NEUROBIOLOGY B4

• Region of photosensitive cells

  • Depressed/folded area allows limited directional sensitivity
  • Pinhole eye allows finer directional sensitivity and limited imaging
  • Transparent humour develops in enclosed chamber
  • Distinct lens develops
  • Iris and separate cornea develop
• Visual acuity


  A measure describing the ability to distinguish two nearby points (dependent on the density of photoreceptor packing and other optical factors)
• Measures of visual acuity are in visual angles (degrees), i.e. object sizes are relative to distance to the eye
• Eye and retina

  • Responds to photons for more than visual processing
  • Superior colliculus (SC) is involved in saccadic eye movements and the oculomotor reflex
  • Coordinates visual, somatic and auditory information, adjusting movement of the head and eyes toward a stimulus
• 5 neuron types involved in retinal information flow

  • Photoreceptor cells: convert light into electrical signals
  • Bipolar cells: transmit signals from photoreceptors to ganglion cells
  • Ganglion cells: send visual information to the brain via the optic nerve
  • Horizontal cells: modulate signals between neighbouring photoreceptors
  • Amacrine cells: modulate signals between other retinal neurons
• Light traverses across a transparent retina (0.25mm thick), which is a displaced portion of the CNS
• The light reaches the photoreceptor later, which has high regenerate and metabolic needs, thus requires close proximity to blood supply
• Rods

  Detect amount of light (monochromatic) (highly sensitive)
• Cones

  Multiple types permit perception of colour (not as sensitive)
• There are ~100 million rods and ~5 million cones (per eye), which transduce light into neural signals (graded)
• Photoreceptors do not generate AP, they depolarise and hyperpolarise in response to light
• Outer Segment of photoreceptors

  • Region of phototransduction, flattened organelles pinched off from outer membrane
  • Rods (night vision) have membrane 'disks' (elongated outer segment) - this is where phototransduction occurs in rods
  • Cones have infolding of plasma membrane - this is where phototransduction occurs in cones
• Inner segment of photoreceptors includes the nucleus and mitochondria
• Terminal of photoreceptors

  • Connected by axon-like process
  • Storage and release of neurotransmitter
• Scotopic vision


  Night vision
• Photopic vision


  Day vision
• Mesopic vision


  Where both rods and cones are active (between low light and bright light)
• Fovea (central vision)


  • Pinhead sized depression
  • Cells other than cones are 'pushed' out of the way
  • Only here is there a 1:1 ratio between cones and ganglion cells (high resolution)
• Peripheral vision

  • Higher ratio of rods to cones
  • Convergence of more photoreceptors to ganglion cells (larger receptive field)
  • More sensitive to light (e.g. night vision)

  
• Cortical magnification


  The disproportionate representation of the central visual field in the primary visual cortex (V1) compared to the peripheral visual field
• Areas of the visual field with higher acuity and finer detail have a larger cortical representation than areas of low acuity
• Rhodopsin

  A light receptor molecule (GPCR) tightly-packed in disk membranes that absorbs photons
• There are 10 billion rhodopsin molecules in a single rod
• Rhodopsin


  • Composed of retinal, a derivative of vitamin A and opsin, a single polypeptide containing 7 transmembrane domains
• Phototransduction

  1. The all-trans isomer of retinal activates opsin which signals to the heterotrimeric G protein transducin, initiating a 2nd messenger cGMP cascade

  2. 1 opsin can activate 8% of the disk's transducin (~1000) with each PDE degrading ~50 cGMP - this equates to the closure of ~200 channels

  3. Each channel closure stops ~10^4 ions which means that a single photon (time course of ~0.5s) stops ~10^6 ions from entering the outer segment

  4. Via signal amplification, 1 photon can produce 1mV hyperpolarisation
• 3 cone subtypes

  • short (blue)
  • medium (green)
  • long (red)
• Colour perception is derived from overlapping but distinct responses (Young-Helmholtz trichromacy theory of colour vision)
• Opsins

  • Absorption of light by 11-cis retinal causes a rotation around double bond to form more stable all-trans retinal
  • Results in conformational change in the opsin
  • Variations of opsins in rods and L, M and S cones permits variation of spectral sensitivity
  • Recombination L and M pigment genes control colour vision
• Photoreceptor responses in light and dark are graded changes (no AP)
• Dark Current

  • Glutamate released
  • Cell depolarised
  • Na+ channels open
  • Rhodopsin inactive
• Light Current

  • Glutamate release decreases
  • Cell hyperpolarised
  • Na+ channels closed
  • Rhodopsin active
• Research Experiments

  • Intracellular recordings from neurons in the insect brain
  • Response to drifting targets of varying size
  • Quantifying photoreceptor noise
• Information Flow in Retina

  Photoreceptors (detect light) -> Horizontal cells (modulate signals) -> Bipolar cells (transmit signals) -> Amacrine cells (refine signals) -> Retinal ganglion cells (relay signals to brain)
• Cone and Rod Signal Circuitry

  • separation of channels into on and off pathways
  • Off and on pathways determined by light stimulation
  • Rods feed into circuitry via amacrine cells