Olfactory and Gustatory Systems

Sensing Chemicals: Smell

  • Nasal Conchae (Turbinates):

    • Shelves of bone within the nasal passage.
    • Shape forces air through passages to control humidity and temperature.

  • Olfactory Epithelium:

    • Epithelium cells in the nasal cavity.
    • Composed of Pseudostratified Columnar Epithelium.
      • Pseudostratified: Appears layered but all cells touch the basement membrane.
      • Stratified: Layered.
      • Transitional: Stretchy and changes shape.

Bowman’s Gland and Olfactory Pathway

  • Bowman’s Gland (Olfactory Gland):

    • Located in the Lamina Propria, beneath the Olfactory Epithelium.
    • Secretes:
      • Lysozyme
      • Amylase
      • IgA
      • Gel-forming Mucin: Gives mucus gel-like properties.

  • Olfactory Pathway:

    • Signal travels from the olfactory bulb via the olfactory tract (CN I).
    • Goes to:
      • Frontal Lobe: For conscious interpretation of smell.
      • Limbic System: For emotional responses triggered by smell.
        • Can be sympathetic (e.g., gas) or parasympathetic (e.g., food).

  • Peripheral Stem Cells:

    • Stem cells in the olfactory system.
    • Guide axon and dendrite growth to match glomeruli (odor).
    • New axons penetrate the olfactory bulb.

Smelling Theories

  • Amoore’s Theory:

    • Focuses on the shape of molecules.
    • Limitation: Only 350 receptors in humans; this theory suggests detecting 1000 different smells, which is inaccurate.

  • Labelled Line Theory:

    • Focuses on receptor type determining the pathway.
    • Limitation: Same as above; 350 receptors are insufficient to detect 1000 smells.

  • Combinatorial Odor Code Theory:

    • Focuses on patterns of receptor activation.
    • Each of the 350 receptors has unique combinations, allowing for nearly infinite possibilities.
    • Combinations of glomeruli are activated.

  • Olfactory Adaptation:

    • Decrease in sensitivity to a continuous odor.

  • Chemoreception Nerves:

    • Olfactory Nerve: Primary nerve for detecting odors.
    • Trigeminal Nerve: Enhances odors by detecting irritants/pain (e.g., smelling something burning).
    • Olfactory nerve function is affected by lack of O2O_2 (e.g., in astronauts or airplanes).

Taste / Gustation

  • Taste Sensations:

    • Sweet: Detected by saccharin.
    • Sour: Detected by acids and hydrogen ions (H+H^+).
    • Salt: Detected by metal ions, such as NaCl.
    • Bitter: Detected by alkaloids, such as nicotine.
    • Umami: Detected by amino acids, such as glutamate (MSG).

  • Taste Transduction and Gustatory Pathway:

    • Food dissolves and contacts gustatory hairs.
    • Gustatory/taste cells (taste buds) depolarize.
    • Nerves involved:
      • Facial nerve (anterior 2/3 of tongue).
      • Glossopharyngeal nerve (posterior 1/3 of tongue).
      • Vagus nerve.
    • Pathway:
      • Solitary Nucleus (Medulla oblongata)
      • Hypothalamus/Limbic system (Taste appreciation, memory)
      • Gustatory Cortex

  • Taste vs. Flavor:

    • Taste: Pure gustation.
    • Flavor: Taste affected by other factors like smell and temperature.