smell

• Smell and Taste

  Related to each other; classified as visceral senses
• Smell and Taste

  • Elicit physiological responses involved in the digestion and utilization of foods
  • Both are chemoreceptors
• Chemoreceptor process

  1. Chemicals get in touch with receptors

  2. Transmit signals to the nerve fiber

  3. Nerve fibers will send signal to the brain for interpretation
• Smell Receptors

  • Telereceptors - 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

  1. Brainstem to thalamus

  2. Subserved by the postcentral gyrus
• Smell

  Evolved as protective mechanism to avoid potentially harmful substances
• How we smell

  1. Receptors are located at the base of the skull, at the olfactory bulb which has fibers that enters at the perforations at the cribriform plate

  2. Scents are contained in the molecules in the air - they reach our nostrils

  3. Turbinates - moist area where molecules of scents attach to

  4. Olfactory mucous membrane

  5. Olfactory epithelium - major component of olfactory mucous membrane

  6. Cribriform plate - part of ethmoid bone which forms the anterior portion of the base of the skull, where the olfactory bulbs are located

  7. Olfactory bulb - fibers jutting into the cribriform plate, continue on as the olfactory tract

  8. Nerve fibers just into the cribriform plate in superior turbinates

  9. Chemicals would send signals into the bulb to the olfactory tract
• Neurons important for olfaction

  • Olfactory sensory neurons
  • Supporting cells
  • Basal stem cells
• Olfactory Mucous Membrane


  • Where olfactory receptors are located, responsible for olfactory transduction
  • Axons of olfactory sensory neurons pass through cribriform plate of ethmoid and enter olfactory bulbs
  • Microsmatic (small, difficult to appreciate) vs macrosmatic (big, easier to appreciate)
  • Contains supporting cells & progenitor cells
  • 10 - 20 million receptor cells located in this area
• Smell process

  1. Molecules in the air attach to the mucous layer

  2. Go through chemoreceptors

  3. Some nerve fibers will converge to form one common axon bundle

  4. Go up to the cribriform plate

  5. Attach themselves to another neuron in the olfactory bulb
• First order neurons


  Get in touch with olfactory bulb (second order) then will go further up to the brain which is the third order neuron
• Nasal area

  • Cilia - where the receptors are located, a lot of cilia becomes glomerulus
  • Supporting cells
  • Dendrites - coming from the mucous membrane in the nasal passages
• Glomeruli


  • Important waystations in the pathway from the nose to the olfactory cortex, critical for odorant signal transduction
• 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 (R) side, asymmetric cortical representation, mediate olfactory discrimination & conscious perception
• Anterior Olfactory Nucleus (AON)


  • Station where you have the connections, a central olfactory cortical structure that has heavy reciprocal connections with both the olfactory bulb (OB) and piriform cortex
• Amygdala


  • When signals are sent to this area, it would elicit emotional responses
• Entorhinal Cortex


  • Associated with olfactory memories, brings back memories when you smell something
• The Olfactory Centers

  • Very Old Olfactory System - located in the medial olfactory area, primitive responses to olfaction
  • The Less Old System - located in the lateral olfactory area, pass thru the hippocampus, most signals feed to the paleocortex
  • The Newer System - passing through the thalamus, to the orbitofrontal cortex, responsible for conscious analysis of odors
• Connections from the olfactory bulb

  • Conscious area - oribitofrontal cortex; near the eye, located on the frontal lobe
  • most would bypass the hypothalamus
  • hippocampus - learn to like or dislike
• Olfactory Thresholds and Discrimination

  • Lowest possible concentration of a chemical that can be detected
  • can recognize and discriminate about 10,000 different odors
  • poor determination of differences in intensity of odor
  • difference in arrival time of odoriferous molecules
  • high water & lipid solubility are present in strong odors
  • respond only that are dissolved in the mucous
• Olfactory Thresholds

  • Ethyl ether - 5.83 mg/L
  • Chloroform - 3.30 mg/L
  • Pyridine (cleansing agents) - 0.03 mg/L
  • Oil of peppermint - 0.02 mg/L
  • Iodoform - 0.02 mg/L
  • Butyric acid (spoiled food, vomit) - 0.009 mg/L
  • Propyl mercaptan (onion smell) - 0.006 mg/L
  • Artificial musk - 0.00004 mg/L
  • Methyl mercaptan (poop smell) - 0.0000004 mg/L
• Olfaction sensitivity

  More acute/sensitive in women, especially during ovulation, toxic substances are essentially odorless - odor detection thresholds higher than lethal concentrations
• Membrane and Action Potentials

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

  • 1,000 odorant receptors, all odorant receptors are diverse, but all are classified as G-protein coupled receptors (GPCRs), act via adenylyl cyclase and cAMP, act via phospholipase C, one type of receptor in each olfactory receptor neuron
• 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
• 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 (tears)
• Adaptation

  • Threshold for other odors are unchanged
  • desensitization - you get adapted to it
  • olfactory dendrites & cilia have high concentration of β - adrenergic receptor kinase 2 & β arrestin - 2
• granule cells

  • inhibitory cells in olfactory bulb, have no axons, make reciprocal synapse with lateral dendrites of mitral cells