Human Eye and Ear Anatomy and Physiology

Posted on Mar 3, 2025 in Biology

Eye Structures and Functions

  • Ciliary Zonule: Secures the lens to the ciliary body.
  • Aqueous Humor: Fluid filling the anterior segment of the eye, providing nutrients to the lens.
  • Sclera: White portion of the fibrous layer.
  • Optic Disc: Area lacking photoreceptors (blind spot).
  • Ciliary Body: Muscular structure that manipulates the lens.
  • Choroid: Nutritive vascular middle layer of the eye.
  • Canal of Schlemm (Scleral Venous Sinus): Drains aqueous humor from the eye.
  • Retina: Inner layer responsible for image formation.
  • Vitreous Humor: Gel-like substance filling the posterior segment of the eye.
  • Ciliary Body and Iris: Composed of smooth muscle structures.
  • Fovea Centralis: Area of acute or discriminatory vision.
  • Refractory Media of the Eye: Cornea, lens, vitreous humor, and aqueous humor.
  • Cornea: Anterior, clear part of the fibrous layer.

Vision Terminology

  • Accommodation: Ability to focus for close vision.
  • Emmetropia: Normal vision.
  • Hyperopia: Inability to focus well on close objects (farsightedness).
  • Cataract: Clouding of the lens.
  • Myopia: Nearsightedness.
  • Astigmatism: Blurred vision due to unequal curvatures of the lens or cornea.
  • Convergence: Medial movement of the eyes to focus on close objects.
  • Pupillary Reflex: Reflex constriction of pupils when viewing close objects or in bright light.

Ear Structures and Functions

External Ear

  • Pinna (Auricle): The visible part of the ear.
  • External Acoustic Meatus: Canal leading to the eardrum.
  • Tympanic Membrane (Eardrum): Vibrates in response to sound waves.

Bony (Osseous) Labyrinth

  • Cochlea: Contains the organ of Corti, responsible for hearing.
  • Semicircular Canals: Involved in balance and equilibrium.
  • Vestibule: Involved in balance and equilibrium.

Middle Ear and Inner Ear

  • Structures *Not* Involved in Hearing: Vestibule and semicircular canals.
  • Pharyngotympanic Tube: Allows middle ear pressure to equalize with atmospheric pressure.
  • Tympanic Membrane: Transmits sound vibrations to the ossicles.
  • Oval Window: Transfers vibrations from the stapes to the fluid in the inner ear.
  • Endolymph: Fluid inside the membranous labyrinth.
  • Perilymph: Fluid inside the osseous labyrinth.
  • Ossicles: Malleus, Incus, Stapes.

Sound Transmission Pathway

Eardrum → Malleus → Incus → Stapes → Oval Window → Perilymph → Endolymph → Hair Cells.

Equilibrium

  • Dynamic Equilibrium: Crista ampullaris of the semicircular canals; detects changes in angular/rotary motion. When motion begins, endolymph fluid lags, bending the cupula and stimulating receptors.
  • Static Equilibrium: Maculae of the saccule and utricle; respond to gravity.

Proprioception and vision are also important for equilibrium.

Physiology of Vision and Hearing

All receptor potentials are *not* strong enough to elicit an action potential.

Eye Layers (Outermost to Innermost)

  1. Cornea & Sclera
  2. Choroid
  3. Retina

A 44-year-old man with sudden pain and vision loss may have decreased reabsorption of aqueous humor.

If the eyeball is too shallow, the image focuses behind the retina.

Retinal Cells

  • Photoreceptor Layer: Contains specialized cells to capture photons.
  • Information Relay: Photoreceptor Cell → Bipolar Cell → Ganglion Cell → Brain.
  • Rods: Specialized for scotopic (dim light) vision.
  • Outer Segment: Portion of photoreceptor cells that captures photons.
  • Photopigments: Opsin + 11-cis-retinal.

Cones are responsible for color vision; rods are responsible for black and white vision. *True*

Visual Transduction

  1. Capture of a photon by rhodopsin and isomerization of retinal.
  2. In the light, the level of cGMP *decreases*.
  3. The physiological response of a photoreceptor cell to light is *hyperpolarization*.
  4. Light → Hyperpolarization → Decreased glutamate release.

Visual Transduction Steps

  1. Capture of a photon, causing 11-cis-retinal isomerization.
  2. Activation of G protein (transducin).
  3. Activation of phosphodiesterase.
  4. Decrease in cGMP.
  5. Closure of nucleotide-gated channels.
  6. Photoreceptor hyperpolarization.
  7. Decreased release of glutamate onto bipolar cells.

Visual transduction is turned off by arrestin binding to rhodopsin.

Ganglion cells can fire action potentials.

Cranial Nerve II: Optic Nerve.

Ganglion cell axons from the right and left eyes *do* crossover. *False*

Sound

  • Amplitude: The total height of a sound wave (peak to valley). *True*
  • Pitch: Related to the frequency of sound waves.
  • Organ of Corti: Location of hair cells responsible for auditory transduction in the inner ear.

Ion channels on stereocilia open, and K+ (not Na+) flows into the cells, causing depolarization. *False*

Vestibular system hair cells have a similar structure and function to those found in the auditory system. *True*