The Special Senses II

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The Special Senses II

• Image formation
• Physiology of Vision

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Image formation Refraction

• Refraction is the bending of light rays at the
  junction of two transparent substances of
  different density.
• In the eye refraction takes place at the surfaces
  of the cornea and the lens.

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Focusing of light on the retina

• Light moves from air?cornea?aqueous humor ?lens ?
  vitreous humor ? through the entire neural layer
  of the retina ? photoreceptors.
• Light is bent 3 times cornea? lens ? vitreous
  humor
• Refractory power of cornea and vitreous humor is
  constant.
• The lens is high elastic and can change its
  light-bending power.

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Focus for distant vision

• Our eyes are best adapted for distance.
• Images are focused upside down and undergo left
  to right reversal
• The brain interprets the visual signals as
  correctly oriented.

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Focus for close vision

• Accommodation of the lens.
• Constriction of the pupils.
• Convergence of the eyeballs.

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Normal refraction

• The normal eye can reflect light rays from an
  object 20 feet away (6 m) and clearly focus it.

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Refraction abnormalities

• Myopia is the ability to see objects that are
  close
• The eyeball is long relative to focusing power of
  cornea and lens

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Refraction abnormalities

• Hyperopia is the ability to see objects that are
  distant
• The eyeball is short relative to focusing power
  of cornea and lens

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Photoreceptors

• Photoreceptors are modified neurons that have
  their tip in the pigmented layer.
• Outer segment
• Inner segment

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Photopigments

• Rods have rhodopsin
• Cones have different photopigments (opsins)
• All photopigments have opsins and retinal which
  is the light absorbing part

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Light response of photopigments

• Cis-retinal absorbs a photon of light and
  isomerizes into a trans-retinal.
• The retinal-opsin combination breaks down leading
  to their separation. This is known as
  photobleaching.
• An ATP-requiring process that uses the enzyme
  retinal isomerase converts the trans-retinal back
  to a cis-retinal.
• The cis-retinal rebinds to opsin forming a
  functional photopigment--regeneration

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Light transduction

• In the dark cGMP holds open cation channels that
  depolarize the cells.
• Known as the dark current. Vm is -30mV.
• Glutamate is continually released and
  hyperpolarizes the bipolar cell.
• Light activate rhodopsin activates transducin and
  transducin activates phosphodiesterase (PDE)
  which breaks down cGMP.
• Cation channels are closed but Kchannels remain
  open. Vm is -70mV
• Glutamate is no longer released and bipolar cell
  releases NT and activates the ganglion cell.

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Light transduction
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Visual pathway

• Photoreceptors
• Outer synaptic layer
• Horizontal cells
• Bipolar cells
• Inner synaptic layer
• Amacrine cells
• Ganglion cells

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Visual pathway

• Axons of retinal ganglion cells make up optic
  nerve and exits at the optic disc.
• At the optic chiasma axons from temporal half of
  the retina do not cross and go to lateral
  geniculate nucleus in thalamus.
• Axons from the nasal half cross at the optic
  chiasma and continue to lateral geniculate on the
  opposite side.

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Visual pathway

• Axon collaterals of the retinal ganglion cells
  project to the midbrain.
• Participate in circuits that govern pupillary
  responses.
• Participate in circuits that coordinate head and
  eyes
• Axon collaterals project to hypothalamus where it
  regulates sleep/wake cycles.
• Axons of thalamic neurons from optic radiations
  that project to the primary visual cortex on the
  same side.

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Visual pathway

• Process information on shape of objects
• Process information on color of objects
• Process information on movement, location, and
  organization.

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Binocular vision

• The focus from two points on a single object
  produces binocular vision.
• Light rays from an object strike the same point
  on both retinas.
• This is produced by medial movement of the
  eyeballs .

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Binocular vision
View from left eye
View from right eye
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