Olfaction/smell Notes

Smell (Olfaction)

Olfactory Receptor Cells

  • Located in the roof of the nasal cavity within the olfactory mucosa.
  • Highly sensitive: Women tend to have slightly more sensitive receptors than men.
  • Capable of detecting around 10,000 distinct odors in humans (less compared to dogs).

Olfactory Neurons

  • Cilia:
    • Long hairs act as the dendrite end of the olfactory neurons.
    • Receptors on the cilia bind to odor molecules.
  • Volatility Requirement:
    • Substances must be volatile (in a gas state) to be smelled.
    • Liquids cannot be directly smelled.
  • Neural Pathway:
    • Axons pass through the cribriform plate (part of the ethmoid bone, which forms the roof of the nasal cavity).
    • Olfactory foramina are holes in the cribriform plate.
    • Neurons extend through these holes into the olfactory bulbs.
    • Second-order neurons continue the olfactory message to the temporal lobe.

Neuron Regeneration

  • Olfactory neurons have a short lifespan of approximately 60 days.
  • Unique in that these neurons regenerate (unlike most neurons, which are amitotic).
  • Regeneration is likely due to exposure to pathogens in the nasal cavity.
  • Research Significance:
    • Studying olfactory neuron regeneration may offer insights into treating spinal cord injuries by understanding how to regenerate damaged tissue.

Mechanism of Smell

  • Odor molecules bind to receptors on the cilia of olfactory neurons.
  • Binding causes the opening of ion channels, leading to depolarization and a local potential.
  • This local potential triggers an action potential.
  • The signal travels through the cribriform plate to the olfactory bulb.

Adaptation

  • Olfactory receptors are highly adaptive and ignore repetitive stimuli.
  • Synaptic inhibition in the olfactory bulb filters out continuous smells.
  • Example: The strong smell in a cadaver lab fades with time due to receptor adaptation.
  • Contrast: Nociceptors (pain receptors) do not adapt, so pain persists with repetitive stimuli.

Olfactory Pathways and Destinations

  • Bulb cells (axons from olfactory mucosa) lead to the olfactory bulb.
  • Olfactory bulbs transition into olfactory tracts (in the CNS).
  • Tracts project to:
    • Temporal Lobes: For conscious perception of smell.
    • Amygdala and Hypothalamus: Elicit reflex responses like sneezing or vomiting due to certain smells; also involved in emotional responses to odors.
  • Amygdala: Connects odors to emotions (e.g., a food smell evoking memories of a loved one).