neurobiology of vision

• how many cells types are present in the retina?


  1. photoreceptors
  2. bipolar cells
  3. horizontal cells
  4. amacrine cells
  5. retinal ganglion cells

  interestingly the rods and cones are the back of the retina and then there is the ganglion cells closer to the centre, with bipolar cells in the middle

  
• what are retinal circuits comprised of


  PR- cones and rods, phototransduction (light to electrical information), work on graded potentials NOT AP, think of it like communication by hyperpolarisations and depolarisations

  biopolar - synapse with PR and retinal ganglion cells, compute electrical information through

  amacrine and horizontal cells - lateral inhibition of PRs and bipolars, allows for CSFO (centres surround functional organisation), modulate signalling through the circuit

  retinal ganglion - only ones to cause action potentials, along the internal surface of the retina

  
• describe the arrangement of PRs (rods and cones) in the retina, how do they synapse onto bipolar cells


  fovea - almost entirely cones

  as move away from the fovea - rods begin to dominate

  rods - convergence signalling, 30:1 many rods synapse onto a single bipolar cells

  cones - synapse in concentration so 1:1 to maximise resolution
• explain how centre surround functional organisation operates in the retina


  1. there are horizontal cells that are activated by photoreceptors
  2. once activated they cause hyperpolarisation of surrounding PRs
  3. amplifies differences in the visual scene
  4. the same applies to amacrine cells but twice as complex as there are ON and OFF pathways
  5. Glutamate release from photoreceptor terminals has a depolarizing effect on horizontal cells (sign-conserving synapse), while horizontal cells have a hyperpolarizing influence on photoreceptor terminals (sign-inverting synapse)

  
• the visual world is projected onto V1 in a TOPOGRAPHIC MAP (note - the visual world is mapped NOT THE RETINA)

  the topographic map is interpreted in the secondary visual corticies

  
• projections of the retinal ganglion cells from the retina mean that the topographical map produced is orientated as...


  Right hand side of the VISUAL WORLD is computed by the Left side of the brain

  what is the top of the VISUAL FEILD/WORLD 's image is transmitted by the inferior retinal ganglion cells and information is spread through the superior nerve fibres
• how do temporal and nasal fibres pass the optic chiasm?


  temporal axons travel ipsilaterally

  nasal axons decussate at the chiasm

  thus info from both eyes goes ton both sides of the brain\
• what is the relevance of the suprachiasmatic nucleus in the central pathway of vision?


  the suprachiasmatic nucleus receives information from non-image-forming retinal ganglion cells -> this information is used to regulate circadian rhythm and arousal/awakening

  delineated as the hypothalamus -> regulation of circadian rhythm

  
• in context, demonstrate the function of the pretectal nucleus/ pretectum:


  pupillary reflex is mediated by the pretectal nucleus

  The primary input to the Edinger–Westphal nuclei is from the pretectal olivary nuclei, which receive direct crossed retinal input.

  
• what is the role of the superior colliculus in central pathways of vision? how does the superior colliculus control the motor nuclei of CN III, IV and VI?

  superior colliculus is associated with saccadic eye movements (rapid eye movements and visual reflexes, movements should be smooth)

  the superior colliculus lies close to the motor nuclei of CN III, IV and IV -> it controls them via nuclei called the horizontal gaze centre and the vertical gaze centre in the brainstem

  
• what is the role of the lateral geniculate nucleus in the central visual pathways?

  the lateral geniculate nucleus is a nucleus in the thalamus which is responsible for transferring information to the visual cortex to be processed

  
  

  the LGN is divided into 6 layers -> nasal RGC project into layers 1,4,6 and temporal RGCs project into 2,3,5 -> therefore information from two different eyes is not mixed until the cortex
• how do different RGCs project to different layers of the LGN - lateral geniculate n nucleus


  different types of RGC go to different layersP RGC -> layers 3-6 and are for colour and fine spatial detailM RGC -> all layers, movement in the visual field, high temporal resolution and have a higher receptive fieldK RGC -> poorly understood project to areas between the layers
• which thalamocortical projection is an important consideration in surgery for temporal lobe epilepsy?


  Meyer's loop

  
• what is the importance of secondary visual/ association cortices in vision?


  Neurons in V1 project to all of these areas, and there are many circuit projections within and between these areas. All of them function to interpret the visual scene.
• what are the two cortical visual pathways? what does each show/represent?


  dorsal stream (to the parietal association cortex) - shows movement and positional information

  ventral stream (to the temporal cortex ) - e
• what are the two cortical visual pathways? what does each show/represent?



  dorsal stream (to the parietal association cortex) - shows movement and positional information ventral stream (to the temporal cortex ) - recognition especially in a complex visual scene

  
• how does the dorsal stream travel to the parietal association cortex?

  In the dorsal stream, V1 neurons/circuits project -> V2, and V1 + V2 -> MT (‘middle temporal’,   sometimes called V5). MT projects to the parietal association cortex and this pathway is particularly associated with movement and positional information.

  
• how does the ventral stream project into the inferior temporal lobe?


  In the ventral stream, V1 -> V2 -> V3 -> V4 -> inferior temporal cortex. This pathway is particularly associated with recognition, especially of complex aspects of a visual scene.    

  
• Conscious vision summary

  here