Olfaction and Gustation Notes

Special Senses Overview

  • Special senses convey stimuli from specialized sensory organs in specific areas:
    • Olfaction (smell)
    • Gustation (taste)
    • Vision
    • Audition (hearing)
    • Vestibular sensation (balance)

Differences Between General and Special Senses

  • Stimulus Detected:
    • General senses: Touch, pain, temperature.
    • Special senses: Light, sound waves, head movement, chemicals (taste and smell).
  • Structure of Sensory Receptors:
    • General senses: Receptive ends of sensory neurons (specialized dendrites).
    • Special senses: Receptors that are not neurons (except olfaction).
  • Location of Sensory Nerves:
    • Special sensory organs are confined to the head and travel via cranial nerves.
    • General sensory information travels via cranial and spinal nerves.

Receptor Types

  • General Senses: Modified dendrites act as receptors.
  • Special Senses: Secondary receptors sense modality and release neurotransmitters to generate action potentials in sensory neurons.
    • Examples: Photoreceptor cells, taste cells, hair cells in the ear.
    • Exception: Olfaction, where olfactory hairs (neurons) directly detect stimuli.

Olfactory Neurons

  • Unique because they are:
    • Neurons exposed to the outside world.
    • Capable of frequent regeneration.

Transduction

  • Conversion of physical or chemical stimulus into an action potential.
  • Both general and special senses utilize transduction to send signals to the CNS.
  • Processed by sensory nuclei, transmitted to the thalamus and primary cortex for awareness, then to association areas for interpretation.

Olfaction (Smell)

  • Detects odorants (chemicals) in the air, perceived as odors.
  • Process starts in the olfactory epithelium at the top of the nasal cavity.
Structures Involved
  • Cribriform Plate: Location where olfactory neurons pass through.
  • Olfactory Neurons: Synapse with mitral cells to form the olfactory tract.
  • Olfactory Cilia: Bind to odorants dissolved in mucus to generate action potentials.
Cells Involved
  • Olfactory Receptor Cells: Modified bipolar neurons that are chemoreceptors.
  • Basal Cells: Stem cells that replace olfactory neurons (lifespan: 30-60 days).
  • Supporting Cells: Columnar cells surrounding olfactory neurons.
Nerves and Tracts
  • Olfactory Nerve (Cranial Nerve I): Combined axons from olfactory neurons.
  • Olfactory Bulb: Located in the cribriform plate.
  • Olfactory Tract: Axons exiting the olfactory bulb traveling to the CNS.
Process of Olfaction
  1. Odorants are inhaled and dissolve in the mucus.
  2. Odorants bind to olfactory cilia.
  3. Action potential is generated and propagated.
Role of G Protein-Coupled Receptors
  • Odorant binding activates a G protein.
  • G protein triggers adenylyl cyclase to convert ATP to cyclic AMP (cAMP).
  • cAMP opens ion channels, allowing sodium and calcium to enter the cell, causing depolarization and generating an action potential.
    • ATP→cAMPATP \rightarrow cAMP
Olfactory Pathway
  1. Odorant dissolves in mucus and binds to cilia.
  2. Action potential generated.
  3. Signal travels down the olfactory nerve to the olfactory tract.
  4. Signal reaches the primary olfactory cortex.
  • Also projects to the amygdala, hippocampus, hypothalamus, and limbic system for emotional and visceral responses.
  • The olfactory tract bypasses the thalamus on its way to the primary olfactory cortex to enable awareness and identification of odors.
Olfactory Disorders
  • Anosmia: Lack of olfaction.
    • Can result in inability to detect dangers (smoke, spoiled food) and malnutrition due to reduced appeal of food.
  • Hyposmia: Reduced olfactory sensitivity.
Causes of Olfactory Dysfunction
  • Blockage of airflow to the olfactory epithelium (nasal polyps, deviated septum, swollen respiratory epithelium).
  • Head injuries damaging neural pathways or the cribriform plate.
  • Age-related decline in olfactory neuron regeneration, noticeable after age 60.

Gustation (Taste)

  • Chemoreceptors stimulated by chemicals in food.
  • Complex process involving olfactory chemoreceptors, thermoreceptors, and nociceptors.
Taste Buds and Papillae
  • Papillae: Structures housing taste buds on the tongue.
  • Taste Buds: Clusters of receptor cells and supporting cells.
    *Each taste bud is associated with a sensory neuron.
Types of Papillae
  • Circumvallate Papillae: Large, dome-shaped, containing hundreds of taste buds.
  • Fungiform Papillae: Mushroom-shaped, containing a few taste buds.
  • Foliate Papillae: Ridges on the side of the tongue, contain taste buds in childhood, sensitive to bitter compounds.
  • Filiform Papillae: Long, thin cylinders scattered across the tongue, do not contain taste buds, detect food texture and temperature.
Cells within Taste Buds
  • Gustatory Cells (Taste Cells): Specialized epithelial cells with microvilli projecting into a taste pore.
    • Associated with sensory neurons (cranial nerves VII, IX, and X).
  • Basal Cells: Stem cells that differentiate into new gustatory cells (lifespan: 10-14 days).
  • Supporting Cells: Surround and support gustatory cells.
Process of Tasting
  1. Chemicals must dissolve in saliva to reach taste buds.
  2. Chemicals bind to microvilli of taste buds.
  3. Signal transduction converts chemical stimuli into electrical signals.
Taste Sensations
  • Sweet: Simple sugars (glucose, fructose), lead paint, ethylene glycol.
  • Sour: Hydrogen ions (citric acids, lemon juice).
  • Salty: Metal ions (sodium, potassium).
  • Bitter: Nitrogen-containing compounds (rancid or poisonous substances).
  • Umami: Meaty or savory (glutamate and other amino acids).
Physiology of Taste
  1. Substances dissolve in saliva and reach taste buds.
  2. Change in ion movements depolarizes gustatory cell’s plasma membrane.
  3. Depolarization opens voltage-gated calcium channels.
  4. Calcium ions enter the cell and trigger the release of neurotransmitters.
  5. Neurotransmitters generate an action potential in a sensory neuron.
Gustatory Pathway
  1. Anterior two-thirds of the tongue (facial nerve).
  2. Posterior one-third of the tongue (glossopharyngeal nerve).
  3. Back of the tongue and pharynx (vagus nerve).
  4. Information travels to the medulla.
  5. Then to the thalamus .
  6. And finally, to the primary gustatory cortex in the parietal lobe.
  • Also projects to other areas for integrating visual and olfactory information, and to the limbic system for emotional reactions to taste.