TSH.1 The Eye - Structure and Function, Trichromatic Theory of Colour Vision, Colour Blindness

Evolution of the Eye

• Light-sensitive cells date back to the Cambrian period (541 million years ago)

• Image formation necessitates a lens, which led to the evolution of eyes

• Eye structures have evolved independently at least three times in different organisms (arthropods, mollusks, vertebrates)

• Example of convergent evolution

Overview of the Eye

• Function: Converts light energy into electrical signals using photoreceptors

• Eye placement allows binocular vision resulting in:

  • Wider field of view

  • Enhanced distance and depth perception

Eye Movement

• Muscular Control:

  • Horizontal movements: Medial and lateral rectus muscles

  • Vertical movements: Superior/inferior rectus and oblique muscles

• Eye movements can be:

  • Conscious (deliberate focus)

  • Unconscious (maintaining focus while head moves - vestibule ocular reflex)

• Eyes generally work together to focus on the same area

Anatomy of the Human Eye

• Three Major Tunics:

  • Surround a hollow sphere filled with fluids (humors)

• Fibrous Layer:

  • Outermost layer composed of dense avascular connective tissue

  • Components:

    • Sclera: Strong protective layer, muscle attachment point

    • Cornea: Transparent, contributes 2/3 of focusing power, can be reshaped or transplanted

Uvea (Vascular Tunic)

• Components:

  • Choroid: Blood vessels supplying nutrients to the retina

  • Iris: Control light entry through pupil; contains muscles for dilation and constriction

  • Ciliary Body: Controls lens shape and supports the lens

Nervous/Sensory Tunic (Retina)

• Layers:

  • Outer pigmented epithelial layer (contains melanin)

  • Inner light-sensitive neural layer with photoreceptors (rods and cones)

• Key cells in the retina:

  • Photoreceptor Cells: Rods (120 million) and cones (6 million)

  • Neurons: Bipolar neurons, Retinal Ganglion cells

Macula and Fovea

• Macula: High concentration of cone photoreceptors for color vision

• Fovea: Area of highest acuity; critical for detailed vision

• Optic Disc: Blood vessels enter/exit; creates a blind spot

Photoreceptor Function

• Rods are high in sensitivity but low in discrimination (scotopic vision)

• Cones provide detailed color vision in well-lit conditions (photopic vision)

• Visual Cycle: Light converts 11-cis-retinal to 11-trans-retinal, triggering signal transmission and response to light

Trichromatic Theory of Color Vision

• Proposed by Thomas Young and expanded by von Helmholtz

• Three types of cones:

  • Blue sensitive (max 430 nm)

  • Green sensitive (max 530 nm)

  • Red sensitive (max 562 nm)

• Color perception results from the relative responses of these three types of cones

Color Blindness

• Genetics: Genes for green and red pigments linked to X chromosome

• 1:12 males and 1:200 females affected

• Commonly involves difficulty distinguishing red and green

• Ishihara color cards used for testing

Conclusion

• Contact Information:

  • Prof. Warren Thomas

  • Email: wathomas@rcsi-mub.com

• References provided from Chirras 9th Ed and Sherwood 9th Ed.