5 Vision II

• Macula lutea


  Part of the retina responsible for sharp, detailed central vision (also called visual acuity)
• Fovea


  Small depression within the neurosensory retina where visual acuity is the highest

  
• Layers of the Retina


  • Pigmented Layer
  • Layer of rods and cones projecting to the pigment the rods
  • Outer nuclear layer containing the cell bodies of the rod and cones
  • Outer plexiform layer
  • Inner nuclear layer
  • Inner plexiform layer
  • Ganglionic layer
  • Layer of optic nerve fibers
  • Inner limiting membrane

  
• Müller cells


  • Play an important role in maintaining the internal geometry of the retina
  • Oriented radially, parallel to the light path through the retina
  • Outer ends form tight junctions with the inner segments of the photoreceptors, forming the outer limiting membrane

  
• Rods


  Narrow and long photoreceptors, not present in the fovea, contain rhodopsin which is light-sensitive, sensitive to light levels and help give good vision in low light (scotopic vision)

  
• Cones


  Conical in shape, shorter compared to rods but longer in the fovea, concentrated in the center of the retina, contain iodopsin or color pigments which are important in day vision (photopic vision), responsible for color vision, less light is needed to activate

  
• There are more rods than cones
• Structure of the photoreceptor


  • Outer segment
  • Inner segment
  • Cell body
  • Synaptic terminals

  
• Rhodopsin


  1. protein coupled receptor, the most abundant protein in the rod cells, found on the outer segment of the rod that projects into the pigment layer of the retina, combination of scotopsin and retinal pigment

  
• Retinol (Vitamin A)


  Important vitamin to make all-trans retinal, important in long-term adaptation of the retina to different light intensities, deficiency leads to night blindness or nyctalopia

  
• Excitation of Photoreceptors


  1. Depolarization: cGMP-gated cation channels are open, allowing steady influx of Na+ and Ca2+, leading to tonic release of glutamate which excites bipolar cells and inhibits ganglion cells

  2. Light absorption: Photoisomerization of rhodopsin activates transducin, which activates phosphodiesterase, lowering cGMP and closing cGMP-gated cation channels, leading to hyperpolarization and reduced neurotransmitter release

  
• Color Vision


  Wavelengths of visible light range from 397 to 723 nm, photochemicals in the cones have almost the same chemical composition as rhodopsin, but the protein portions (opsins) are different and called photopsins

  
• Theories of Color Vision


  • Trichromasy theory
  • Young Helmholtz Theory
  • Opponent process theory
  • Retinex Theory

  
• Protanope (red-color blindness)


  Due to loss of red cones, occurs exclusively in males, genes for cones are on the X chromosome

  
• Attributes of Color


  • Hue
  • Intensity
  • Saturation
• Types of Color Vision Deficiencies


  • Trichromats (Normal Vision)
  • Dichromats
  • Monochromats
  • Achromatopsia

  
• Tests used to screen for color blindness include Ishihara plates
• Colors


  • Orange
  • Blue
  • Yellow
  • Red
  • White
• Attributes of color


  • Hue = name of color
  • Intensity = quality of color (bright or dull)
  • Saturation = relative purity of color/luminance
• Color Blindness


  • x-linked or sex-linked recessive condition
  • 8% normal males
  • 2% normal females
  • Due to abnormality in one or three cones

  
• Suffixes for color vision disorders


  • Anomaly or anomalous (color weakness)
  • Anopia or anope (Color Blindness)

  
• Prefixes for color vision disorders


  • Prot = red (common in males)
  • Deuter = green
  • Tri = blue

  
• Trichromats


  All cone systems are functioning (normal vision)

  
• Dichromats


  Two cone systems are functioning, one is missing
• Monochromats


  Only one cone system is functioning, two are missing
• Achromatopsia


  Could only see black, white, and gray
• Tests used to screen color blindness


  • Ishihara Test
  • Holmgren wool Test
  • Spilling's Test (pseudoisochromatic plates)
  • Rayleigh match test (Anomaloscope)
  • Matching of spectral colors

  
• Ishihara Test


  • Color vision test for detection of red–green color deficiencies

  
• Holmgren wool Test


  • The numbers on the pieces of wool have codes to determine the type of colour blindness
• Spilling's Test


  • Colored plates that appear isochromatic to individuals with color-vision abnormality

  
• Rayleigh match test


  • Definitive clinical instrument for classifying phenotypic variations in X-linked color-vision disorders

  
• Matching of spectral colors

  • Attempts to give two colors the same spectral reflectance characteristic, making them a good metameric match
• Types of ganglion cells


  • W cells
  • X cells
  • Y cells
• W cells


  40% of all the ganglion cells, receive most of their excitation from rod, detect directional movement, important for crude rode vision

  
• X cells


  Most numerous, 55% of total ganglion cells, small fields, transmit fine details of visual image, responsible for all color vision

  
• Y cells


  Largest but least numerous (5%), respond to rapid changes in light intensities, apprise central nervous system almost instantaneously when a new visual event occurs

  
• Changes in light adaptation


  • Pupillary constriction
  • Utilization of central portion of retina
  • Bleaching of rhodopsin
  • Greater activation of cones

  
• Changes in dark adaptation


  • Pupillary dilation
  • Utilization of peripheral portion of retina
  • Synthesis of Rhodopsin
  • Greater activation of rods

  
• Visual field


  Area seen by an eye at a given instant, nasal field and temporal field

  
• Perimetry


  Systematic measurement of differential light sensitivity in the visual field by detection of presence of test target on a defined background