Vision (Lecture) 

  • The eye has 6 muscles:
    • 4 rectus: superior, inferior, medial, lateral
    • 2 oblique: superior and inferior
  • The optic nerve is millions of axons merged together
  • Fovea Centralis: The sharpest vision due to high density of cones
  • Optic disk is the blind spot, where the optic nerve exits
  • Refraction is the bending of light rays as they pass through the eye
  • The anterior chamber is BEFORE the lens
  • The optic chiasm is a SPACE not a structure, where fibers from each eye go to the left and right side of the brain, basically where fibers cross
    • In the blue/green picture, it is where the optic tract from the nasal side of the eye cross to go to the other hemisphere, which the “safety mechanism”
    • After the optic chiasm are the optic tracts that lead to the thalamus (neurons of the thalamus form the projection fibers/optic radiation)
  • The retina has two layers:
    • Pigmented
    • Associated with the choroid, separates sensory cells
    • Reduces light scattering, absorbs extra light rays
    • Neural
    • Has 3 types of cells/neurons
      • Photoreceptors
      • Photoreceptors → Bipolar Cells → Ganglionic
      • Rods (dark and light; black and white)
      • Type of bipolar photoreceptor, not found in fovea centralis, but found almost all over the retina
      • More sensitive to light
      • Contains rhodopsin (found in disks)
        • Rhodopsin can be broken into opsin and retinal (contains vitamin A) when light hits
        • They combine in the absence of light
      • Rods are DEPOLARIZED at rest, and when light hits, they become HYPERPOLARIZED
      • Na+ is ALWAYS flowing into the outer segment of the rod
      • Cones (color)
      • Have iodopsin (red, blue, green pigments) instead of rhodopsin
      • Mostly in the fovea centralis, where the highest visual acuity is (due to cones)
      • When light density goes down, there is less sight of colors
    • EXTRA INFO ON RODS AND CONES
      • BOTH RODS AND CONES HAVE OUTER AND INNER SEGMENTS AND DISKS
      • THE INNER SEGMENTS HAVE ORGANELLES, THIS PART PUMPS OUT NA+
      • THE OUTER SEGMENT TAKES IN NA+
      • BOTH ALSO HAVE THE DARK CURRENT CYCLE
      • CONES HAVE IODOPSIN; RODS HAVE RHODOPSIN
      • Disks contain rhodopsin, and when rhodopsin runs out, the top disk gets phagocytized, and the disks get replenished
  • Light and dark adaptation: the ability of the eyes to adjust to light and dark situations by the movement of the pupil, the availability of rhodopsin, and the changes in photoreceptor function
  • When rods/cones are depolarized, it causes bipolar and ganglionic cells to be depolarized too
    • The neurotransmitter glutamate (released by photoreceptors) get picked up by bipolar cells
    • Glutamate decreases with light absorption
    • Bipolar cells release a neurotransmitter that gets picked up by ganglionic cells
    • Then the ganglionic cell produces an action potential that goes to the optic disk
  • Rhodopsin (opsin big, retinol small) is embedded into the phospholipid bilayer of the disk, and G protein (transducin, and alpha, beta, and gamma) is beside the rhodopsin
  • The sodium gated channels are open during rest (allowing the dark current) because of cGMP
  • There are two halves to each eye, the nasal and temporal sided
    • This is basically a safety mechanism because the two halves go to different sides of the brain, so if you have a stroke, you won’t completely lose vision
  • ]]RHODOPSIN CYCLE (when light hits, it created a cascade effect):]]
  1. (In the outer segment disk) Retinol changes shape (bent [11-cis-shape] → linear [all-trans-shape])

   

  1. Opsin changes shape because of that
  2. That activates transducin (G protein, 3 subunits)
  3. Which activates the enzyme cyclic GMP phosphodiesterase (THE ENZYME)
  4. Catalyzes cGMP to GMP (NEITHER ARE THE SAME AS ENZYME)
  5. The sodium channels close and the dark current stops
  6. Rod cells are now hyperpolarized
    1. Retinol and opsin separate

   

  1. The G protein reunites (3 subunits)
  2. The enzyme deactivates, the channels are closed
    1. In the absence of light, the retinol goes back to a bent shape

   

  1. Then it binds to opsin
  2. cGMP returns to the base of the sodium gated channel, opening it, causing it to depolarize again
  3. The dark current returns