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  • Overview
  • Vision
  • Hearing
  • Vestibular
  • Taste and smell
  • Worked micro‑examples
  • Pitfalls
  • Practice prompts
  • References

Anatomy and Physiology - Sense Organs

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Type: Study
Divisions: B, C
Participants: Up to 2
Approx. Time: 50 minutes
Allowed Resources: Binders/notes allowed per rules; non‑programmable calculator as permitted.

Overview

Special senses transduce physical/chemical stimuli to neural signals: light (vision), sound/head motion (hearing/balance), and tastants/odorants (gustation/olfaction).

Vision

  • Optics: cornea (major refraction), lens (accommodation), pupil/iris (light control), retina (photoreceptors → bipolar → ganglion cells). Fovea for high acuity.
  • Phototransduction: in darkness, cGMP‑gated Na⁺/Ca²⁺ channels open; light → rhodopsin activation → transducin → PDE → ↓cGMP → channel closure → hyperpolarization; graded release onto bipolar cells.
  • Adaptation: rods for scotopic, cones for photopic color vision; bleaching and regeneration dynamics.
  • Pathways: optic nerve → chiasm (nasal fibers cross) → tract → LGN → visual cortex; homonymous defects map to lesions.

Hearing

  • Outer/middle/inner ear: tympanic membrane, ossicles (impedance matching), oval window, cochlea.
  • Transduction: basilar membrane tonotopy; hair cell stereocilia deflection opens mechanically gated K⁺ channels (endolymph high K⁺) → depolarization → neurotransmitter release onto auditory nerve.
  • Sound encoding: place coding by frequency; intensity by firing rate/population; temporal coding at low frequencies.

Vestibular

  • Semicircular canals (angular acceleration); otolith organs (utricle/saccule for linear acceleration/tilt). Cupula/inertia principles.

Taste and smell

  • Taste: receptors for sweet, bitter, umami (GPCR), salty/sour (ion channels); cranial nerves VII, IX, X to nucleus tractus solitarius.
  • Olfaction: odorant → GPCR on olfactory neurons → cAMP → cation influx; strong adaptation; direct projection to olfactory bulb/cortex (limbic links).

Worked micro‑examples

  1. Visual field defect
  • Left homonymous hemianopia suggests right optic tract/optic radiation/occipital lesion; macular sparing implies PCA with MCA collateral.
  1. Audiogram reasoning
  • High‑frequency hearing loss begins at base (stiff) of cochlea; noise‑induced notch at 4 kHz common.
  1. Vestibular test
  • Head impulse abnormal to left suggests left horizontal canal hypofunction; nystagmus direction helps lateralize.

Pitfalls

  • Confusing phototransduction sign (light hyperpolarizes rods/cones).
  • Misattributing which taste modalities use GPCRs vs channels.
  • Ignoring decussation at optic chiasm when localizing field cuts.

Practice prompts

  • Trace visual pathways and predict field defects for specific lesions.
  • Explain cochlear tonotopy and effects of selective hair cell damage.
  • Compare semicircular canal vs otolith responses to motion.

References

  • SciOly Wiki – Anatomy & Physiology (Special senses)
  • OpenStax Anatomy & Physiology (Special senses)