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The Organization of the Retina and the Visual System - Part 1; Foundations (by Helga Kolb)

I. Introduction

• Retina - piece of brain tissue that gets direct stimulation from the outside world's light and images
• Visual purple
  • rhodopsin
  • biological pigment in photoreceptor cells
  • G-protein-coupled-receptor (GPCR) family
  • extremely light sensitive; enable vision in low-light conditions

II. Gross Anatomy of the Eye

• Pupil - allows light to enter the eye
• Iris - colored circular muscle that controls the size of the pupil
  • Eye color is due to variable amounts of eumelanin (black/brown) and pheomelanin (red/yellow)
  • Melanocortin-1 receptor gene regulates eumelanin production
    • Point mutations affect melanogenesis (common in light skinned and blue/green eyed people)
• Cornea - transparent external surface that covers the pupil and the iris
  • With the crystalline lens, produces sharp image at the retinal photoreceptor level
• Sclera - white of the eye
• Eye is a slightly asymmetrical sphere
• Cross sectional view shows three layers:
  • External layer formed by the sclera and cornea
  • Intermediate layer
    • Interior: iris an ciliary body (controls the shape of the lens)
    • Posterior: choroid (vascular layer of the eye containing connective tissue, lying between the retina and the sclera)
  • Internal/sensory layer: Retina
• Three chambers of fluid
  • Anterior (between cornea and iris)
  • Posterior (between iris, zonule fibers (ring of fibrous strands connecting the ciliary body with the crystalline lens), and the lens)
  • Vitreous chamber (between the lens and the retina)
  • First two chambers are filled with aqueous humor, vitreous chamber is filled with vitreous humor, a more viscous fluid.
• Accommodation - changing the shape of the lens that allows us to form a sharp image on the retina
• Fovea - visual axis/central point for image focus in human retina
• Optic axis - more nasally than the visual, projects closer to the optic nerve head.
  • Longest sagittal distance between the vertex of the cornea and the furthest posterior part of the eyeball
• Some vertebrates have a specialization of the central retina known as an area centralis or a visual streak (instead of a fovea)
• Extraocular Muscles
  • Three pairs of muscles inserted into the sclera
    • 2 pairs: rectus muscles
      • run straight to the bony orbit of the skull orthogonal to each other (the superior rectus and inferior rectus, the lateral rectus and medial rectus.)
    • Third pair: oblique (inferior and superior)
  • Rotate the eyeball in the orbits and allow the image to be focused on the fovea at all times
• Development of the eye
  • Derives from the neural tube (embryo's precursor to the CNS)
  • Retina forms during the development of the embryo from optic vesicles that form pouches from two sides of the neural tube
  • Optic vesicles fold back on themselves to form the optic cup
    • Inside of the cup becomes the retina
    • Outside of the cup remains a monolayer of epithelium known as the retinal pigment epithelium
  • Sensory retinal development begins in the optic vesicle stage
  • "Additional development is characterized by the formation of further layers arising from cell division and subsequent cell migration."
  • Develops in an inside to outside manner (ganglion cells first, photoreceptors last)
    • "Ganglion cells collect visual information in their dendrites from bipolar cells."
  • 5 months of gestation: most basic neural connections of the retina have been established
  • Fovea is the last part to mature

III. Simple Anatomy of the Retina

• Optic nerve in center of retina
  • Circular/oval area measuring about 2 x 1.5 mm across
  • Center of optic nerve radiates major retinal blood vessels
• About 4.5 - 5 mm away is the fovea (macula)
• Retina is about 0.5 mm thick and lines the back of the eye
• Ganglion cells (output neurons) lie innermost in the retina closest to the lens and the front of the eye
• Photosensors (rods and cones) lie outermost against the pigment epithelium and choroid.
• This means that light must travel through the thickness of the retina before striking and activating the rods and cones
  • Message is translated into biochemical message and then electrical message that can stimulate the succeeding neurons of the retina
• Vertical section of the retina reveals many layers of neurons between the rods/cones and the ganglion cells
• Vertebrate retinas are composed of three layers of nerve cell bodies and two layers of synapses
  • Outer layer: cell bodies of photoreceptors
  • Inner nuclear layer: cell bodies of the bipolar, horizontal, and amacrine cells
  • Ganglion layer: cell bodies of ganglion cells and displaced amacrine cells
  • *Amacrine cells: inhibitory neurons interacting at the second synaptic level of the vertically direct pathways of the photoreceptor-bipolar-ganglion cell chain
• Dividing the nerve cell layers are two neurophils where synaptic contacts occur
  • *Neuropil: a broad term defined as any area in the nervous system composed of mostly unmyelinated axons, dendrites, and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies
  • First area of neuropil: the outer plexiform layer (OPL) where connection between rods and cones, and vertically running bipolar cells and horizontally oriented horizontal cells occur
  • Second neuropil: inner plexiform layer (IPL) functions as a relay station for the vertical-information-carrying nerve cells, the bipolar cells, to connect to ganglion cells
  • Different varieties of horizontally- and vertically- directed amacrine cells somehow interact in further networks to influence and integrate ganglion cell signals
• Culmination of all this neural processing causes the image to be transmitted to the brain along the optic nerve
• Central and Peripheral Retina Compared:
  • Central is cone dominated; peripheral is rod dominated
  • ONL (outer nuclear layer) is about the same thickness in both.
  • INL (inner nuclear layer) is thicker in central than peripheral
  • difference between central and peripheral is in the relative thicknesses of IPL, GCL (ganglion cell layers), and NFL (nerve fibre layer).
    • Due to the greater numbers and increased packing-density of ganglion cells needed for the cone pathways
    • Greater number of ganglion cells means more synaptic interaction in a thicker IPL
• Muller Glial Cells
  • Radial glial cells of the retina
  • Outer limiting membrane (OLM) is formed from adherens junctions between Muller cells and photoreceptor cell inner segments.
    • NO IDEA WHAT THAT SENTENCE MEANS. LOOK UP LATER.
  • OLM forms a barrier between the subretinal space and the neural retina proper.
  • inner limiting membrane (ILM) composed of laterally contracting Muller cells; borders the vitreous humor and forms a diffusion barrier between neural retina and vitreous humor
  • Major blood vessels of the retinal vasculature supply the capillaries that run into the neural tissue
  • Nutrients from vasculature supply photoreceptor layer
• Foveal Structure
  • Photoreceptors concentrated to maximum density (not rods)
    • Most efficient packing method: hexagonal mosaic
  • Fundus: Base; interior surface opposite the lens, including the retina, optic disc, macula, fovea, posterior pole
  • OCT: Optical coherence tomography
  • Foveal rim is the most thick portion of the entire retina b/c of the ganglion cells packed into six layers
• Macula Lutea
• Ganglion Cell Fiber Layer
• Blood Supply to the Retina
• Degenerative Diseases of the Human Retina

IV. How the Retina Works