Neurobiology of Vision

• The retina is embryologically part of the CNS (diencephalon).
• Fill in the blanks

  A) Forebrain

  B) Telencephalic vesicles

  C) Diencephalon

  D) Optic vesicles

  E) Midbrain

  F) Hindbrain

  
• What does the lens develop from?

  Lens placode (specialised thickening of ectoderm)
• What is Pax6?

  Homeobox transcription factor

  Controls all aspects of eye development

  Can elicit eye development on its own
• What are the 5 principle neuronal cell types in the retina?

  Photoreceptors

  Bipolar cells

  Horizontal cells

  Amacrine cells

  Retinal ganglion cells
• We have no photoreceptors where the optic nerve is (this is where the blindspot is).
• The visual world (NOT the retina) is projected onto the primary visual cortex in a topographic map.

  Interpretiation of information in the map is done in secondary visual cortices.
• Fill in the blanks

  A) Optic nerve

  B) Optic chiasm

  C) Lateral geniculate nucleus

  D) Optic tract

  E) Optic radiation

  F) Striate cortex

  G) Hypothalamus

  H) Pretectum

  I) Superior colliculus

  
• What layers in the lateral geniculate nucleus of the thalamus do nasal retinal fibres project to?

  1, 4, 6
• What layers in the lateral geniculate nucleus of the thalamus do temporal retinal fibres project to?

  2, 3, 5
• What Brodmann area is V1?

  17
• What is Meyer's loop?

  Projection of inferior fibres of LGN of thalamus, through temporal lobe to occipital cortex

  Clincial importance in surgery for temporal epilepsy
• Vast majority of thalamocortical neurones project to layer 4 of cortex.
• What is another name for the primary visual cortex?

  Striate cortex

  IT IS NOT IN THE STRIATUM
• Eye development is a combination of retinal & lens development.
• What are photoreceptors comprised of?

  Rods & cones
• What do photoreceptors do?

  Convert light into electrical information (NOT APs)

  Synapse with & signal to bipolar cells
• What do horizontal cells do?

  Provide lateral inhibition to photoreceptors
• What do amacrine cells do?

  Provide lateral inhibition to bipolar cells
• Fill in the blanks

  A) Photoreceptors

  B) Horizontal cells

  C) Bipolar cells

  D) Amacrine cells

  E) Ganglion cells

  
• Only retinal ganglion cells send action potentials, everything else consists of subtle balances of polarisation & depolarisation.
• What happens when light hits photoreceptors?

  Phototransduction
• Phototransduction

  Light converts 11-cis retinal to all-trans retinal in opsin -> conformational change in opsin -> activates transducin (heterotrimeric G protein) -> activates phosphodiesterase -> converts cGMP to GMP -> closure of cGMP-gated Na+ channels -> less Na+ influx -> K+ channels still open -> hyperpolarisation of photoreceptor
• What does the fovea consist of?

  Cones
• Synapsing with bipolar cells

  Cones = 1:1 -> maximum acuity/resolution

  Rods = up to 30:1 -> lower acuity, but bigger amplification of circuits
• What are the inhibitory receptor for glutamate?

  mGlu6
• What are the excitatory receptors for glutamate?

  AMPA kainate

  NMDA
• As humans, we are 'better' at perceiving differences than reality.

  We perceive it in comparison to everything around it, due to horizontal cells -> inhibit surrounding photoreceptors

  Note differences, rather than actual reality

  
• What is the pupilarry reflex mediated by?

  Pretectal nucleus
• What is Pax6? What is its role?

  Homeobox transcription factor

  Controls all aspects of eye development
• What is aniridia?

  Absence of iris

  Heterozygous mutation is Pax6
• What happens if you have a homozygous mutation in Pax6?

  Incompatible with development

  Foetus will die before 2nd trimester
• Describe centre-surround function organisation. (in terms of light in the centre)

  If light is in centre -> hyperpolarised PR in centre -> inhibit bipolar ON cell less AND will activate horizontal cells less -> horizontal cells normally inhibits all of the surrounding photoreceptors, but because they are being activated LESS, they will inhibit LESS -> activation of surrounding PRs 

  PRs in dark -> depolarisation due to darkness & less inhibition from horizontal cells -> more excitation 

  ^^ Lateral inhibition 

  
• Horizontal cells recieve synapses from & to multiple photoreceptors.

  They are GABA-ergic, and usually inhibit surrounding photoreceptors.
• It is the visual world that is mapped in V1 (primary visual cortex), not the retina.
• Due to nasal fibres decussating at the optic chiasm, this means that information from both eyes goes to both sides of the brain.
• Fill in the blanks

  A) Optic nerve

  B) Optic chiasm

  C) Optic tract

  D) Lateral geniculate nucleus

  E) Optic radiation

  F) Striate cortex

  G) Hypothalamus

  H) Pretectum

  I) Superior colliculus

  
• How is the hypothalamus involved in the visual pathway?

  Suprachiasmatic (above chiasm) nucleus in hypothalamus

  Receieves projections from retinal ganglion cells to regulate arousal (how much light there is? -> why we wake up in the morning)
• How is the pretectum involved in the visual pathway?

  Pupillary reflex

  Information travels down optic nerve to pretectal nucleus (anterior midbrain) -> projections bilaterally -> synapses with EW nucleus (parasympathetic CN3) -> sends parasympathetic signals down BOTH oculomotor nerves -> synapse at ciliary ganglion -> projects into muscles of the iris (constriction = light, dilation = dark)
• What is the importance of the superior colliculus in the visual system?

  Involved in coordination of eye movement for visual reflexes (eyes drawn to moving/bright object in a scene)