Lab Nine: Sensory Systems - Gustation and Olfaction

Lab Nine Checklist Overview

  • Use the lab manual in conjunction with Chapter 16 to complete the lab practical.
  • The lab practical consists of multiple choice questions and labeling tasks focused on:
    • Taste buds
    • Olfactory system

Transduction Process

  • Definition: Transduction refers to the process by which a sensory receptor converts a stimulus into an action potential that the brain can interpret.
    • Involves translating a physical stimulus into an electrical signal that the nervous system can understand.

Resources for Lab Nine

  • Access the Lab Nine Checklist, a PDF document approximately three to four pages long.
  • Utilize the lab folder under "Lab Nine through 11 - Special Senses" for supplementary resources.
  • Important pages: Page 575 in the textbook as a reference for definitions related to transduction in olfaction (smell) and gustation (taste).

Sensory Systems Overview

  • Special Senses vs General Senses:
    • Special senses include vision (sight), olfaction (smell), gustation (taste), hearing (audition), and balance (vestibular).
    • General senses include tactile (touch), temperature, pain, proprioception (position in space).
  • Receptor Classification:
    • Receptors may be classified by location or type of stimulus (mechanical, chemical, thermal, etc.).
    • General senses typically involve unipolar or pseudo-unipolar neurons.
    • Special senses predominantly utilize bipolar neurons.

Types of Sensation and Receptors

  • General Sensation Examples:
    • Tactile
    • Temperature
    • Pain
    • Proprioception
  • Special Senses:
    • Vision (retina)
    • Olfaction
    • Gustation
    • Hearing

Sensory Pathways

  1. General sensations:

    • Specifically handled by pathways in the spinal cord.
    • Sensory messages (e.g., touch, pain) can travel through designated tracks (e.g., spinothalamic tract, gracilis, cunatus).
    • **Example Tracks:
      • Gracilis Track: Sensation from lower limbs, below T6.
      • Cunatus Track: Sensation from upper body, T6 and above.

  2. Spinal Cord Integration:

    • Sensory pathways ascend toward the brain along spinal cord tracts.
    • Typically, pain, temperature, and vibration messages are routed through the thalamus to the cerebral cortex, particularly the parietal lobe.
    • Postcentral gyrus: Primary somatosensory cortex location, integrating sensory input.

Proprioception

  • Definition: Awareness of body position and movement achieved through specialized receptors in muscles, tendons, and joints.
  • Special receptors provide feedback about the position of limbs and any movements made, which aids in coordination and balance.

Classification by Location

  • Exteroceptors: Receptors sensing external stimuli (e.g., touch).
  • Interoceptors: Receptors for internal stimuli in viscera (e.g., hunger).
  • Proprioceptors: Receptors responding to internal stimuli concerning the body's position (e.g., limb position).

Structural Classification of Receptors

Types of Receptors by Function:

  • Mechanical Receptors:
    • Respond to physical stimuli (e.g., touch, vibration).
  • Thermoreceptors:
    • Respond to temperature changes.
  • Photoreceptors:
    • Exclusive to the retina for detecting light.
  • Chemoreceptors:
    • Detect chemical stimuli; involved in taste and smell.
  • Nociceptors:
    • Respond to painful stimuli (extreme temperatures, mechanical damage).

Gustation (Taste)

Taste Bud Overview:

  • Taste Bud: The organ responsible for taste, made up of gustatory epithelial cells.
    • Surrounded by stratified squamous epithelial tissue.
    • Contains microvilli (gustatory hairs) that bind chemicals for taste.

Five Primary Tastes:

  • Salty: Sodium ion concentration.
  • Sour: Acidity (low pH, high hydrogen ion concentration).
  • Bitter: Alkaline substances (high pH, low hydrogen ion concentration).
  • Sweet: Sugars and organic compounds.
  • Umami: Amino acids (e.g., glutamate) – found in aged cheese and meat.

Taste Transduction Process:

  1. Chemical taste stimuli enter the taste bud and mix with saliva.
  2. Saliva breaks down food and allows it to bind with gustatory hairs (microvilli).
  3. Activation of gustatory hairs causes local depolarization and the release of neurotransmitters.
  4. Action potentials travel along the cranial nerves to the insular cortex for taste integration.

Cranial Nerves Involved in Gustation:

  • **Cranial Nerves Responsible for Taste:
  1. Facial Nerve (CN VII) - Anterior two-thirds of the tongue.
  2. Glossopharyngeal Nerve (CN IX) - Posterior one-third of the tongue.
  3. Vagus Nerve (CN X) - Posterior pharynx and epiglottis.**

Olfaction (Smell)

  • Olfactory System Overview:
    • Sensory input processed by Cranial Nerve I.
    • Location: Medial aspect of the temporal lobe (olfactory cortex).
  • Olfactory Conduction Pathway:
    • Odorants bind to olfactory receptors (modified chemoreceptors).
    • Involves a secondary messenger (cyclic AMP) for creating an action potential.

Main Structures Involved:

  • Olfactory Epithelium: Contains olfactory sensory neurons.
  • Cribriform plate of the ethmoid bone: Axons pass through this to reach the olfactory bulb.
  • Mitral Cells: Second-order olfactory neurons that transmit impulses to the brain.

Conclusion

  • Combining textbook resources with lab manuals is essential for effective preparation.
  • Understanding the anatomical structures and processes of gustation and olfaction, as well as how these sensory modalities are integrated within the nervous system, is crucial for success in lab practical assessments.
  • Students are encouraged to review the relevant chapters and familiarize themselves with brain anatomy, cranial nerves related to taste, and the pathways of sensory transduction to prepare for examinations effectively.