Smell and Sense

• Smell and taste are both forms of chemoreception
• chemoreception is old - some bacteria use it to guide their movements and animals without brains use them to find food and mates
• chemoreception may have evolved into chemical synaptic communication
• Olfactory receptors are in the olfactory epithelium
• epithelium lies at the top of nasal cavity, covers ~3cm2 in each of the 2 sides, contains ~10 million receptor cells total
• epithelium is pigmented, don’t know why but richness in colour relates with olfactory sensitivity. Humans = pale yellow, cats = dark mustard brown
• Receptor cells for olfaction are ciliated neurons
• each receptor cell has a single dendrite that extends into the olfactory epithelium. There it branches to form a nonmotile cilia that increases the SA of the cell, greater chance of catching odorant molecules
• ~400 kinds of receptors ie ~400 primary odors
• Olfactory receptor cells have G protein coupled receptor molecules in their membranes
• when an odorant molecules binds its receptor, it activates a G protein Golf which increases the local concentration of cAMP
• Receptor cells are sensitive

  • some can detect a single molecules of preferred chemical, ~40 have to react before we experience a smell
• Olfactory receptor cells have unusual properties

  • they are pinocytotic, continually sipping in fluid and sending it along the nerves into the brain. don’t know why
  • they are short lived, degenerating after a month or 2, to be replaced by new ones from below
  • they send their axons into the brain through tiny holes in the cribriform plate - the plate at the base of the cranial cavity
• Olfactory receptor cells have unusual properties

  • they are pinocytotic, continually sipping in fluid and sending it along the nerves into the brain. don’t know why
  • they are short lived, degenerating after a month or 2, to be replaced by new ones from below
  • they send their axons into the brain through tiny holes in the cribriform plate - the plate at the base of the cranial cavity
• Receptor cells project to the olfactory bulb

  • bulb is an extension of the cerebrum and lies on the underside of the frontal lobes
  • the projection from the receptors to the bulb is called the olfactory nerve or cranial nerve 1
• Many receptor cells converge on each bulb neuron

  • as with rods converging on ganglion cells, this arrangement enhances sensitivity but discards spatial information
• Bulb projects directly to olfactory cortex, bypasses thalamus

  • olfactory cortex found in the frontal and temporal lobes
• Bulb also projects to the limbic system
• old group of brain regions concerned with motivation and emotion. For early animals, motivation was tightly linked to smell, used to identify food and poisons, mates and predators
• emotions are no longer smell related but they are still handled by these olfactory areas. maybe why odors call up emotional memories
• Olfactory sense is phasic, adapts slowly but completely
• Rodents and maybe humans have pheromones
• Main taste receptor cells are clustered in taste buds
• Typical taste bud contains at least 5 kinds of receptor cells
• • sweet and umami receptor cells detect sugar (energy) and the AA glutamate (indicates protein), respectively
  • bitter receptor cells detect poison
  • salty and sour receptor cells detect Na+ and H+ - important ions
  • the tongue may also have receptors for fatty acids
• Receptors cells of all 5 kinds all over the top of tongue

  • different areas of the tongue do vary slightly in their thresholds for different flavours
• Taste receptor cells are grouped into 3 types

  • only type 3 cells form synapses with sensory neurons, activating them with serotonin
  • type 2 cells release ATP, which acts on neurons and type 3s
• Different kinds of cell employ different membrane proteins

  • cells for sweet, umami, and bitter have receptor molecules coupled to a G protein called gustducin, which activates signal pathways, increasing intracellular Ca2+ and triggering release of ATP
  • detection of salt and sour involves ion channels which are not linked with G proteins
• Experience of food depends on other sensors besides the taste buds
• nerve endings in the walls of the mouth have TRP channels sensitive to temperature and chemicals. ex. vanilloid receptors respond to heat and to capsaicin in chillies; TRPM8 channels respond to cold and to menthol
• Taste signals take several paths to the brain
• receptor cells in taste buds excite fibers of cranial nerves 7, 9 and 10, the facial glossopharyngeal, and vagus nerves. Pathways synapse in medulla and thalamus en route to the cortex
• TRP receptors in the walls of the mouth excite cranial nerve 5, the trigeminal