Olfactory Perception and Action Potential

Action Potential and Olfactory System
  • Action Potential: The brain’s method for exchanging information, represented in frequency and pattern.
  • Sensory Stimuli: Airborne chemicals interact with receptors, encoded into electrical signals (action potentials) and transmitted to higher brain centers.
Odorant Perception in Mammals
  • Factors affecting olfactory perception in mammals include:
    • Number of olfactory receptor neurons.
    • Variety of odorant receptor proteins.
    • Size of the olfactory epithelium.
    • Proportion of the forebrain focused on olfaction.
  • Human Olfactory Neurons: Humans have fewer olfactory receptor neurons compared to bloodhounds.
Sensitivity to Odors in Humans

  1. Detection Thresholds:

    • Ozone detected at ~10 parts per billion.
    • D-limonene (citrus) detectable at ~15 parts per billion.

  2. Concentration-Dependent Perception:

    • Indole: Pleasant floral smell in low concentrations; putrid smell in high concentrations.
    • D and L carvone enantiomers produce different scents (spearmint vs caraway).

  3. Functional MRI Findings:

    • Pleasant and unpleasant odors activate distinct regions in the orbitofrontal and cingulate cortex.
Loss of Odor Sensitivity
  • Anosmia: A condition where individuals cannot identify one or more odors, either congenital or acquired through various means:
    • Chronic sinus infection/inflammation.
    • Traumatic head injury.
    • Ageing and neurodegenerative diseases.
    • Exposure to toxins, chemotherapy, and specific disorders.
  • Anosmic individuals struggle to identify scents like baby powder or chocolate.
  • Olfactory sensitivity declines with age, especially in identifying common odorants.
Olfactory Epithelium and Receptor Neurons

  • Olfactory Epithelium:

    • Contains olfactory receptor cells, supporting cells, and basal cells.
    • Odorants dissolve in mucus and activate receptor cilia.

  • Transduction Mechanisms:

    • Odorant binding activates specific G-proteins (Golf) and generates cAMP.
    • cAMP opens cation channels (Na+, Ca2+), leading to depolarization and olfactory receptor potential.
Odorant Transduction Flow
  1. Odorants create receptor potential in the cilia.
  2. This potential propagates and triggers action potentials in the olfactory receptor cell.
  3. Action potentials travel down the olfactory nerve.
Distinguishing Odors
  1. Population Coding: Each odor represented by a combination of neuron activity.
  2. Spatial Maps: Axons from neurons expressing a specific receptor gene converge on the same glomeruli, forming a spatial map in the olfactory bulb.
  3. Temporal Coding: Timing of action potentials provides additional information for odor discrimination.
Neural Activation in Olfactory Pathways
  • Axons from olfactory receptor neurons converge in glomeruli:
    • Each glomerulus receives inputs from about 25,000 olfactory axons.
  • Mapping of activated neurons in the olfactory bulb occurs depending on the nature and concentration of the odorant.