VS142 Visual Neuroscience

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VS142 Visual Neuroscience

• Neural Retina
• Basic Pathways

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Televisions, video monitors and digital cameras
use regular arrays of red, green, and blue
phospors/sensors to record or reproduce an image.
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But not the retina!Blue cones are sparse, mostly
red and green, but they are located in patches
not in regular repeating arrays.
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For form vision the red and green cones appear
to be used almost exclusively (and perhaps
interchangeably similar wavelength
sensitivity).The blue cones are rare and
sparsely localized absent in the fovea! used
mostly only for color vision.
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• Synaptic Contacts of Mammalian Photoreceptors
• Pedicles terminals of cones. Foot-shaped.
• Spherules terminals of rods. Ball-shaped.
• -gt In the mammalian system, any given bipolar
  cell contacts either rods or cones but not both.
  
• Triad dendrite of a cone bipolar cell
  invaginates the cone pedicle and is flanked by
  two other invaginating processes from horizontal
  cells.
• Invaginating Bipolar Cell bipolar cell part of a
  triad.
• Flat Bipolar Cells contact cone pedicels without
  invaginating.
• Midget Bipolar Cells in primates, both
  invaginating and flat, can be postsynaptic to a
  single cone in the central retina, provide
  exclusive bipolar input to a single midget
  ganglion cell.

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• Synaptic Contacts of Mammalian Photoreceptors
  (cont)
• Rod bipolar cells of all mammals are strictly
  invaginating, form triads with rod spherules.
• Bipolar cells do not generate action potentials
  all graded potentials.
• -gt Light depolarizes invaginating bipolar cells,
  thus increasing their neurotransmitte release.
  On bipolar cell.
• -gt Light hyperpolarizes flat bipolar cells, thus
  decreasing their neurotransmitter release. Off
  bipolar cell.
• Invaginating On
• Flat Off

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• -gt All photoreceptors are hyperpolarized by light
  and decrease their transmitter (glutamate)
  release in light.
• -gt So the on- and off- classes of bipolars are
  created by different kinds of glutamate
  receptors.
• Sign-inverting synapse cone/rod and on-bipolar
  cell. Cone hyperpolarizes and bipolar cell
  depolarizes. Transmitter affects ion channels
  indirectly through a second messenger (cGMP)
  system.
• Sign-conserving synapse cone only (never rod)
  and off-bipolar cell. Transmitter affects ion
  channels directly.
• This splitting up into on- and off- pathways will
  continue throughout much of the visual system.

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• RECEPTIVE FIELD (of a neuron) (sound effects)
• The region of the retina (or the visual field
  visual space) where a stimulus must be placed
  for the neuron to be affected. For all cells
  other than the photoreceptors, the receptive
  field (RF) is a funnction of its functional
  connections to the photoreceptors.

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• Receptive fields of bipolar cells
• On-bipolar cells are depolarized by a spot of
  light in the center, but hyperpolarized by light
  in the surround. This is an on-center,
  off-surround receptive field.
• Off-bipolar cells have the opposite effects, are
  said to have off-center, on-surround receptive
  fields.
• Effects thought to be due to horizontal cells.
  Lateral inhibition.
• Effects of center and surround tend to cancel
  with uniform illumination, though typically some
  imbalance.

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• Synaptic terminals of bipolar cells make
  characteristic dyad junctions in the inner
  plexiform layer. One contact is usually an
  amacrine cell, the other may be either an
  amacrine or a ganglion cell. Can have a
  reciprocal synapse between amacrine and bipolar.
  Feedback circuit?
• BT bipolar terminal
• A amacrine cell
• G ganglion cell
• Large open arrow reciprocal synapse from
  amacrine cell to bipolar cell terminal.

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• Cartoon of Dyad Junction

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• Amacrine cells an enormous variety of types.
  Function(s) still not very well understood. An
  area of intensive research.
• -gt some amacrine cells can generate action
  potentials.
• -gt many different neurotransmitters
• Interplexiform cells similar in some ways to
  amacrine cells, but also send processes into the
  outer plexiform layer where they contact
  horizontal cells. Not clear what the point of
  this is.

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• Ganglion Cells the final total output of the
  retina!
• -gt The action potentials of ganglion cells are
  the ONLY thing that make it to the rest of the
  brain. Retinal activity is only important for
  visual perception to the extent that it affects
  the pattern of firing of action potentials for
  ganglion cells.
• -gt Transmission is one-way. The brain does not
  send signals to the retina. (About the only part
  of the visual system that is a one-way street).
• -gt Usually have center-surround receptive fields,
  both on- and off-types.

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• Neurotransmitters and Neuromodulators in the
  retina
• Rapid signaling of information in the retina is
  conveyed by classical small-molecule
  neurotransmitters (like glutamate and GABA-) and
  also by electrical synapses.
• Slower processes are mediated by peptides and
  also by dopamine. We dont really know what is
  being regulated by these slower mechanisms, so
  you wont have to memorize them (yet!).

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• GABA Gabara Gamera

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• Light and dark adaptation the human visual
  system can operate over an incredibly large
  dynamic range. The difference in light energy
  between a dark and stormy night and a sunny day
  at the beach can be over 10 billion to 1.
• -gt Iris/pupil size changes
• -gt Photoreceptor adaptation (slower). Non-linear
  effects of bleached photopigment?
• -gt Network adaptation (faster). Switching from
  rods to cones (AII amacine cells)

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• Scotopic vision dark-adapted vision
• Photopic vision light-adapted vision
• Purkinje shift shift of visual sensitivity
  towards longer wavelengths with light adaptation,
  because the balance of cones are responsive to
  longer wavelengths than are rods.

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• Midget Ganglion cells both on- and off-types.
  About 80 of the ganglion cells in primates.
  Connect to midget bipolar cells. Four basic
  types
• Red-ON
• Red-OFF
• Green-ON
• Green-OFF
• -gt Remember, blue cones not really used for
  detailed pattern vision, therefore blue does not
  (mostly, maybe) need a direct line to the brain.
• -gt In central retina midget bipolars get their
  input from just one cone. A little more diffuse
  (I.e., more than two cones/midget) in the
  periphery but not by much.

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• -gt There are lots of different types of ganglion
  cells, for example bistratified (connections in
  both on and off layers). We are not sure of
  the functional role of many of these types of
  ganglion cells.
• -gt Directly photosensitive melanopsin-containing
  ganglion cells! (more later on)
• -gt SMALL RELATIVE NUMBERS OF NEURONS DOES NOT
  MEAN THAT THEIR FUNCTIONS ARE NOT IMPORTANT!!!!
• Technical Jargon
• P-Cell (parvo) midget ganglion cell
• M-Cell (magno) parasol ganglion cell

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