Comprehensive Notes on Eye Anatomy and Vision

The Eye and Vision

Lacrimal Apparatus

  • Consists of a gland and duct leading to the nasal cavity.
  • The gland secretes tears.

Extrinsic Eye Muscles

  • Rectus Muscles (4):
    • Superior, Inferior, Lateral, Medial.
    • Responsible for eye movement and lateral turning.
  • Oblique Muscles (2):
    • Superior, Inferior.
    • Responsible for vertical eye movement and medial turning.

Close and Distant Vision

  • Distant Vision:
    • Flat lens.
    • Sympathetic nervous system control.
  • Close Vision:
    • Round lens.
    • Parasympathetic nervous system control.

Lens and Refraction

  • The cornea is primarily responsible for refraction.
  • Refraction adjusts the focal point to meet on the retina.
  • Glasses correct refraction.

Structure of the Eye Wall

  • Comprised of three layers:
    • Fibrous
    • Vascular
    • Sensory
Fibrous Layer
  • Mostly connective tissue.
  • Contains the sclera and cornea.
    • Sclera:
      • The white part of the eye.
      • Shapes the eye.
    • Cornea:
      • Transparent.
      • Bends light.
      • Contains pain receptors.
Chambers of the Eye
  • Vitreous Chamber (Vitreous Humor):
    • Located between the lens and retina.
    • Transmits light.
    • Holds the retina against the pigmented layer.
  • Anterior Segment (Aqueous Humor):
    • Produces aqueous humor.
    • Supplies nutrients to the cornea and lens.
      • Anterior Chamber: Between the cornea and iris.
      • Posterior Chamber: Between the iris and lens.
Glaucoma
  • Damage to the retina and optic nerve due to blockage in the drainage of aqueous humor.
  • Causes tunnel vision.
    • Progressive Open-Angle Glaucoma: Slow exit of aqueous humor.
    • Closed-Angle Glaucoma: Iris blocks exit.
Vascular Layer (Uvea)
  • Divided into:
    • Choroid Region
    • Ciliary Body
    • Iris
Choroid Region
  • Supplies blood to all layers.
Ciliary Body
  • Ring of tissue surrounding the lens.
  • Controls lens shape (focus).
  • Holds lens in position.
Iris
  • The colored part of the eye.
  • Contains the pupil, which regulates the amount of light entering the eye.
  • Includes muscles that constrict and dilate the pupil.
    • Radial Muscles (Dilator Pupillae):
      • Dilation.
      • Sympathetic control.
    • Circular Muscles (Sphincter Pupillae):
      • Constriction.
      • Parasympathetic control.
Sensory Layer (Retina)
  • Divided into:
    • Pigmented Layer
    • Neural Layer
Pigmented Layer
  • Absorbs light.
Neural Layer
  • Transmits and processes signals through photoreceptors.
    • Photoreceptors: Convert light to electricity.
      • Rods and Cones.
    • Bipolar Cells: Transmit signals from photoreceptors to ganglion cells.
    • Ganglion Cells: Transmit signals to the brain via the optic nerve.
    • Horizontal Cells: Interneurons that connect multiple photoreceptors and allow for light adjustment.
    • Amacrine Cells: Interneurons that transmit signals to ganglion cells.
Neural Blood Supply to Retina
  • Choroid: Supplies the outer layer (photoreceptors).
  • Central Artery and Vein: Supply the inner layer.
Photoreceptors
  • Outer Segment: Contains visual pigments (photopigments).
  • Inner Segment: Synapse with horizontal / bipolar cells.
Rods
  • Peripheral, away from the macula lutea (area of focus).
  • Night vision.
  • Peripheral vision.
  • Converging pathways (fuzzy / indistinct images).
Cones
  • Found in the macula lutea.
  • Bright light vision.
  • Three pigments: Red (10%), Green (45%), Blue (45%).
  • Nonconverging pathways (detailed images).
Chemistry of Visual Pigments
  • Retinal is the light-absorbing molecule that combines with opsin.
  • Has two isomers: 11-cis-retinal and all-trans-retinal.
  • Conversion from 11-cis-retinal to all-trans-retinal causes an electric impulse.
Excitation of Rods
  • Rhodopsin = 11-cis-retinal + Opsin
  • When light is absorbed:
    Rhodopsin→all-trans retinol+opsinRhodopsin \rightarrow all\text{-}trans\ retinol + opsin
  • When dark:
    all-trans retinol+opsin→Rhodopsinall\text{-}trans\ retinol + opsin \rightarrow Rhodopsin
Visual Pathway
  • Most ganglion cells (nerves) go to the lateral geniculate body of the thalamus.
  • Ganglion cells with Melanopsin go to:
    • Pretectal Nuclei
    • Suprachiasmatic Nucleus of the hypothalamus.
  • Stereoscopic Vision – Both eyes view the same object.
Lateral Inhibition
  • When cells inhibit signals from neighboring cells.
  • Mainly done by Horizontal + Amacrine cells.
  • Allows for detection of edges and sharp images.
Fovea Centralis
  • In the macula lutea, it has the greatest acuity (visual clearness).