Smell

• Cribriform plate of ethmoid bone


  Part of the olfactory system
• Olfactory tract


  • Carries olfactory signals to the brain
• Olfactory recess


  • Part of the nasal cavity
• Nasopharynx


  • Part of the nasal cavity
• Frontal bone


  • Bone that the olfactory bulb is located on
• Olfactory bulb


  • Receives olfactory signals from the olfactory nerve
• Olfactory nerve fascicles


  • Bundles of olfactory nerve fibers
• Nasal cavity


  • Where the olfactory membrane is located
• Palate


  • Part of the nasal cavity
• Smell is the least understood of our senses
• This is partly because the sense of smell is a subjective phenomenon that cannot be studied with ease in lower animals
• Another complicating problem is that the sense of smell is poorly developed in human beings (Microsmatic) in comparison with the sense of smell in many lower animals (Macrosmatic)
• Sense of Smell


  Important for pleasure, enjoying the taste of food, and alerting us to potential dangers
• Olfactory Membrane


  • Lies in the superior part of each nostril
  • Has a surface area of about 2.4 square centimeters in each nostril
• Olfactory Cells


  • Bipolar nerve cells derived originally from the CNS
  • About 100 million of these cells in the olfactory epithelium interspersed among sustentacular cells
• Olfactory Cilia


  • 4 to 25 olfactory hairs that project into the mucus and react to odors
• Bowman's Glands


  • Secrete mucus onto the surface of the olfactory membrane
• Olfactory cells are constantly being replaced with a half-time of a few weeks
• Mitral Cells


  • Cells in the olfactory bulb that receive input from the olfactory cells
• Glomeruli


  • Globular structures in the olfactory bulb where olfactory cell axons terminate
• Sustentacular Cells


  • Support cells in the olfactory epithelium
• Excitation of the Olfactory Cells

  1. Odorant substance diffuses into the mucus

  2. Binds with receptor proteins in the olfactory cilia membrane

  3. Activates G protein-cAMP pathway

  4. Opens Na channels

  5. Generates action potential to excite the olfactory neuron
• Volatile substances


  Only substances that can be sniffed into the nostrils can be smelled
• Odorant solubility


  • Substance must be at least slightly water soluble to pass through the mucus
  • Substance must be at least slightly lipid soluble to pass through the lipid constituents of the cilia
• Sniffing


  A semi-reflex response that occurs when a new odor attracts attention
• Adaptation


  • Olfactory receptors adapt about 50% in the first second, then very little and slowly
  • Most additional adaptation occurs within the CNS
• Granule Cells


  Inhibitory cells in the olfactory bulb that receive input from the olfactory tract and inhibit the olfactory system
• Primary Smell Sensations


  • Camphoraceous
  • Musky
  • Floral
  • Pepperminty
  • Ethereal
  • Pungent
  • Putrid
• Affective Nature of Smell

  • Smell has an affective quality of either pleasantness or unpleasantness
  • Smell is probably even more important than taste for the selection of food
  • A person can become nauseated by the smell of food that previously disagreed with them
• Smell Threshold


  • The minute quantity of stimulating agent in the air can elicit a smell sensation
  • The substance methylmercaptan can be smelled when only one 25 trillionth of a gram is present in each milliliter of air
• Smell Pathway


  • Olfactory mucous membrane -> Cribriform plate -> Olfactory bulb -> Olfactory tract -> CNS
• Olfactory Bulb


  • Lies over the cribriform plate
  • Has multiple small perforations through which olfactory nerves pass
• Smell Pathway in Olfactory Bulb

  1. Olfactory cell axons terminate in glomeruli

  2. Dendrites of mitral and tufted cells in the bulb receive input from the glomeruli

  3. Mitral and tufted cells send axons through the olfactory tract to transmit olfactory signals to higher levels in the CNS
• Medial Olfactory Area


  • Very old olfactory system that projects to the septal nuclei, hypothalamus, and limbic system
• Lateral Olfactory Area


  • Newer and less old olfactory system that projects to the prepyriform and pyriform cortex, amygdala, and hippocampus
• Newer Olfactory Pathway


  • Passes through the thalamus to the dorsomedial orbitofrontal cortex for conscious analysis of odors
• Vomeronasal Organ


  • Found in rodents and other mammals, concerned with the perception of pheromones
  • Not well developed in humans
• Evidence for the existence of pheromones in humans is unclear
• The sense of smell is said to be more acute in women than in men, and in women it is most acute at the time of ovulation
• Smell and taste have a unique ability to trigger long-term memories