Perception: The Chemical Senses

Chemical Senses Overview

- Chemical Senses: Includes olfaction (smell) and gustation (taste).

Part 1: Olfaction

  • Odor: Perceptual experience created by an odorant.
    • Odorants: Volatile molecules activating olfactory receptors.
    • Requirements for Olfaction:
    • Presence of an odorant.
    • Sufficient concentration for detection.
      • A single source can emit hundreds of different odorants.
Pathways to the Nose
  • Orthonasal Pathway: Odorants enter through nostrils.

- Retronasal Pathway: Odorants enter through the pharynx from the oral cavity.

The Nose Structure

  • Nasal Septum: Divides the two nasal cavities.

- Turbinates: Bony structures guiding airflow to the olfactory epithelium.

Olfactory Epithelium

  • Contains approximately 20 million olfactory receptor neurons (ORNs) in about 2 cm².
  • Cilia: Extend into olfactory mucus produced by Bowman’s glands, replaced every 10 minutes.
  • Axons of ORNs: Form the olfactory nerve; basal cells are precursors for ORNs and are replaced monthly.

- Odorants dissolve in olfactory mucus, interacting with surface cilia containing olfactory receptors.

Odor Detection Mechanisms

  • Ligand-Gated Ion Channels: Open via a lock-and-key mechanism, unlike voltage-gated channels.

- G-Protein Coupled Receptors: Involve a series of intracellular messages where ions flow across the membrane due to receptor activation.

Neural Code for Odors

  • Humans possess about 350 active genes for ORN receptors but can discriminate over 10,000 to 1 trillion odors using population coding.

- Multiple types of ORNs respond to individual odorants, with identity coded by patterns.

Olfactory System Structures

  • Olfactory Bulb: Where the olfactory nerve synapses with mitral and tufted cells in glomeruli, organized for chemotopy based on similar molecules.
  • Main Targets of Olfactory Signals:
    • Amygdala: Emotion regulation.
    • Piriform Cortex: Primary olfactory cortex.

- Entorhinal Cortex: Gateway to hippocampus for memory formation.

Representing Odors in the Brain

  • Anterior Piriform Cortex (APC): Represents molecular structure; narrow tuning.

- Posterior Piriform Cortex (PPC): Represents subjective qualities of odors, lacks chemotopic organization.

Odor Detection and Identification

  • Higher concentration = higher perceived intensity (measured in ppm).
  • Threshold Detection:
    • Requires careful controls of concentrations.
    • Strong odorants detected at lower concentrations than weak ones.

- Humans can identify few odors without context; training can improve performance (e.g., wine experts).

Adaptation and Memory

  • Adaptation: Diminishing sensitivity to odors, important for detecting new stimuli.

- Odor and Memory: Odor information is deeply tied to emotional memories and often evokes vivid recollections from early life.

Part 2: Gustation (Taste)

  • Taste: Perception of a tastant, which stimulates taste receptors.
    • Five Basic Tastes:
    • Sweet (sugars)
    • Salty (sodium, potassium)
    • Umami (amino acids)
    • Sour (acids)
    • Bitter (toxins)
Flavor Experience

- Flavor: Full sensory experience combining taste, odor, and mouth feel, influenced by trigeminal sense (irritants).

Taste Mechanisms

  • Taste Receptor Cells (TRCs): Detect tastants, similar to ORNs but with fewer varieties and do not synapse with axons.
  • Taste Buds: Structures with multiple TRCs found predominantly on the tongue.
    • Types:
    • Presynaptic Cells: Detect salty and sour.
    • Receptor Cells: Taste specific (sweet, bitter, umami).
Neuroanatomy of Taste
  • Cranial Nerves: Different parts of the oral cavity send signals to the gustatory nucleus, then to the cortex.

- Gustatory Cortex: Located in the insula and frontal operculum, processes taste and other sensory properties.

Orbitofrontal Cortex (OFC) and Cognitive Influences

  • OFC: Responds to the reward value of food and integrates flavor perception.

- Cognitive expectations can markedly affect flavor perception, influencing enjoyment and preference.

Individual Differences in Taste Perception

  • Variations in T2R38 bitter receptors lead to different sensitivity levels to bitter tastes.
    • “Supertasters” have heightened sensitivity, affecting responses to various foods and flavors.