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visual system function - Coggle Diagram

- - - - innermost layer of the eye
      - contains neurons that are sensitive to light and transmit visual signals to central targets
      - located at the back of the eye
      - the retina - neurons
        
        five basic classes of neurons
        
        photoreceptors
        
        bipolar cells
        
        retinal ganglion cells
        
        horizontal cells
        
        amacrine cells
        
        interneurons
        
        secrete neuromodulators to modulate
        
        connectivity
        
        amount of signal coming from photoreceptors
        
        comodulate how much signal comes from excitatory cells
        
        all different types of neurons born early in neurogenesis
        
        signalling eg. delta notch etc. is important in specification
        
        outer plexiform layer and inner plexiform layer - synaptic
        
        ganglion cell layer, inner nuclear layer, outer nuclear layer - neuronal
    - - adjusts the refractive power of the lens
      - controls how much light can come into the eye
      - in darker environments increases the opening
      - in brighter conditions it shrinks
      - reflex control of lens
    - - the opening through the iris that allows light to pass through the lens
    - - small depression in the retina with the highest visual acuity
      - spatial resolution - when we want to detect something with high spatial resolution we focus directly on it
    - - where the optic nerve leaves the eye
      - contains no photoreceptors - also called blind spot
- - - - low spatial resoluton
      - high sensitivity to light
      - effective in dim light but not for seeing detail
      - EVENLY DISTRIBUTED THROUGHOUT RETINA EXCEPT AT FOVEA
      - rod phototransduction
        
        the discs in the outer segment of rods contain the photo-pigment rhodopsin
        
        a photon of light activates the GCPR rhodopsin by changing the configuration of the 11-12 double bond at the light sensitive retina (cis to trans)
        
        GPCR-activated cascade
        
        decreases depolarisation by closing ion channels through a reduction of cGMP
        
        in response to a photon of light
        
        rhodopsin activates G protein (Ggamma, transducin)
        
        Galpha translocates in the membrane to activate PDE
        
        phosphodiesterase (PDE) hydrolyses cGMP -> GMP
        
        decrease in levels of cGMP leads to closure of Na+ channels and hyper polarisation
    - - very high spatial resolution
      - relatively low sensitivity to light
      - colour sensitive
      - acuity at the expense of sensitivity
      - HIGH DENSITY OF CONES AT FOVEA
      - CONCENTRATION OF CONES V LOW ELSEWHERE
    - - membrane infolds increase outer SA of plasma membrane
        
        allows them to contain larger amounts of photosensitive molecules (cones)
  - - - efflux potassium channels depolarise membrane - leads to a steady release of glutamate at terminal
    - - cyclic GMP gated channels are closed by light
      - potassium remains open - hyper polarises receptors, decreasing glutamate release
- - - - the glutamate signals are reversed
        
        excitaition is decreased
        
        inhibition is relieve
    - - all other neurons in the retina release transmitter in a graded fashion (not all or none) so synapses act like dimmer switches, giving graded responses
  - - - at the fovea, receptive fields are very small (~1 photoreceptor -> bipolar cell)..... smaller receptive field = higher visual acuity
      - at the periphery, receptive fields of bipolar are large (10s of photoreceptors -> 1 bipolar cell)
  - - - a circular area of retina proving direct photoreceptor input - the receptive field centre
        
        centre can be activated or inhibited by light striking the photoreceptors in this region, depending on which glutamate receptors are present on the bipolar cell, so they are designated ON OR OFF CENTRE BIPOLARS
      - a surrounding area of the retina providing indirect input from adjacent photoreceptors - the receptive field surround
        
        the bipolar cells response to light on the surround is opposite to that falling on the centre of its receptive field: antagonistic centre-surround receptive field
        
        horizontal cells are responsible for antagonistic surround via lateral connections from photoreceptors in the surround area of the receptor field of this particular bipolar cell
    - - horizontal cells are thought to exert their influence via action on photoreceptor terminals, regulating the amount of transmitter that the photoreceptors release onto bipolar cell dendrites
  - - - tunes the visual system to detecting contrast in light (luminance) levels (between centre and surround of receptive field)
      - this is the basis for detecting edges - a basic requirement for 'seeing' a scene
  - - - genes involved in bipolar cell targeting to photoreceptors
        
        ELFN1: synaptic adhesion protein - regulates connectivity between growth photoreceptors and bipolar cells
        
        Pikachurin/dystroglycan: attachment to the ECM
        
        LRIT3: synaptic adhesion protein
      - genes involved in horizontal cell neurone targeting
        
        NGL2: synaptic adhesion protein
        
        Sema6A: contact repulsive cue
- - - - genetically encoded calcium indicator
      - fluorescees when calcium binds to calmodulin
      - used in research as a transgene
  - - - direction selective retinal ganglion cells (DSGCs)
        
        combining different on, on/off, off dsgcs give variety
      - direction sensitivity is incredibly complex - selectivity is produced because DSRGC receive inputs from gutamatergic bipolar cells and GABAergic/cholinergic starburst amarcine cells
        
        SACs are inhibitory to DSRGCs
        
        SACs on the null side of DSGCs make more inhibitory synapses onto DSGC dendrites compared to SACs on the preferred side
        
        when an object moves into preferred direction of RGCs excitation comes before inhibition but in the null inhibition comes before excitation
        
        earlier models show that this is not as simple - more inhibitory synapses on the null side than on the preferred side, some sort of asymmetry but we don;t know much more than that
  - - - very important physiologically