SMELL

• Smell


  One of the visceral senses, related to taste, both are chemoreceptors

  
• Smell and Taste


  Chemicals get in touch with receptors, then transmit signals to the nerve fiber, then nerve fibers will send signal to the brain for interpretation

  
• Smell Receptors


  • Teleceptors - you can smell from afar, no relay in the thalamus, goes directly to the smell center, you cannot choose not to smell when your nostrils are open, no neocortical representation for olfaction

  
• Taste Pathways


  • Brainstem to thalamus, more interpretation, subserved by the postcentral gyrus along with touch & pressure sensibility from mouth
• How we smell


  Scents are contained in the molecules in the air, reach our nostrils, attach to the olfactory mucous membrane, signals sent to the olfactory bulb and tract

  
• Olfactory Mucous Membrane


  • Where olfactory receptors are located, microsmatic vs. macrosmatic species, contains supporting cells & progenitor cells

  
• Smell Pathway


  Molecules in the air attach to the mucous layer, go through chemoreceptors, nerve fibers converge to form one common axon bundle, go up to the cribriform plate, attach to neurons in the olfactory bulb

  
• First Order Neuron


  • Nasal cavity, dendrites, axons that converge into a bundle, go to the cribriform plate, synapse with Second Order Neurons in the olfactory bulb

  
• Olfactory Bulbs


  • Glomeruli are important waystations, mitral cells make up the olfactory glomeruli, 26,000 receptor cells would converge on each glomerulus

  
• Olfactory Cortex


  • Piriform cortex bilaterally, activate orbitofrontal cortex only on the right side, responsible for conscious analysis of odors

  
• Olfactory Centers


  • Very Old Olfactory System (medial olfactory area, primitive responses)
  • The Less Old System (lateral olfactory area, prepyriform cortex, pyriform cortex, cortical portion of amygdaloid nuclei, pass thru the hippocampus)
  • The Newer System (passing through the thalamus to the orbitofrontal cortex, responsible for conscious analysis of odors)

  
• Olfactory Bulbs


  • Olfactory Cortex (asymmetric cortical representation, only right side is activated, mediate olfactory discrimination & conscious perception), Anterior Olfactory Nucleus AON (station where you have the connections), Amygdala (elicit emotional responses), Entorhinal Cortex (associated with olfactory memories)

  
• Olfactory Thresholds and Discrimination

  Can recognize 10,000 different odors, poor determination, concentration of a substance must be changed 30% in order for you to appreciate a change in the smell, difference in arrival time of odoriferous molecules, high water & lipid solubility are strong odors
• Membrane and Action Potentials


  RMP -55mV, cells can generate continuous action potentials at a very slow rate, once per 20 secs to 23 per second, depolarization to -30mV, increased frequency of action potentials (20 per sec)
• Signal Transduction


  • 1,000 odorant receptors, coupled to G proteins, act via adenylyl cyclase and cAMP, act via phospholipase C, one type of receptor in each olfactory receptor neuron, odorants must stimulate more than one type of receptor

  
• Odorant-Binding Proteins OBP


  Concentrate odorants, responsible for transferring odorants (scents) to receptors

  
• Relation to Sex and Memory


  Mediated by vomeronasal organ, utilized by perfume ads, more acute in women during ovulation, sense of smell in women who are ovulating are heightened

  
• Pheromones


  Chemicals that when humans get to appreciate it, their sensuality is excited, clear liquid, induce a sexual response, not present only in humans

  
• Sniffing


  Eddy currents, movement of air where you facilitate the scents molecules in the nasal passages, semi-reflex
• Pain Fibers


  • Subserved by naked nerve endings in olfactory mucus membrane, mediated by CN V Trigeminal Nerve, e.g. peppermint, chlorine, initiating sneezing, lacrimation
• Adaptation


  Threshold for other odors are unchanged, desensitization - you get adapted to it, inhibitory cells in olfactory bulb, granule cells, high concentration of β - adrenergic receptor kinase 2 & β arrestin 2