel to the olfactory bulb </li></ul>
  23. 23. Mechanisms of Olfactory Transduction <ul><li>Odorant binds to G-protein coupled receptor protein </li></ul><ul><ul><li>There are 500 - 1000 different olfactory receptor proteins coded genetically </li></ul></ul><ul><li>Effector protein is adenylyl cyclase </li></ul><ul><li>cAMP produced and binds to cation channel (sodium + calcium) </li></ul><ul><li>Calcium influx opens calcium-sensitive chloride channel </li></ul><ul><li>Calcium and chloride cause membrane depolarization (receptor potential) </li></ul><ul><li>Action potentials in olfactory receptor fibers </li></ul>
  24. 24. The Olfactory Bulb <ul><li>A pre-processing center that sorts the signals before they travel along the olfactory tract to the brain </li></ul><ul><li>Axons from receptor cells project to the olfactory bulbs </li></ul><ul><li>Here the glomeruli , receive signals from 26,000 receptors </li></ul><ul><li>The olfactory bulbs on either side are cross-connected. </li></ul>
  25. 25. Pathways to the Brain <ul><li>Nerve fibers project from the olfactory bulb to the olfactory tubercle </li></ul><ul><li>Olfactory tubercle neurons project to medial dorsal nucleus of thalamus </li></ul><ul><li>Thalamic neurons project to orbitofrontal cortex </li></ul><ul><li>reach the olfactory areas in the neocortex for the sensation of smell </li></ul>
  26. 26. Neural Coding in the Olfactory System <ul><li>Olfactory receptors respond to a variety of odorants </li></ul><ul><li>Population code is used </li></ul><ul><li>Both spatial distribution and timing of action potentials is important </li></ul>
  27. 27. Other Pathways <ul><li>Olfactory bulb neurons also end in other places </li></ul><ul><li>Olfactory cortex that is not part of neocortex </li></ul><ul><ul><li>old system </li></ul></ul><ul><li>Effects many brain systems </li></ul><ul><ul><li>Odor discrimination </li></ul></ul><ul><ul><li>Odor perception </li></ul></ul><ul><ul><li>Motivation </li></ul></ul><ul><ul><li>Emotions </li></ul></ul><ul><ul><li>Reproduction, feeding imprinting, memory </li></ul></ul>
  28. 28. Olfactory Senses in Other Animals <ul><li>In insects, the olfactory sense is located on the antennae. </li></ul><ul><li>Snakes and lizards possess a Jacobson's organ in the front of the mouth that is directly connected to the olfactory center in the brain. </li></ul><ul><ul><li>The flicking tongue transfers scents to this organ for analysis. </li></ul></ul><ul><li>Scents seem to have a strong influence on the social interactions of many animals </li></ul><ul><li>Birds have a well-developed olfactory sense, which was not appreciated until recently. </li></ul>
  29. 29. Smell in a Blow-fly
  30. 30. Some Facts About Smell <ul><li>The olfactory sensations are given as fruity, flowery, resinous, spicy, foul, and burned. </li></ul><ul><li>Smell is 10,000 times as sensitive as taste </li></ul><ul><li>Smell is primarily responsible food flavors. </li></ul><ul><li>The two nostrils receive slightly different chemical signatures </li></ul><ul><ul><li>allows finer discrimination of odors. </li></ul></ul><ul><li>There is strong adaptation; one soon becomes accustomed to an odor and unaware of it </li></ul><ul><li>One odor can be masked by another </li></ul><ul><ul><li>the theory of perfume. </li></ul></ul><ul><li>Need a 20% increase in concentration to cause a perceptible increase in the strength of perception. </li></ul>
  31. 31. Odor & Memory <ul><li>Odors call up memory. </li></ul><ul><li>Smell is the only sense with direct access to the amygdala, the 'emotional center' of the brain. </li></ul>
  32. 32. Interaction of Taste & Smell <ul><li>Information on taste is organized in the brain separately from that of smell </li></ul><ul><li>When the brain processes this information, nerve signals from the two senses unite and create a third, different representation. </li></ul><ul><li>The latter represents flavor the combination of taste and smell. </li></ul><ul><li>Flavor is handled in a distinct region of the brain, separate from those where smell and taste are processed. </li></ul>
  33. 33. Taste & Smell
  34. 34. Poisons <ul><li>Taste and smell are not reliable guides to poisons </li></ul><ul><ul><li>only to identification of known substances. </li></ul></ul><ul><li>Some innocuous substances taste terrible, while some poisons taste delightful. </li></ul><ul><ul><li>Lead acetate, or sugar of lead, tastes pleasantly sweet, but is a powerful cumulative poison. </li></ul></ul><ul><ul><li>The aromatic compounds benzene and toluene are fragrant, but benzene is dangerously carcinogenic, while toluene is relatively safe. </li></ul></ul>
  35. 35. Taste, Smell, & Memory <ul><li>The chemical senses are sometimes associated with vivid mental images and recollections </li></ul><ul><ul><li>an unexpected connection to higher mental processes. </li></ul></ul><ul><li>Scientists investigated the way in which both taste and smell sensations are stored in memory. </li></ul>

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