SMELL

• Smell and taste receptors are chemoreceptors that are stimulated by chemical molecules in solution in mucus in the nose (odorants) and saliva in the mouth (tastants)
• olfactory epithelium


  yellowish pigmented, a specialized portion of the nasal mucosa that covers an area of 10 cm2 in the roof of the nasal cavity near the septum in humans
• olfactory epithelium


  place in the body where the nervous system is closest to the external world
• three types of neurons important for olfaction:

  olfactory sensory neurons

  supporting cells

  basal stem cells
• olfactory receptor cells


  bipolar olfactory sensory neurons
• olfactory receptor cells


  responsible for olfactory transduction
• olfactory receptor cells


  have a short, thick dendrite that projects into the nasal cavity where it terminates in a knob containing 6–12 cilia that protrude into the thin layer of mucus overlying the epithelium
• olfactory receptor cells


  pass through the cribriform plate of the ethmoid bone to enter the olfactory bulbs
• olfactory receptor cells generally survive for only 1-2 months
• about 1000 olfactory genes in humans, accounting for 3% of the human genome; approximately 400 of these genes function as odorant receptors
• odorant receptors are g-protein coupled receptor
• Each olfactory sensory neuron expresses only one of the 400 functional olfactory genes
• Each odorant can bind to a large pool of odorant receptors
• In the olfactory bulb, the axons of the olfactory sensory neurons synapse on the primary dendrites of the mitral cells and tufted cells: olfactory glomeruli
• Each olfactory sensory neuron projects to only one or two glomeruli
• The mitral cells with their glomeruli project to different parts of the olfactory cortex.
• The central olfactory system is able to decode the pattern of receptor-cell activity that signals the identity of the odorant.
• periglomerular cells

  inhibitory neurons connecting one glomerulus to another
• granule cells


  no axons and make reciprocal synapses with the lateral dendrites of the mitral and tufted cells
• The axons of the mitral and tufted cells pass posteriorly through the lateral olfactory stria
• Conscious discrimination of odors is dependent on the pathway to the orbitofrontal cortex.
• pathway to the amygdala mediates the emotional responses to olfactory stimuli
• pathway to the entorhinal cortex is concerned with olfactory memories.
• hydrogen sulfide (0.0005 parts per million, ppm)
• acetic acid (0.016 ppm)
• kerosene (0.1 ppm)
• gasoline (0.3 ppm)
• Some toxic substances are essentially odorless
• they have odor detection thresholds higher than lethal concentrations.
• carbon dioxide is detected at 74,000 ppm but is lethal at 50,000 ppm.
• Sense of smell is said to be more acute in women than in men, most acute at the time of ovulation.
• Determination of differences in the intensity of any given odor is poor. The concentration of an odor-producing substance must be changed by about 30% before a difference can be detected. The comparable visual discrimination threshold is a 1% change in light intensity
• anosmia


  inability to smell
• hyposmia or hypesthesia


  diminished olfactory sensitivity
• Anosmia is associated with hypogeusia
• hyperosmia


  enhanced olfactory sensitivity (pregnant)
• dysosmia


  distorted sense of smell