Visual pathways 1

• Vitreous humour


  Fills the main cavity behind the lens, gives the eyeball its shape and keeps the retina pinned to the back of the eye
• The retina develops from a pouch of the embryonic forebrain, and the retina is therefore considered part of the brain
• Light rays must pass through the entire retina before reaching pigment molecules to excite
• This is because the pigment-bearing membranes of the photoreceptors have to be in contact with the eye's pigment epithelial layer
• Main cell types in the retina


  • Photoreceptors (rods and cones)
  • Bipolar cells
  • Ganglion cells
  • Horizontal Cells
  • Amacrine cells
• Rods


  Outer segments are generally rod-shaped
• Cones

  Outer segments are generally cone-shaped
• When the outer segments of cones or rods are magnified, stacked membrane disks are visible inside. These disks are studded with thousands of rhodopsin complexes
• Each rhodopsin consists of a membrane-traversing protein with a retinal molecule embedded in its core
• When exposed to light

  One of the bonds in the retinal molecule rotates, changing the shape of the protein
• Both rods and cones hyperpolarize to light. That is rods and cones release neurotransmitter (are excited) in the dark
• When exposed to light, ion channels in the cell membranes close and no neurotransmitter is released for as long as the photoreceptors are exposed to light
• Absorption spectra


  Photoreceptors are more or less sensitive to different wavelengths of light
• Different lights have a different emission spectra or emit different wavelengths of light and each photoreceptor is sensitive to a different range of wavelengths
• Scotopic vision


  When only rods are active
• Cones


  Come in three main sorts: long-wave, middle-wave, and short-wave which reflects the visible wavelengths the different cone opsins are most sensitive to
• Photopic vision


  When it is so bright that the rods cannot function
• Mesopic vision


  The region in between when both rod and cone receptor types play a role
• Cones are heavily concentrated in the fovea. Rods are completely absent from the central fovea and are most densely packed some12–15° into the periphery
• Congenitally colour-blind people have a problem with either their long-wave or middle-wave (or very rarely their short-wave) cones
• People with normal color vision can have VERY different distributions/relative numbers of middle and long-wave cones
• Layers of the retina


  • Photoreceptor Cell Bodies
  • Synapse between photoreceptors and bipolar and horizontal cells
  • Cell bodies of bipolar and horizontal and amacrine cells
  • Synapses between bipolar, ganglion and amacrine cells
  • Ganglion cell bodies – provides outputs that form the optic nerve
• Retinal pigment epithelium (RPE)


  Photoreceptors make contact with RPE, rods and cones shed and renew the discs from their outer segments, discs are absorbed by RPE hence central retina highly metabolically active, cycle may become disrupted in older eyes -> Age Related Maculopathy (ARM)
• The retina contains around 5M cones and 120M rods
• In the centre of the fovea, the foveola, there are no rods and 25,000 cones. They are tightly packed into a hexagonal arrangement to optimise visual acuity
• Receptive field


  The region of the visual field in which a stimulus can modulate the firing of a neuron
• Bipolar cells


  Cell bodies located in inner nuclear layer, synapse with photoreceptors located in the outer plexiform layer, synapse with ganglion cells and amacrine cells in the inner plexiform layer
• ON bipolar cells


  Have inhibitory glutamate receptors therefore responding to light images against a dark background
• OFF bipolar cells


  Have excitatory glutamate receptors therefore respond to dark images on a bright background
• ON bipolar cells connect with ON ganglion cells and OFF bipolar cells connect with OFF ganglion cells
• If these were the only pathways through the retina the output would be low resolution and blurry
• ON and OFF ganglion cells form the output signals from the retina
• Horizontal processing


  Accomplished by the Horizontal cells (at the photoreceptor synapse) and by Amacrine cells (at the Bipolar/Ganglion cell synapse)
• Horizontal cells


  Each horizontal cell actually receives input from many cones, so its collection area or receptive field is large
• Center-surround organization


  Horizontal cells add an opponent signal that is spatially constrictive, giving the bipolar cell a center-surround organization
• Amacrine cells


  Operate at the inner plexiform layer, the second synaptic retinal layer where bipolar cells and retinal ganglion cells form synapses, there are at least 33 different subtypes based on their dendrite morphology and stratification
• The amacrine-cell circuitry in the inner plexiform layer conveys additional information to the ganglion cell – possibly sharpening the center-surround antagonism
• Ganglion cells


  Output neurons of the retina, only cells in retina that generate action potentials, intensity is coded by firing rate (impulses/sec), many different types but two main methods to classify them: Anatomically and Physiologically, main target is the Lateral Geniculate Nucleus (LGN) but others project to different parts of the brain eg Superior Colliculus, SCN, and the pretectum
• Center-surround receptive field


  Maximum response when light fills only the centre, activation of surround suppresses response