Structure of the Eyeball

Introduction

• Importance of understanding eye anatomy before discussing phototransduction and retinal pathways.
• Overview of the lecture topics.

Outline of Topics

1. Tunics of the Eye
2. Segments: Anterior and Posterior
3. Aqueous Humor Flow
4. Lacrimal Flow
5. Clinical Correlations

Tunics of the Eye

1. Fibrous Tunic

• Composed of:
  • Sclera:
    • Structure: Outer part of the eye, made of three layers:
      • Episclera: Outermost, made of dense fibrous connective tissue.
      • Sclera Proper: Middle layer, rich in collagen, tough and resilient.
      • Lamina Fusca: Innermost layer, pigmented.
    • Clinical correlation: Accumulation of bilirubin can cause yellowing (jaundice).
  • Cornea:
    • Structure: Made of five layers, allowing light to pass through.
      • Epithelial Layer: Thin, rich in pain receptors.
      • Bowman's Membrane: Connective tissue layer.
      • Stromal Layer: Thick connective tissue layer.
      • Descemet's Membrane: Another connective tissue layer.
      • Endothelial Layer: Inner layer, important for nutrient supply.
    • Function: Allows light to pass, important for vision.

2. Vascular Tunic (Uvea)

• Composed of three components:
  • Choroid:
    • Structure: Dark pigmented membrane with blood vessels.
    • Function: Absorbs light to prevent scattering, supplies blood to retina.
  • Iris:
    • Structure: Controls pupil size, contains two muscles:
      • Dilator Pupillae: Dilates pupil (sympathetic innervation).
      • Sphincter Pupillae: Constricts pupil (parasympathetic innervation).
    • Function: Regulates light entry into the eye.
  • Ciliary Body:
    • Structure: Contains ciliary muscles and processes.
    • Function: Alters lens shape for accommodation, produces aqueous humor.

3. Sensory Tunic

• Composed mainly of Retina:
  • Contains two layers:
    • Outer Pigmented Layer: Rich in melanin, absorbs light, acts as a barrier for choroid vessels.
    • Inner Neural Layer: Contains photoreceptors (rods and cones), bipolar cells, ganglion cells, and supporting cells for visual processing.

Aqueous Humor Flow

• Produced by ciliary processes.
• Flows through:
  • Posterior Chamber (behind the iris) to Anterior Chamber (in front of the iris).
  • Drains into Canal of Schlemm (Scleral Venous Sinus).

Lacrimal Flow

• Involves tear production and drainage:
  1. Lacrimal Glands secrete tears.
  2. Tears spread across the cornea.
  3. Drain through Lacrimal Puncta into Canaliculi.
  4. Collected in Lacrimal Sac.
  5. Drains into the nasal cavity via Nasal Lacrimal Duct.

Eyelids and Conjunctiva

• Palpebral (eyelids) and Bulbar Conjunctiva cover the eye, providing lubrication and protection.
• Infection can lead to Conjunctivitis (pink eye).

Extraocular Muscles

• Responsible for eye movement, contribute to intraocular pressure through contraction.

Clinical Correlations

• Cataracts: Accumulation of crystallin proteins in the lens, affecting vision. Causes include diabetes, smoking, and excessive Vitamin C.
• Retinal Detachment: Leakage of vitous humor between retinal layers can lead to blindness if untreated.

Optic Nerve

• Cranial Nerve II: Responsible for visual signal transmission.
• Optic Disc: Where the optic nerve exits the eye, observed during fundoscopy.
• Lamina Cribrosa: Holes in the sclera where optic nerve fibers pass.

Conclusion

• Summary of eye structure and functions.
• Next video will focus on retinal layers and phototransduction.