CHAPTER 17

• What is olfaction?

  The sense of smell
• Where are the olfactory organs located?

  In the nasal cavity
• What are the two layers that make up the olfactory organs?

  • Olfactory epithelium
  • Lamina propria
• What are odorants?

  Small airborne organic water-soluble or lipid-soluble substances that stimulate olfactory receptors
• What are the special senses?

  • Olfaction (smell)
  • Gustation (taste)
  • Vision
  • Equilibrium (balance)
  • Hearing
• What does the olfactory epithelium cover?

  The inferior surface of the cribriform plate, the superior portion of the perpendicular plate, and the superior nasal conchae of the ethmoid
• What does the lamina propria contain?

  • Areolar tissue
  • Blood vessels
  • Nerves
  • Olfactory glands (secrete mucus)
• What are olfactory sensory neurons?

  Highly modified nerve cells found in the olfactory epithelium
• What is the function of the dendritic bulb in olfactory sensory neurons?

  It contains dendrites that extend into the surrounding mucus
• Where are olfactory receptors located?

  On the surface of the dendrites of olfactory sensory neurons
• What do the axons of olfactory sensory neurons form?

  The olfactory nerves
• What are the types of cells found in the olfactory epithelium?

  • Olfactory sensory neurons
  • Supporting cells
  • Basal epithelial cells (stem cells)
• What happens when an odorant binds to a G protein–coupled receptor?

  A second messenger cAMP opens sodium ion channels, leading to depolarization of the olfactory neuron
• What triggers action potentials in the axon of the olfactory neuron?

  Sufficient depolarization
• Describe the olfactory pathways.

  • Axons of olfactory sensory neurons penetrate the cribriform plate
  • Synapse with neurons in the olfactory bulbs
  • Form olfactory tracts to the olfactory cortex, hypothalamus, and limbic system
• How does olfactory information reach the cerebral cortex?

  Without passing through the thalamus first
• What causes olfactory adaptation?

  Central adaptation via innervation of the olfactory bulbs by other brain nuclei
• What is gustation?

  The sense of taste
• What do taste receptor cells provide information about?

  Foods and liquids consumed
• Where are taste receptor cells found?

  • In taste buds
  • Distributed on the superior surface of the tongue and portions of the pharynx and larynx
• How many odorants can humans distinguish?

  Between 2000 and 4000
• What are the primary smells interpreted by the nervous system?

  More than 50 “primary smells” based on receptor activity patterns
• How does sensitivity to different smells vary?

  It varies widely among individuals
• How often are olfactory neurons replaced?

  Frequently from basal cells in the olfactory epithelium
• What happens to the total number of olfactory neurons with age?

  It declines
• What are lingual papillae?

  • Epithelial projections on the surface of the tongue
  • Types include:
  • Filiform papillae
  • Fungiform papillae
  • Vallate papillae
  • Foliate papillae
• Where are filiform papillae found?

  In the anterior two-thirds of the tongue
• What is the function of filiform papillae?

  Provide friction to move food around the mouth
• Where are fungiform papillae located?

  Scattered around the tongue with higher concentrations along the tip and sides
• How many taste buds do fungiform papillae contain?

  About five taste buds each
• Where are vallate papillae found?

  In a V-shaped pattern along the posterior margin of the tongue
• How many taste buds do vallate papillae contain?

  About 100 taste buds each
• Where are foliate papillae located?

  In folds along the lateral margin of the tongue
• What do taste buds contain?

  Basal epithelial cells, transitional cells, and taste receptor cells
• What do taste receptor cells extend into the surrounding fluid?

  Microvilli (taste hairs) containing receptor proteins
• What do taste receptor cells do when activated?

  Release neurotransmitters that stimulate sensory neurons
• Describe the gustatory pathways.

  • Taste receptor cells release neurotransmitters
  • Stimulate sensory neurons forming:
  • Facial nerve (anterior two-thirds of the tongue)
  • Glossopharyngeal nerve (vallate papillae)
  • Vagus nerve (extralingual taste buds)
  • Sensory fibers synapse in the solitary nucleus of the medulla oblongata
• What happens to the axons of postsynaptic neurons from the medulla oblongata?

  They enter the medial lemniscus and synapse in the thalamus
• Where do thalamic neurons relay gustatory information?

  To the gustatory cortex in the insula
• What factors influence taste perception?

  • Sensory data about the texture of food
  • Olfactory information
  • Inflammation of the nasal mucosa