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Chemical Senses

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Transcript

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