The Retina

• The Eye as a Window to the Brain:

  • Retina is the only directly observable part of CNS
  • Detectable changes in retina parallel and even precede changes in the brain - diseases can be spotted before they occur

  *Neurodegenerative = Alzheimers , Parkinsons

  *Psychiatric = Bipolar Disorder, Schizophrenia

  *Vascular = heart disease

  • Today, diseases can only be detected after showing symptoms (CT,X-ray,MR,PET scans)
  • For retinal scans, lack of resolution means all these diseases look the same = lack
  • Adaptive Optics allows imaging of structure and function of individual retinal cells
• Main Components of the Eye:

  • Vitreous Humour = main cavity behind lens, gives eyeball its shape and keeps retina pinned to the back of the eye
  • Fovea = highest concentration of cone cells here, therefore highest resolution image
  • Optic Disc = where ganglion cell axons leave retina to form optic nerve
• Why is the Retina Back to Front?

  • Photoreceptors found at the back layer of retina - these cells must be in contact w/ Retinal Pigment Epithelium layer for nourishment
  • RPE layer is not transparent - must be at the back to avoid blocking incoming light from reaching photoreceptors

  
• The Vertical Processing Pathway:

  • Incoming light travels through retina to reach photoreceptors
  • Processing: signal generated by photoreceptors travels to bipolar cells then ganglion cells

  8 Retinal Layers:RPE = nourishes photoreceptors
• Photoreceptors: Rod Cells

  • Achromatic - only detects intensity of light (no colour)
  • Outer segments are rod-shaped
  • Scoptic = when only rods are active (night-vision), only active in dim light levels
  • Completely absent from fovea and most densely packed 12-15 degs into periphery
  • 90 million rods
  • Maximum sensitivity at 498nm
  • Photopigment = rhodopsin in outer segments

  ! All rods are the same

  
• Photoreceptors: Cone Cells

  • Chromatic - detects colour + less sensitive to light
  • Outer segments are cone-shaped
  • Photopic = when only cones are active (day-vision), only active in bright light levels
  • Highest concentrations in fovea = maximises vision by moving cell layers out way and reducing light scatter to give best image
  • 5 million cones - distribution differ b/w people
  • Photopigment = opsin in outer segments (varies to give max sensitivity at a particular wavelength)

  *Long-wave (red) = 564 nm

  *Middle-wave (green) = 534 nm

  *Short-wave (blue) = 420 nm

  ! 3 types of cone cells

  
• The Purkinje Shift:

  • Mesopic = both photoreceptor types play a role (occurs at twilight)
  • Purkinje shift = sensitivity b/w light and dark vision

  *Photopic Peak Sensitivity (cone-based) = 555 nm

  *Scotopic Peak Sensitivity (rod-based) = 507 nm

  How do you know which Cone is which?

  • When a cone is illuminated w/ a wavelength that matches its type, it lengthens
  • People w/ normal colour vision can have very different distribution of each cone type
  • More detail seen with Adaptive Optics
• Intrinsically Photosensitive Retinal Ganglion Cells:

  • maximum sensitivity at 480 nm
  • 2% of ganglion cells are sensitive to light (not just photoreceptors)

  *Contain pigment , melanopsin , embedded throughout the cell

  • Impacts circadian rhythms, mood , cognition

  Why are they hard to image?

  • With adaptive optics, we don't know which are IPRGC's as staining not safe in human eye - has only been done on dead (cells appear red when stained)
  • Can transform glaucoma diagnosis - by counting cells one by one instead of visual fields test
  • Also affected in psychiatric and neurodegenerative diseases
• Bipolar Cells:

  • 2 types - ON (OPPOSES) and OFF (COPIES)
  • 2 terminals = 2 synapses

  IN THE LIGHT:

  • Photoreceptor releases less glutamate = ON releases more glutamate
  • Photoreceptor releases less glutamate = OFF releases less glutamate

  IN THE DARK:

  • Photoreceptor releases more glutamate = ON releases less glutamate
  • Photoreceptor releases more glutamate = OFF releases more glutamate

  CONNECTION TO GANGLION CELLS:

  • ON TO ON
  • OFF TO OFF

  
• Ganglion Cells:

  • 2 types - ON and OFF (connect to ON and OFF bipolar cells respond respectively)
  • Axons carry info from retina to brain
  • Only cell in retina that fire action potentials - intensity is coded for by firing rate
  • Main targets: LGN , Superior Colliculus (eye movement), Suprachiasmatic Nucleus (circadian rhythm) , Pretectum (pupil size)

  INCREASE IN GLUTAMATE:

  • Both cells increase firing rate

  DECREASE IN GLUTAMATE:

  • Both cells decrease firing rate

  
• 8 Retinal Layers:

  1)RPE = nourishes photoreceptors

  2)INNER-SEGMENT OUTER-SEGMENT = photoreceptor segments

  3)OUTER NUCLEAR = photoreceptor bodies

  4)OUTER PLEXIFORM = connections b/w photoreceptors + bipolar cells (horizontal cells)

  5)INNER NUCLEAR = bipolar cell bodies

  6)INNER PLEXIFORM = connections b/w bipolar + ganglion (amacrine cells)

  7)GANGLION CELL = ganglion cell bodies

  8)NERVE FIBER = ganglion cell axons
• The Vertical Processing Pathway:

  • Incoming light: travels through retina to reach photoreceptors
  • Processing: signal generated by photoreceptors travels to bipolar cells and then ganglion cells
  • Photoreceptors release a neurotransmitter called glutamate

  The Horizontal Processing Pathway:

  • Horizontal cells connect photoreceptors and bipolar cells
  • Amacrine cells connect bipolar cells and ganglion cells
  • Both use GABA
  • Both impact a phenomenon called Receptive Field Centre-Surround Opponency
• What is a Receptive Field?

  • A group of photoreceptors that affect the firing rate of an individual ganglion cell

  *When an image falls on the retina, each part is analysed by a receptive field

  • Increase in diameter as you move further away from fovea - more photoreceptors can be pooled together to increase light sensitivity

  *This is why rods are more sensitive to light than cones - most are located in periphery so have larger receptive fields

  • At fovea, there are no receptive fields - each ganglion cell is affected by a single photoreceptor

  *This is why high acuity vision occurs here
• Centre-Surround Opponency:

  • Receptive fields have 2 parts - centre & surround

  *Opposing firing rates of ganglion cell - mediated by horizontal processing pathways

  • 2 types - ON Centre & OFF Centre

  *ON Centre = Light in centre increases firing, light in surround decreases firing

  *OFF Centre = Light in centre decreases firing, light in surround increases firing

  • Firing rate depends on how much light falls on centre compared to surround - overall effect is dominated by centre
• Achromatic Centre-Surround Opponency:

  • Light/No light above centre & surround = equal illumination so effects cancel each other out and no change in firing rate (ganglion cells are ALWAYS firing)

  ON CENTRE:

  • Maximum Firing = centre is fully illuminated, surround is not illuminated at all
  • Minimum Firing = centre is not illuminated at all, surround is fully illuminated

  OFF CENTRE:

  • Maximum Firing = centre is not illuminated at all, surround is fully illuminated
  • Minimum Firing = centre is fully illuminated, surround is not illuminated at all

  
• Chromatic Receptive Fields: FOR COLOUR PROCESSING

  • Red and Green:

  *Red centre , green surround

  *Green centre , red surround

  • Blue and Yellow

  *Blue centre, yellow surround

  *Yellow centre, blue surround