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.