- Olfaction

Function of Sensory Systems

  • Role: Provide the Central Nervous System (CNS) with information about the environment.
  • Receptors: Specialized cells that respond to stimuli.
  • They convert stimuli into signals (membrane voltages) for neuronal transmission.
  • Receptors are specific, often responding to only one type of stimulus.

Chemical Senses: Olfaction (Sense of Smell)

  • Olfactory System Circuitry:
  • Each Olfactory Sensory Neuron (OSN) has one type of olfactory receptor.
  • Axons of OSN types converge to form synapses in glomeruli of the main olfactory bulb (MOB).
  • Responses to odors are spatially represented in the MOB, facilitating lateral inhibition and oscillation.
  • Central projections go to: olfactory cortex, thalamus, neocortex, amygdala, hypothalamus.
  • Function: Detect low concentrations of medium-sized molecules in the air.

Key Discoveries in Olfaction

  • 2004 Nobel Prize in Medicine awarded to Dr. Richard Axel and Dr. Linda Buck for the discovery of a "single family of olfactory receptors".
  • Olfactory Receptor Genes:
  • Humans: 800 genes
  • Mice: over 1400 genes
  • Recognition of complex odors is due to the combinatorial activation of receptor patterns – capable of distinguishing > 10,000 odors.

Organization of the Human Olfactory System

  • Includes olfactory bulb, cribriform plate, olfactory epithelium, and olfactory nerve.
  • Ciliary Structures:
  • Olfactory sensory neurons have cilia with receptor molecules capable of odor detection.
  • Cribriform Plate: Separates the nasal cavity from the cranial cavity; facilitates odorant access to sensory neurons.

Neural Processing of Smells

  • Odors are often complex mixtures.
  • Detection capability varies widely among species:
  • Bloodhound: ~4 billion olfactory receptor neurons (ORNs)
  • Humans: ~12 million ORNs
  • Increasing odor concentration yields stronger glomerular responses within the olfactory bulb.
  • Lateral Inhibition: Enhances the selectivity of odor responses.

Adaptation and Sensitivity

  • Olfactory Adaptation: Initially, neurons show rapid desensitization to continuous stimulation, influenced by calcium mechanisms.
  • Types of Adaptation: Immediate, short-term, and long-term adaptations reduce sensitivity through receptor phosphorylation and calcium interactions.

Molecular Mechanisms of Odorant Transduction

  • Basic Steps:
  1. Odorant binds to a G protein-coupled receptor.
  2. Activates G proteins, leading to increased levels of cAMP.
  3. cAMP binds to channels, resulting in sodium and calcium influx, which depolarizes the cell.
  4. Calcium influx leads to the release of chloride through channels, amplifying the signal.

Vomeronasal Chemoreception

  • Detects specific chemical signals often related to social communication in other species, although its role in humans is less clear.
  • The vomeronasal organ (VNO) responds to pheromones and other chemical cues, supporting social behaviors in mammals.
  • Two receptor families (V1R and V2R) are involved in chemosensory signaling, projecting to the accessory olfactory bulb (AOB).
  • Response Specificity: Vomeronasal sensory neurons maintain high specificity and sensitivity even at low concentrations of substances.