Principles of Anatomy and Physiology: The Special Senses

Olfaction: Sense of Smell

• Supporting Cells:

  • Columnar epithelium in the mucous membrane of the nose.
  • Provide physical support, nourishment, and electrical insulation to olfactory receptor cells.

• Basal Stem Cells:

  • Undergo mitosis to replace olfactory receptor cells.

• Olfactory Glands (Bowman's Glands):

  • Produce mucus to dissolve odor molecules, facilitating transduction.

Olfactory Transduction

• Pathway:

  • Odorant molecules are dissolved in mucus, bind to G-protein-coupled receptors (GPCRs), leading to activation of adenylyl cyclase and a cascade that results in depolarization of the olfactory sensory neuron.

• Electrical Signal Generation:

  • The binding of odorants opens cation channels, allowing sodium (Na^+) and calcium (Ca^{2+}) influx, generating a receptor potential that triggers nerve impulses.

Odor Transduction Pathway

1. Odorant molecules dissolve in mucus.
2. Bind to olfactory receptors (GPCR).
3. Activate CAMP cascade leading to depolarization of olfactory neuron.
4. Signal sent via olfactory nerve (CN I) to olfactory bulb, then to the temporal lobe of the brain.

Anosmia in COVID-19

• Approximately 40% prevalence during the acute phase of infection.
• Impacts olfactory sensory neurons leading to acute loss of olfactory function.
• Persistent olfactory function loss noted in some patients after recovery due to viral infections.

Gustation: Sense of Taste

• Taste Buds:
  • Located on the tongue, soft palate, pharynx, and epiglottis.
  • Roughly 10,000 taste buds contain three types of epithelial cells:
  • Supporting Cells
  • Gustatory Receptor Cells
  • Basal Stem Cells

Structure of Taste Buds

• Types of Lingual Papillae:
  • Vallate papillae, Fungiform papillae, and Filiform papillae.
  • Filiform Papillae:
  • Cover the tongue surface, contain tactile receptors but no taste buds.
  • Aid in food manipulation.

Vision

• Electromagnetic Spectrum:
  • Visible light: Wavelengths from approximately 400 to 700 nm.
• Refraction:
  • Light bent at the junction of two transparent substances with different densities.

Eye Structures and Functions

• Iris:
  • Regulates the amount of light entering the eyeball;
  • Pupil constricts in bright light (sphincter pupillae muscles) and dilates in dim light (dilator pupillae muscles).
• Lens:
  • Refracts light and alters shape for focusing (accommodation).
• Retina:
  • Converts light into receptor potentials and nerve impulses to send signals to the brain via the optic nerve (CN II).

Visual Processing

• Light focused on the retina is inverted and reversed; the brain corrects the image.
• Images are focused on the fovea centralis for sharpest vision.

Aging and the Special Senses

• Smell and Taste: Affected by aging around age 50; receptors decline and regeneration slows.
• Vision: Lens begins to lose elasticity around age 40 (presbyopia); iris muscles weaken, impacting light adaptation.
• Hearing: By age 60, ~25% experience presbycusis (hearing loss); tinnitus and vestibular imbalance are common.

Disorders in the Special Senses

• Cataracts: Loss of transparency in the lens.
• Glaucoma: Leading cause of blindness due to high intraocular pressure.
• Deafness: Significant or total hearing loss.