Physio chemical senses


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Physio chemical senses

  1. 1. Dr. Mohanad R. Alwan
  2. 2. <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 and smell (chemical receptors) tend to influence each other Taste (gustation) Taste buds- epithelial cells that act like neurotransmitters Innervated by cranial nerves
  4. 4. <ul><li>Taste (‘gustation’) and smell (‘olfaction’) are often referred to as the chemical senses </li></ul><ul><li>They are mediated in the first instance by receptors that are stimulated by chemical substances. Receptors are called ‘ chemoreceptors’ </li></ul><ul><li>Taste: Chemicals dissolve in our mouth (must be water soluble) and stimulate the taste buds in the oral cavity (tongue, soft palate, cheek, etc.). </li></ul>
  5. 5. <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>
  6. 6. Four major sensations of taste Sweet- G-protein activates second messenger system Bitter- also G-protein activated specific receptor Sour- proton channel (acids taste sour) Salty- sodium channels Tastes seem to be enhanced in different parts of the tongue
  7. 8. <ul><li>The tongue is the major organ of gustation </li></ul><ul><li>There are also some taste buds 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>
  8. 11. <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>
  9. 12. <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>
  10. 13. <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>
  11. 14. <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>
  12. 15. <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>
  13. 16. receptor cells in olfactory epithelium sense of smell transmitted directly to cerebral cortex (all others go through thalamus) receptors in medial temporal lobes and in limbic system (emotional response to odors) Molecular basis for smell is complex- humans can distinguish up to 10,000 different odors Smell (olfaction)
  14. 18. <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>
  15. 19. <ul><li>Smell: Volatile (gaseous) chemicals are inhaled into the nasal passages (or enter via the mouth) where olfactory receptors line the membranes </li></ul><ul><li>Taste and smell are closely linked in that they are both usually involved in many activities such as food seeking and sampling (flavour involves both) </li></ul>
  17. 21. <ul><li>Smell also conveys important non-nutritive information such as the presence of prey, predators and in some species mates (pheromones regulate sexual activity) </li></ul><ul><li>- Taste aids in the regulation of nutrients and enables the organism to ‘test’ substances prior to ingestion (important for identifying both nutrious foods and harmful substances) </li></ul>
  18. 22. <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>
  19. 24. <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>
  20. 25. <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>
  21. 26. <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>
  22. 27. <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>
  23. 29. The End