Photoreception

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Photoreception

• Rods and Cones

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Rods and Cones

• The retina
• The delicate inner layer of the eye contains the
  photoreceptor cells.
• There are two types the rods and the cones

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Rods and Cones

• Both transduce light into chemical energy
• Each contains a photochemical pigment in its
  outer segment

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Rods and Cones

• Cones are high light intensity colour receptors
• Located mainly near the yellow spot
• There are no rods here
• Fact they are packed so closely (125 000 per mm3
  ) results in high definition

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Rods and Cones

• Each cone has its own single connection to the
  optic nerve
• So for each cone stimulated an impulse is passed
  to the brain
• This results in high visual acuity

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Rods and Cones

• Many rods connect with a single bipolar neurone
• So impulses from a number of rods summate before
  triggering an impulse to the brain giving low
  visual acuity
• This feature of rods is called retinal convergence

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Rods and Cones

• Rods are distributed more or less evenly over the
  retina
• They are highly concentrated in the periphery.

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Rods and Cones

• Rods and cones are made up of an inner and outer
  segment
• The outer segment contains many membranes
  carrying a photosensitive pigment rhodopsin in
  rods, iodopsin in cones
• The inner segment contains the nucleus and
  mitochondria

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Rods and Cones

• Rods are sensitive to low light
• Rhodopsin (or visual purple) is made up of a
  protein, opsin and a light absorbing Vit. A
  derivative, retinine
• When hit by light rhodopsin splits into its
  components a process called bleaching
• This depolarises the membrane of the rod and
  results in an impulse sent to the brain.

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Rods and Cones

• Before the rod can be used again the rhodopsin
  has to be reformed
• This requires energy in the form of ATP

A truck load of ATP!
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Rods and Cones

• Cones are high light intensity colour receptors
• Their visual pigment is less easily broken down
  and takes longer to remake.

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Rods and Cones

• In bright light most of the rhodopsin in rods is
  bleached and the eye is said to be light adapted
• In low light vision is poor as it takes the rods
  time to resynthesise rhodopsin.

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Rods and Cones

• This is why your eyes need time to adjust when
  you enter a dark cinema on a sunny day.
• Once the rods have resynthesised rhodopsin your
  retina is said to be dark adapted

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About 120 X 106
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About 120 X 106
About 6 X 106
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
Affected by low light intensities
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
Affected by low light intensities
Affected only by light of high intensities
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
Affected by low light intensities
Affected only by light of high intensities
Unable to distinguish colour
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
Affected by low light intensities
Affected only by light of high intensities
Unable to distinguish colour
Able to distinguish colour
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
Affected by low light intensities
Affected only by light of high intensities
Unable to distinguish colour
Able to distinguish colour
Synapse in groups
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About 120 X 106
About 6 X 106
Evenly spread throughout the retina - except for
the fovea (yellow spot)
Tightly packed at fovea
Rod shaped
Cone shaped
Single pigment
Three pigments (but each cone only contains one)
Affected by low light intensities
Affected only by light of high intensities
Unable to distinguish colour
Able to distinguish colour
Synapse in groups
Synapse individually
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Yes No No
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red
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Yes No No
red No Yes
No
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Yes No No
red No Yes
No
green
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Yes No No
red No Yes
No
green No
No Yes

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red No Yes
No
green No
No Yes
blue
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow No
Yes Yes
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow No
Yes Yes
cyan
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow No
Yes Yes
cyan Yes
No Yes
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow No
Yes Yes
cyan Yes
No Yes
magenta
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow No
Yes Yes
cyan Yes
No Yes
magenta
Yes Yes
Yes
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Yes No No
red No Yes
No
green No
No Yes
blue Yes
Yes No
orange / yellow No
Yes Yes
cyan Yes
No Yes
magenta
Yes Yes
Yes
white