The protective mechanism in the Eye

 

The protective mechanism in the Eye





Mechanism of potential damages to the eye can ocular as:

  1. Mechanical
  2. Chemical
  3. Biological
  4. Electromagnetic radiation

1.Protection from mechanical damage

The orbit:

  • The eye and orbital tissues are supported and shielded by the fat and bone walls of the orbit.
  • The orbital fat acts as a semi-fluid padding that cushions the eye, providing shock absorption.

The eyelids:

  • The eyelids act as a mechanical barrier between the eye and the outside world, quickly closing in response to reflexive or deliberate blinking.
  • When triggered, the cilia (modified tiny hairs) on the eyelids’ skin provide a stimulus to blink in response to airborne particles.

The corneoscleral shell

  • The corneoscleral shell gives the globe tensile strength.
  • The cornea’s dense innervation enables quick blinking and withdrawal reflexes.
  • Furthermore, corneal innervation supplies trophic elements that encourage epithelial recovery.

2.Protection from chemical damage

Eyelid closure

  • Reflex blinking provides rapid closure of the eye in response to splash or foreign body sensation. 

Bell’s phenomenon

  • A normal Bell’s phenomenon provides involuntary upward and inward rotation of the globe on lid closure, removing the cornea from noxious stimuli . 

Tears

  • Tear flow increases in response to mechanical or noxious stimuli.
  • This causes dilution and washout of the irritant.

Corneal epithelial barrier

  • The corneal epithelium is 5-7 layers deep, with cells adjoined by desmosomes.
  • Tight connections (zonulae occludens) surround the most superficial corneal epithelial cells, creating a low conductance barrier to fluid and solutes.

3.Protection from biological damage

Tear film and conjunctiva:

  • Glycocalyx and mucous layer: A physical barrier against infections is provided by mucins in the glycocalyx (conjunctival cell membrane-bound mucin) and the mucous layer of the tear film, which can also trap bacteria.
  • Aqueous layer: Many antibacterial substances, including as secretory immunoglobulin A (IgA), lysozyme, and lactoferrin, are present in the aqueous layer.
  • Normal conjunctival flora: The normal bacterial flora may inhibit survival of more pathogenic species.
  • Natural killer cells: Present in the conjunctiva, natural killer cells may have a role in restricting the spread of viral infection or tumors.

Corneal epithelium and Bowman’s layer:

  • They serve as physical barriers to preventing microbiological infections.

Descemet’s membrane:

  • In cases of severe corneal infections, Descemet’s membrane resists proteolysis, preserving the integrity of the globe.

4.Protection from electromagnetic radiation

Eyelid closure:

  • The dazzle reflex causes reflexive blinking in response to bright light.

Pupil constriction:

  • Rapid pupil constriction in response to bright light limits excessive radiation exposure to the ocular media internal to the iris.

Light absorption by ocular tissues:

  • The cornea and sclera absorb ultraviolet (UV)-B, UV-C, infrared (IR)-B, and IR-C .
  • The crystalline lens absorbs UV-A.
  • Excessive UV-induced oxidative damage is prevented by antioxidants in the lens and macula.
  • Short-wavelength light is absorbed by the yellow macular carotenoid xanthophyll pigments in Henle’s fibre layer.
  • Melanin and hemoglobin, which are primarily present in the choroid, absorb too much IR radiation and light.

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