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Nervous Tunic

  • Definition and Function

    • The nervous tunic, also known as the retina, is a mostly transparent layer of tissue that forms the inside lining of the globe (the eyeball).
    • It is an extension of the brain and consists mostly of nerve cells.
    • Composed of 10 layers, of which nine are normally transparent.
    • Contains light receptors primarily consisting of rods and cones.
    • If damaged, the retina is unable to repair itself.

  • Photoreceptors

    • Definition: Specialized nerve endings that produce an electrical nervous impulse when stimulated by light.

    Rods

    • Structure: Photoreceptors with flat or rod-shaped outer segments.
    • Quantity: Approximately 120 million rods in each eye.
    • Contains the chemical rhodopsin, which reacts to light and triggers nerve impulses.
    • Sensitivity: More sensitive to light than cones.
    • Function: Responsible for black and white vision and night or low light (scotopic) vision.

    Cones

    • Structure: Photoreceptors with pointed or cone-shaped outer segments.
    • Quantity: Approximately 6 million cones in each eye.
    • Functionality: Each cone contains one of three separate chemicals sensitive to different wavelengths of light:
    • Blue
    • Green
    • Red
    • Sensitivity: Less sensitive to light than rods.
    • Function: Responsible for color vision and visual acuity; cones function best in bright (photopic) conditions.

Two Areas of the Retina

  • The retina can be divided into two distinct areas:

    1. Pars Ceca
    • Location: Anterior to the ora serrata, which lines the posterior border of the ciliary body.
    • Composition: Does not contain rods and cones.
    1. Pars Optica
    • Also known as: The sensory retina.
    • Location: Extends posteriorly from the ora serrata and constitutes the largest part of the retina.
    • Composition: Contains photoreceptors (rods and cones) and lines the inner surface of the choroid.

Macula

  • Definition: The macula, or macula lutea, is the central area of the retina.
  • Function: It is used for central vision.
  • Composition: Dense with photoreceptors, mostly cones, making it crucial for high-resolution vision.

Fovea

  • Definition: The fovea, or fovea centralis, is a small depression in the retina located at the very center of the macula.
  • Functionality: Acts like a hyperbolic mirror; reflects light when observed with an ophthalmoscope, creating a small, bright spot of light known as the foveal reflex.
  • Foveola: A small region within the fovea that contains only cones, responsible for fine discrimination and sharp visual acuity.

Optic Disc

  • Definition: The optic disc, or optic nerve head, is the area where about one million nerve fibers from the rods and cones exit the eye.
  • Location: Found in the nasal retinal area, within the posterior pole, medial to the macula.
  • Functionality: This area serves as the entrance of the retinal artery and the exit for the retinal vein.
  • Physiologic Blind Spot: The optic disc causes a blind spot in each eye due to the absence of rods and cones in this area.

Fundus

  • Definition: The inner, posterior surface of the eye, encompassing the retina, optic disc, and macula.
  • Diagnostic Tools:
    • Ocular Photos (Fundus Photography): Essential for the non-invasive in vivo analysis of retinal tissue.
    • OCT (Optical Coherence Tomography): Another important tool for diagnosing and managing retinal diseases.