VISION

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VISION

• Eye StructureDetector Cells (Receptors)Transduct
  ionBrain Pathways

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Eye Structure

• Cornea
• Transparent dome which serves as the outer
  window of the eye. The cornea is the primary
  (most powerful) structure focusing light entering
  the eye composed mostly of epithelium tissue.
• Iris
• Colored disc inside the eye, thin diaphragm
  composed mostly of connective tissue and smooth
  muscle fibers.
• LensComposed of fibers that come from epithelial
  (hormone-producing) cells.
• Ciliary muscleSmooth muscle connected to zonule
  fibers to lens - keep tension
• Retina
• innermost layer of the eye, comparable to the
  film inside of a camera It is composed of nerve
  tissue which senses the light entering the eye.
  This complex system of nerves sends impulses
  through optic never to the brain

• Optic disk
• Location where ganglion cell axons exit the eye
  to form the optic nerve. There are no light
  sensitive rods and cones to respond to a light
  stimulus at this point thus it is also known as
  "the blind spot"
• Optic nerveCranial nerve that composed of
  retinal ganglion cell axons and support cells. It
  leaves the eye via the optic canal.
• PupilCentral transparent area of iris the size
  is involuntarily controlled by the contraction
  and dilation of iris.
• Fovea
• The centermost part of the macula (without blood
  vessels) and is responsible for our central,
  sharpest vision. ONLY cone receptors.

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Eye Structure
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Photoreception

• Electromagnetic radiation
• Vibration characterized as sine waves (nm)
• 400-700nm (violet to red)
• Human visual spectrum
• Long bands red
• Extra long bands infrared (IR)
• Short bands blue / purple
• Extra short bands ultraviolet (UV)

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Visual Perception

• Visual field
• Visual acuity
• Blind spot
• Lateral inhibition

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Retinal Anatomy - I

• Rods (scotopic) and cones (photopic)
• 100 million rods, 4 million cones
• Rods (3 cone opsins), cones (rhodopsin)
• Release NT (generator potential) to modulate
  bipolar cells
• Bipolar cells
• Connects rods and cones to ganglion
• Generator potential
• Horizontal (receptor cells)
• Involved in lateral inhibition
• Generator potential
• Amacrine cells
• Contact bipolar and ganglion cells
• Ganglion cells
• Make up optic nerve
• Action potential

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Retinal Anatomy - II

• ConvergenceMore receptors than
• gangliaAmacrines and horizontalsGreate
  r for rods ScotopicRod dependentDim
  lightLow acuity
• PhototopicCone dependentBrighter lightHigh
  acuityColor vision

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Hyperpolarization of Photoreceptors (Rods)

• Photon splits rhodopsin (photopigment receptor)
• G-protein (transducin 500 molecules) liberated
• GTP replaces GDP at binding of alpha subunit
• GTP-alpha complex activates phosphodiesterase
  (PDE)
• PDE hydrolyzes gt2000 molecules of cGMP (2nd
  messenger)
• Na channels closed (reduced Na influx),
  hyperpolarization

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Photoreceptor Excitation /Inhibition (Cones)

• Cone and rods (release Glu at rest)
• Off-center bipolar cells
• Turn off light, depolarize photoreceptors, Glu
  release increased
• Depolarizes bipolar cells, increased glutamate
  release (to ganglion)
• On-center bipolar cells
• Turn on light, hyperpolarize photoreceptors,
  decrease Glu release
• Hyperpolarizes bipolar cells, decreased glutamate
  release (to ganglion)
• Cone contacts
• 1 on-center midget bipolar, 1 off-center midget
  bipolar cell, several diffuse on- and off-center
  bipolar cells

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Visual Pathway

• Retinal image inverted and reversed
• Axons of ganglion (optic nerve) enter brain
• Decussation of nasal sides
• Lateral geniculate nucleus (LGN) (terminus for
  many)
• Visual thalamus
• Superior colliculus (terminus)
• LGN and striate cortex (visual cortex)
  connections back and forth

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Take Home Message

• Select receptors for vision
• Stimulus quality / other characteristics
• Transduction
• Brain pathways