Chapter 17: The Special Senses – Olfaction and Gustation

Chapter 17: The Special Senses

Introduction to Special Senses

The special senses are specialized sensory systems that allow organisms to interact with their environment through highly developed organs. These senses include olfaction (smell), gustation (taste), vision, equilibrium (balance), and hearing. Each sense is associated with distinct anatomical structures and physiological pathways.

• Olfaction – Sense of smell

• Gustation – Sense of taste

• Vision – Sense of sight

• Equilibrium – Sense of balance

• Hearing – Sense of sound

Olfaction (Sense of Smell)

Olfactory Organs

Olfactory organs are paired structures located in the nasal cavity, on either side of the nasal septum, and inferior to the cribriform plate of the ethmoid bone. They are responsible for detecting airborne chemical stimuli (odorants).

• Composed of two main layers:

  • Lamina propria

  • Olfactory epithelium

Lamina Propria

• Contains areolar connective tissue

• Houses blood vessels, nerves, and olfactory glands (Bowman's glands)

• Olfactory glands produce mucus that surrounds and protects olfactory sensory neurons

Olfactory Epithelium

• Contains three main cell types:

  • Olfactory sensory neurons – Detect odorants

  • Supporting cells – Provide physical support and detoxification

  • Basal (stem) cells – Replace worn-out olfactory neurons (neurogenesis)

Olfactory Sensory Neurons

• Approximately 400 different types of olfactory sensory neurons

• About 20 million olfactory sensory neurons in humans

• Dendrites are covered with cilia and mucus

• Airborne, water-soluble, or lipid-soluble substances (odorants) diffuse and bind to cilia

• Olfactory neurons are frequently replaced by basal cells (rare example of adult neurogenesis)

• Number of olfactory receptors declines with age

Physiology of Olfaction

• Odorant molecules bind to G protein-coupled receptors on olfactory cilia

• This activates adenylate cyclase, which converts ATP into cyclic AMP (cAMP)

• cAMP acts as a second messenger, opening Na+ channels

• Influx of Na+ ions depolarizes the membrane, generating a generator potential

• If threshold is reached, action potentials (APs) are generated and transmitted to the CNS

Key Equations:

• ATP → (adenylate cyclase) → cAMP

$\text{ATP} \xrightarrow{\text{adenylate cyclase}} \text{cAMP}$

Adaptation

• No peripheral adaptation; only central adaptation occurs

• Olfactory bulbs can be inhibited by brain nuclei, reducing sensation of a given odor but remaining sensitive to new ones

Olfactory Pathways to the Brain

• Olfactory sensory neurons penetrate the cribriform plate of the ethmoid bone

• Synapse with olfactory bulbs

• Travel along the olfactory tract to reach the olfactory cortex of the cerebrum, hypothalamus, and limbic system

• Olfaction is strongly linked to emotion and memory due to connections with the limbic system

Olfaction & Sexual Selection

• Body odors are influenced by the Major Histocompatibility Complex (MHC), a group of genes essential for immune function

• Greater diversity in MHC genes leads to a stronger immune system

• Women tend to prefer partners with different MHC genes (increased offspring fitness)

• Pheromones play a role in sexual attraction and mate selection

• Hypotheses:

  • Good gene hypothesis

  • Inbreeding avoidance hypothesis

  • Parasite hypothesis

Olfactory Discrimination

• Humans can distinguish between 2,000 to 4,000 chemical stimuli

• About 50 primary smell sensations are known

• Humans can detect odorants at concentrations as low as a few parts per billion

• Mercaptan is added to odorless natural gas for leak detection

• Human nose can theoretically detect up to 1 trillion odors (pattern recognition)

Gustation (Sense of Taste)

Sensory Organs of Taste

Taste is detected by specialized sensory organs called taste buds, located primarily on the dorsal surface of the tongue, but also in the pharynx and larynx.

• Taste buds are found within structures called lingual papillae:

  • Filiform papillae: Provide friction, contain no taste buds

  • Fungiform papillae: Mushroom-shaped, contain taste buds, concentrated at tip and sides of tongue

  • Foliate papillae: Contain taste buds, located on lateral margins of tongue

  • Vallate papillae: V-shaped row at back of tongue, contain many taste buds

Structure of Taste Buds

• Composed of three main cell types:

  • Basal (stem) cells: Divide and differentiate into transitional cells

  • Transitional cells: Mature into gustatory epithelial cells

  • Gustatory epithelial cells: Have microvilli (taste hairs) that extend into taste pores

• Taste buds are replaced every 10 days by basal cell division

Salivary Glands and Taste

• Saliva dissolves food, allowing chemicals to diffuse into taste pores

• Saliva contains water, electrolytes, mucus, white blood cells, epithelial cells, amylase, lipase, and lysozyme (antimicrobial)

Function of the Tongue

• Manipulates and mixes food with saliva

• Contains taste buds for gustatory sensation

• Involved in communication (speech, social behaviors)

Primary Taste Sensations

• Five primary tastes:

  • Sweet

  • Sour

  • Salty

  • Bitter

  • Umami (savory, taste of glutamate)

• Water may also be considered an additional taste sensation

Umami

• Japanese for "delicious"

• Triggered by free glutamates (e.g., in meats, cheese, soy sauce)

Physiology of Gustation

• Chemicals dissolve in saliva and bind to receptors on gustatory microvilli

• Different tastes use different receptor mechanisms:

  • Salty: Na+ influx through leak channels causes depolarization

  • Sour: H+ ions enter and cause depolarization

  • Sweet, Bitter, Umami: Bind to G protein-coupled receptors, activate second messengers (e.g., cAMP), leading to depolarization and neurotransmitter release

Key Equation (for sweet, bitter, umami):

$\text{Ligand} + \text{Receptor} \rightarrow \text{G protein activation} \rightarrow \text{Second messenger (e.g., cAMP)}$

Gustatory Pathways

• Taste signals travel from taste buds via cranial nerves:

  • Facial nerve (VII)

  • Glossopharyngeal nerve (IX)

  • Vagus nerve (X)

• Signals synapse in the medulla oblongata, then relay to the thalamus and gustatory cortex

Integration of Olfaction and Gustation

• Flavor perception is a combination of taste and smell

• Olfactory input greatly enhances taste discrimination

Sensitivity and Age

• Humans start with about 10,000 taste buds

• With age, both olfactory and gustatory sensitivity decline due to reduced receptor numbers and stem cell activity

• Reduction in taste and smell can affect appetite and nutrition in older adults

Summary Table: Comparison of Olfactory and Gustatory Systems

Feature	Olfaction (Smell)	Gustation (Taste)
Receptor Location	Nasal cavity (olfactory epithelium)	Taste buds (tongue, pharynx, larynx)
Stimulus Type	Airborne chemicals (odorants)	Chemicals dissolved in saliva
Receptor Cells	Olfactory sensory neurons	Gustatory epithelial cells
Pathway to Brain	Olfactory bulb → olfactory tract → cortex/limbic system	Cranial nerves VII, IX, X → medulla → thalamus → cortex
Adaptation	Central adaptation only	Peripheral and central adaptation
Regeneration	Frequent (basal cells)	Frequent (basal cells)

Additional info: Vision, hearing, and equilibrium are also special senses but are not covered in detail in these notes. The notes focus on olfaction and gustation as per the provided materials.