Vision

Eyeball consists of 3 layers: 

  • Outer fibrous layer 

    • Sclera: white of eyes 

      • Protection, muscle insertions 

      • Tough dense connective tissue, Type I collagen 

      • Relatively avascular 

    • Cornea: transparent, anterior 1/6th 

      • Avascular 

      • Six layers 

      • Aqueous humour drainage  

    • Corneoscleral junction: where cornea and sclera meet 

      • Drain fluid behind cornea to preserve the pressure  

      • No drainage -> fluid and pressure build up -> glaucoma  

  • Inner vascular layer 

    • Choroid: posterior 2/3rd of the eyeball 

      • Highly vascular  

      • Loose CT with lots of collagen and elastic fibers 

      • Abundant melanocytes in addition to standard CT cells 

    • Cilary body: thickened ring of tissue anterior to choroid 

      • Cilary muscle 

      • Suspensory ligaments 

      • Stratified columnar epithelium 

      • Form aqueous humour  

  • Vascular layer of the eyeball 

    • Iris 

      • Opening in the center forms the pupil 

      • Partial covers lens 

      • Radially oriented myoepithelial cells form dilator pupillae m. 

      • Circumferential smooth muscle fibers form sphincter pupillae m. 

      • Melanocytes in iris keep stray light rays from interfering with image 

      • Parasympathetic and sympathetic effects on the iris 

    • Lens 

      • Transparent biconvex structure posterior to iris 

      • Covered by thick capsule rich in type IV collagen 

      • Single layer of cuboidal epithelial cells 

      • Lens fibers form perfectly transparent tissue for light refraction 

      • Vascular layers (outermost to innermost) 

        • Lens capsule, lens epithelium, differentiating lens fibers, mature lens fibers 

  • Retina: the nervous layer 

    • Between the choroid and the rest of the retina you have the pigmented layer, an epithelial layer containing melanocytes 

    • Below the pigmented layer you have the neural layer (outer to innermost) 

      • Rods and cones 

      • Horizontal cell 

      • Bipolar cells 

      • Amacrine cells 

      • Ganglion cells 

      • Axons of the ganglion cells to optic nerve (CN II) 

    • Macula of the retina: spot where you focus your vision 

  • The Vitreous 

    • Gel-like 

    • 99% water bound to hyaluronate 

  • Neglect syndrome 

    • Perceptual disorder in which they see the entire visual field but don't perceive some of it 

    • Happens because of the visual pathways 

  • Visual Pathways 

    • The right and left visual fields are processed in the contralateral portion of the brain (basically the opposite side) 

    • Easier to visualize 

      •  

      • Both nerves in the right and left visual fields are present in both eyes, but at the optic chiasm, they join together to their respective opposite of the midbrain to be processed; disturbances in this pathway cause neglect syndrome 

  • Extraocular muscles 

    • 6 muscles 

      • 4 recti 

        • Superior: elevates and adducts 

        • Inferior: depresses and adducts 

        • Lateral: abducts eye 

          • CN VI 

        • Medial: adducts eye 

      • 2 oblique 

        • Superior: elevates and abducts 

          • CN IV 

        • Inferior: elevates and abducts 

      • All of the other muscles above are innervated by CN III 

  • Refraction of light in eyeball and autonomic innervation 

    - oculomotor nerve (CN 1) 
    - Trochlear nerve (CN 4) 
    - Abducens nerve (CN 6)