Vision and visual pathways

• What is the retina?


  Inner most layer of the eyeball. Has a laminar structure - 3 main layers of cells
• What is the fovea?


  Centre of retina, thinnest part, densest concentration of photoreceptors.
• Macula surrounds fovea.
• What is special about the macula?


  Hardly any blood vessels.
• Label the different parts of the retina.

  A) Optic disk

  B) Blood vessels

  C) Nasal retina

  D) Temporal retina

  E) Fovea

  F) Macula

  
• Where is the densest concentration of photoreceptors in the retina?

  Fovea
• What is the optic disk?


  Point where the optic nerve originates, here optic fibers exit the retina.
• What are the three types of cells in the retina?

  Photoreceptors (rods and cones)

  Bipolar cells

  Retina ganglion cells
• Rods and cones are types of photoreceptor cells.
• Are there more rods or cones in the retina?

  Rods
• What are the only cells in the retina sensitive to light?

  Rods and cones (photoreceptors).
• What is the distribution of rods and cones in the retina?

  Rods: Peripheral retina Cones: Central retina
• What are the four regions of photoreceptors?


  Synaptic terminal, inner segment, cell body and outer segment.
• What does the outer segment of a photoreceptor contain?


  Photosensitive disks - that contain photopigment.
• What is meant by rods being monochromatic?


  They don't change the way they respond to different wavelengths of light, colours.
• Rods


  • Monochromatic
  • Only function in dim light
  • Peripherally distributed in retina
• Cones


  • Maximally sensitive to red, green and blue light (short, middle and longer wavelengths)
  • Maximally concentrated in centre of retina
  • Visual detail and colour vision
• What is this image showing?

  How cells part ways to allow light to reach the photosensitive disks of the cones and rods - lateral displacement around the fovea.

  
• Why is the fovea thinner?


  Lateral displacement of other cells above photoreceptors to allow light to reach cones.

  
• What is the visual pathway through the retina?


  Photoreceptors -> bipolar cells -> ganglion cells -> optic nerve
• Where does 90% of visual information synapse?


  LGN (Lateral geniculate nucleus)
• Where does 10% of visual information synapse?


  Superior colliculus.
• Why is some visual information synapsed at the superior colliculus?


  Important for visual reflexes and entrainment of circadian rhythms.
• Fill in the pathway

  A) Lateral geniculate nucleus

  B) Superior colliculus

  C) Primary visual cortex

  
• What is the binocular field?

  Where the right visual field of the right eye and the left visual field of the left eye overlap.

  
• What is the nasal portion of the retina?

  Middle to nose.
• What is the temporal portion of the retina?


  Middle outwards.
• What happens to axons from nasal retina at the optic chiasm?


  They decussate and continue as the contralateral optic tract.
• What happens to axons from the temporal retina at the optic chiasm?


  They remain on the ipsilateral optic tract, they do not decussate.
• Why is the partial decussation at the optic chiasm so important?

  Means that the left hemisphere processes information from left visual field and right hemisphere that of the right visual field.
• How do photoreceptors behave in the dark?


  Rods and cones steadily release glutamate - depolarised.
• How do photoreceptors behave in the light?


  Release of glutamate decreases - hyperpolarised.
• What do changing glutamate levels affect?


  Bipolar cells.
• What are the three stages of visual processing?

  1. Photoreceptors in dark or light
  2. Photoreceptors -> bipolar cells
  3. Bipolar cells -> ganglion cells and action potentials
• Two types of bipolar cell

  'ON' and 'OFF' bipolar cells.
• How does glutamate affect on and off bipolar cells?

  Hyperpolarises the on bipolar cells.

  Depolarises the off bipolar cells.
• How does an ON bipolar cell respond to light?


  Depolarises

  In light photoreceptors do not produce glutamate - glutamate therefore cannot hyperpolarise the ON bipolar cell.
• How does an OFF bipolar cell respond to light

  Hyperpolarises

  In light photoreceptors do not produce glutamate - glutamate therefore cannot depolarise the OFF bipolar cell so it is hyperpolarised.
• What is the only retinal cell that fires action potentials?

  Retinal ganglion cells
• Light stimulation of the photoreceptor causes bipolar cells to depolarise or hyperpolarise depending on whether they are ‘on’ or ‘off’ bipolar cells.