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

Question 1

Words to include

Answer 1

• 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

Question 2

Morphology of the eye

Answer 2

• 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)

Question 3

Morphology of the eye

Retina

Answer 3

• 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)

Question 4

Morphology of the eye

Uvea

Answer 4

• Middle layer
• Consists of:
  
  • Iris
  • Choroid
  • Ciliary body

Question 5

Morphology of the eye

Sclera

Answer 5

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

Question 6

Morphology of the eye

Cornea

Answer 6

• Outer layer (together with sclera)
• Avascular

Question 7

Morphology of the eye

Fovea

Answer 7

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

Question 8

Morphology of the eye

N. opticus

Answer 8

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

Question 9

Morphology of the eye

Lens

Answer 9

• 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

Question 10

Retraction

Answer 10

• 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

Question 11

Imaging

Answer 11

• 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

Question 12

Cellular mechanism of light perception

Answer 12

• 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

Question 13

Receptive fields

Answer 13

• 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​ ↓

Question 14

Color vision

Answer 14

• 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