Topic 109 - Vision, Cellular mechanism of light perception, color vision   Flashcards

1
Q

Words to include

A
  • Optic system
  • Photon
  • Sensation of light
    • Spatial orientation

Morphology of the eye

  • Eye bulb
  • Fibrous sclera
  • Cornea
  • Limbus
  • Vascular choroid
  • Retina
    • 10 layers
    • Light-sensing sensory epithelium
    • Ganglion cells (neurons)
    • Pigment cells
      • Light absorbing
      • Rods
      • Cones
    • Synaptic part
    • Receptor cell
    • Bipolar cells
    • Ganglion cells
    • Horizontal cells
    • Amacrine cells
  • Fovea
    • Sharp vision
    • Cones
    • Macula lutea
    • Blue light (ø absorbed)
  • Optic disc
    • N. opticus
    • Blind spot
  • Lens
    • Accommodation
      • Crystalline lens
      • Optic axis
    • Suspension of lens:
      • Ciliary muscles
      • Zonular fibers
      • Pupil
        • Sphinter muscles
        • Dilator muscles
      • Iris
        • Anterior chamber
        • Posterior chamber
        • Aqueous humor
        • Choroid plexus

Retraction of eye

  • Air - cornea
  • Cornea - aqueous humor
  • Aqueous humor - lens
  • Lens - viterous humor
  • Total refractive power

Imaging

  • Rear focal point
    • Beam intersect
  • Hypermetropia
    • Elasticity (ø)
  • Myopia
    • Near-sightedness
  • Emmetropic

Cellular mechanism of the light perception

  • Light sensation
    • Rods
    • Cones
  • Darkness
    • Cation channels
      • Depolarization
    • Dark current
      • Rest
      • Glutamate transmission
        • Abolition of glutamate release (inhibition of dark current)
    • cGMP
    • Plasma membrane
      • Sodium channels
        • Sodium ion flow (→ cell)
      • Sustained glutamatergic transmission in synapse
        • Hyperpolarization
        • Depolarization
      • Glutamate release
        • Bipolar cells
    • Light
      • Photopigment
        • 11-cis-retinal bound
        • All-trans-retinal
        • Transretinal protein opsin
        • Opsin
          • Conformation change
        • G-protein
        • Phosphodiesterase enzyme (PDE)
        • Membrane
          • Hyperpolarization
        • Glutamate transmitter release (↓)

Receptive fields

  • Illumination state
  • Ganglion cells
    • ON-type ganglion cells
      • Frequency of AP ↑
    • OFF-type ganglion cell
      • Frequency of AP ↓

Color vision

  • Cone (at least 2)
  • Blue
  • Green
  • Red
  • Retina
    • Photopigments
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2
Q

Morphology of the eye

A
  • The eye bulb is surrounded by fibrous sclera, within reaches the transparent cornea at a borderline called limbus
  • Under this layer is the vascular choroid
  • The inner layer of the eye is retina which contains light-sensing sensory epithelium and neurons (ganglion cells)
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3
Q

Morphology of the eye

Retina

A
  • 10 layers
  • Innermost layer
  • Cosists of:
    • Pigment cells (light absorbing)
    • Rods (light sensitive)
    • Cones ​(light sensitive)
    • Horizontal cells
    • Bipolar cells (nervous element)
    • Amacrine cells
    • Ganglion cells ​(nervous element)
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4
Q

Morphology of the eye

Uvea

A
  • Middle layer
  • Consists of:
    • Iris
    • Choroid
    • Ciliary body
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5
Q

Morphology of the eye

Sclera

A
  • Outer layer (together with cornea)
  • “White of the eye”
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6
Q

Morphology of the eye

Cornea

A
  • Outer layer (together with sclera)
  • Avascular
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7
Q

Morphology of the eye

Fovea

A
  • The place for sharp vision
  • Does not contain cones
  • To ensure sharp vision, the fovea does not absorp blue light
  • Location: macula lutea (humans)
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8
Q

Morphology of the eye

N. opticus

A
  • Optic disc / n. opticus
  • Where the n. opticus leaves the eye
    • Blind spot
      • Ø rods and cones
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9
Q

Morphology of the eye

Lens

A
  • Transparent
  • Biconvex
  • Role: helps to refract light to be focused on the retina
    • Accommodation: By changing chape, functions to change the focal distance of the eye so that it can focus on objects at various distances

Suspension of the lens

  • Ciliary muscles
  • Iris: regulates diameter of the pupil, separates the anterior chamber from the posterior chamber
    • Pupil
      • Mm. sphinter
      • M. dialator
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10
Q

Retraction

A
  • The eye is able to receive stimulus from a relatively wide visual field by focusing the incoming light with retractive elements:
    • Air - cornea
    • Cornea - aqueous humor
    • Aqueous humor - anterior surface of the lens
    • Anterior surface of the lens - vitreous humor
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11
Q

Imaging

A
  • Rear focal point: where the beam intersect
  • Acommodation: the eye changes optical power to maintain a clear image or focus on an object as its distance varies
    • Ciliary muscles can contract, it relaxes the zonula fibers and the convexity of the lens increases
  • Emmetropic: eye with perfect accommodation
  • Hypermetropia: loss of elasticity of the lens in elderly people
  • Myopia: near-sightedness
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12
Q

Cellular mechanism of light perception

A
  • Light sensation is triggered in the rods and cones
    • In darkness the continuously open cation channels (depolarization) maintain a dark current
  • In rest both rods and cones manitain a dark current, which sustains a continuous glutamate transmission

In darkness:

  • The system of the membranes of discs of outer segments is inactive (R, G, PDE)
  • The continuously produced cGMP keeps the sodium channels open in the plasma membrane
  • Through it sodium ions flow continuously into the cell: the dark current typical for cones and rods emerges
  • This latest (rods?) generates a sustained glutamatergic transmission in the synapse
  • The glutamate released activates or inhibits the bipolar cells, depending on the type of glutamate receptor in those bipolar cells
  • Hence the glutamatergic transmission depolarizes some cells and hyperpolarizes others

Light activates the photopigment:

  • This (rhodopsin or visual purple) is 11- cis-retinal bound to a transretinal protein, opsin
  • In the photopigment light transforms 11-cis-retinal to all-trans-retinal. This change of conformation results in conformation change of opsin and that activates the G-protein
  • G-protein activates the phosphodiesterase enzyme (PDE).
  • PDE degrades cGMP, which holds open the sodium channels in the plasma membrane
  • Hence the sodium channels close
  • This hyperpolarizes the membrane:
  • The glutamate transmitter release ceases
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13
Q

Receptive fields

A
  • The AP frequency of ganglion cells depends on the illumination state of that retinal section from which the ganglion cell receives information
  • Two types of ganglion cells are known:
    1. ON-type ganglion cells
      • Freequency of AP ↑
    2. OFF-type ganglion cells
      • ​​Freequency of AP​ ↓
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14
Q

Color vision

A
  • One cone alone is unable to code the color. It contributes the increased contrast among objects and thus the better visual acuity:
    • All colors can be generated by mixing the three colors, blue, green and red, sensed by the three photopigments of the retina
    • The basis for the sensation of mixed colors is that the three types of cones detect the light not only of a single wavelength but in a wider range
    • If from the three kinds of cones is only one type present, color vision is impossible
    • The basis of color vision is the presence of at least two cones absorbing light rays of selected wavelengths with higher efficiency
  • Scotopic vision: Rods react to the change of light intensity in the viewabel spectrum independently from the wavelength (color vision is the function of cones)
  • Role of color vision: Increased contrast among objects → better visual acuity
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