Lecture Notes: Olfactory System and Sense of Smell

Importance of Smell in Taste

• Smell and Taste Connection:
  • When experiencing a cold, the nose becomes stuffy, impacting the ability to taste.
  • Eating involves breaking down cellular components that release molecules, traveling through the throat and into the nose.
  • The sense of smell complements the sense of taste, enriching flavors.
• Effect of Losing Smell:
  • Without smell, reliance is solely on taste, reducing the richness of flavors.
  • Experiment: Close your nose while eating to notice the difference in taste.

Anatomy of the Olfactory System

• Olfactory Epithelium:

  • Located in the nasal passage, not visible externally.
  • Functions as a region where smell-related processes begin.

• Cribriform Plate:

  • A bone that separates the olfactory epithelium from the brain.

• Olfactory Bulb:

  • An extension from the brain situated above the cribriform plate.
  • Composed of a bundle of nerves that project through the cribriform plate into the olfactory epithelium.

• Olfactory Sensory Neurons:

  • Nerve projections with receptors sensitive to specific molecules.
  • Thousands of neurons send signals through the cribriform plate to the olfactory bulb.

Sensory Processing

• Receptors and Molecules:

  • Each receptor is sensitive to a particular type of molecule. E.g., benzene rings are aromatic and detectable by smell.
  • Molecules bind to receptors, triggering a cascade of cellular events.

• Action Potential and Signal Transmission:

  • Binding of molecules causes receptors to induce action potentials.
  • Action potentials travel to the olfactory bulb, synapsing onto a glomerulus.
  • Glomerulus serves as a convergence point for sensory neurons sensitive to the same molecule.

Signal Pathways and Organization

• Mitral/Tufted Cells:

  • Synapse at the glomerulus to transmit signals to the brain.
  • Single cell projections are more efficient than multiple neuron projections to the brain.

• Receptor Specificity and Diversity:

  • Different receptors and glomeruli correspond to different molecular structures (e.g., single vs. double benzene rings).
  • Each type of receptor cell synapses at a specific glomerulus.

Molecular Binding and Signal Cascade

• G Protein-Coupled Receptors (GPCRs):
  • Odor molecule binding leads to GPCR activation.
  • Activation results in G protein disassociation and intracellular cascade.
  • Causes ion channels to open, allowing ion influx and depolarization.
  • Depolarization triggers action potential transmission to the brain.

Summary

• Odor molecules bind to GPCRs, initiating a signal cascade that results in action potentials reaching the brain.
• The organized pathway from molecule binding to brain perception highlights the complexity and efficiency of the olfactory system.