Visual system 1

Visual system 1

1. Visual pathways

   • Right hemifield activates left brain - and vice versa
   • The main (but not only) function of the retina is image acquisition
   • LGN preprocesses the visual information
   • Main processing of information happens in the visual cortex

2. Retinal mosaic

   • Pupil regulates the amount of light that falls on the retina
   • Lens focuses image on the fovea
     • Fovea is the part of the retina with the highest visual acuity
       • The rest of the retina has smaller acuity and contains primarily rods

3. Laminar retinal organisation

   • Retina consists of :
     • 3 layers of neurons
       • Feedforward neurons
         • Photoreceptors
           • Photoreceptor synapses
             • Have special “ribbon synapses” able to keep vesicle release permanently
             • Do not spike (i.e. have “graded potentials”)
             • Release glutamate
             • Therefore, when the light goes up, the release rate goes down and vice versa.
         • Bipolar cells
           • Centre cell organization of receptive field
             • Receptive field is an area in the retina which when illuminated activates a visual neuron
             • Many visual neurons have centre-surround organisation of the RF, meaning that illumination of the centre and the surround leads to responses in opposite polarities
             • Mechanism
               • Many photoreceptors converge to 1 bipolar cell
                 • Some activate directly (centre) while others activate via horizontal cells(surround) and therefore cause surround photoreceptors to depolarise
           • Some bipolar cells hyperpolarise while others depolarise in response to light
             • Why do ON cells depolarise?
               • Sign inverting synapse in On bipolar cell
                 • Photoreceptors release glutamate
                 • OFF cells express ionotropic glutamate receptor
                   • Excitatory
                 • ON cells express metabotropic glutamate receptor
                   • Inhibitory
           • Stratification of bipolar cells in the IPL
             • Like many other neurons, BC terminate in different sublayers of the IPLS
             • Recall, that this is very precisely regulated during development
         • Ganglion cells
           • Also send their dendrites to different layers of the IPL
           • Dendritic trees
             • Ganglion cells have diverse but distinct morphology of the dendritic trees
             • Note that some have very small but very dense dendritic trees
             • Some cells have larger and diffuse dendritic trees
             • How does this determine function?
               • FIND OUT
           • Receptive field
             • Centre surround orginization
               • Illumination of the whole receptive field does not activate ganglion cells
                 • Appears that they are designed to respond to differences in illumination that occur within the receptive field
           • Two major classes
             • Parvocellular
             • Magnocellular
             • In LGN
               • Axons from each eye remain segregated
               • P- and M-Ganglion Cells remain segregated
       • Feedback neurons
         • Horizontal cells
         • Amacrine cells
     • 2 layers of synapses
       • Inner plexiform layer
       • Outer plexiform layer

4. Convergence, divergence and information bottleneck

5. Rods and Cones

   • Rods
     • Active at dim light
   • Cones
     • Active at bright light
   • Phototransduction happens in the outer segment
   • Photoreceptors respond to light flashes by hyperpolarisation (decrease in membrane potential)

6. Phototransduction cascade

   • Stimulus
     • Light
   • Receptor activation
     • Change in protein conformation
   • G-protein response
     • Binds GTP
   • Second messenger
     • cGMP
     • Change
       • Decrease second messenger
   • Ion channel
     • cGMP gated channel
     • Response
       • Decrease na+ conductance

7. Retina

   • Colour processing
     • All rods have same spectral sensitivity
     • Cones
       • Red
       • Blue
         • Much less of these in the fovea
       • Green
   • Rod pathways
     • No direct rod-ganglion cell pathway
     • Rod bipolar cells activate special class of amacrine cell
     • AII form gap junctions with ON cone bipolar cell and inhibitory (glycinergic) synapse with OFF ganglion cell

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