23. Biophysics of vision

Question 1

Reaction steps of light sensation.

Answer 1

light travels from the cornea
→ to the anterior chamber
→ to the lens
→ to the vitreous humor
→ to the optic nerve
→ to the photoreceptor cells.

Question 2

PHOTORECEPTORS OF THE RETINA

Answer 2

Cones are responsible for the daylight (photopic, color vision) and rods for the night (scotopic) vision.

Question 3

Structure of retina

Answer 3

• Outer layer of the eye is the sclera which at the cornea of the eye
• The middle layer - ciliary body and choroid
• The lens is held in place by the ciliary muscle fibres
• The pupil of the iris it the aperture in the middle of the iris
• The innermost layer is embedded with photoreceptors

Question 4

Function of retina

Answer 4

• Part of CNS
• Register light stimuli and transducer the excitation produced to the brain
• Interpret the visual information

Question 5

Molecular mechanism of light-sensing of rods

Answer 5

• Outer membrane contain photoreceptive disks of rods that are responsible for the absorption of light
• It is mediated by rhodopsin - a class of G protein coupled receptors
• The light-absorbing pigment of rhodopsin is the 11-cis-retinal A
  
  • Absorption maximum in solution is in the UV range
    *

Question 6

Molecular mechanism of light-sensing of rods

Answer 6

• Outer membrane contain photoreceptive disks of rods that are responsible for the absorption of light
• It is mediated by photopsins
• There are 3 different cone types with different spectral properties

Question 7

How can human eye distinguished between different colors?

Answer 7

The human eye can distinguish between different colours i.e. different frequencies of visible light

Question 8

Is color a physical property? Why?

Answer 8

Colour is a sensation, not a physical property of light i.e. the relationship between colour and frequency is not a bijective (one-one) mapping

Question 9

the Young-Helmholtz theory

Answer 9

the perception of colour is made possible by three types of spectrally different cone cells that are sensitive to red, green, and blue light

Question 10

Can all colors determine the frequency uniquely? Why?

Answer 10

Under given circumstances we can associate a colour to each frequency but not all colours determine the frequency uniquely.

Question 11

depending on the circumstances, and earlier experience, light beams of equal frequencies can produce ___ depending on the light intensity and illumination of the background.

Answer 11

different colour sensations

Question 12

depending on the circumstances, and earlier experience, light beams of equal frequencies can produce different colour sensations depending on the ___ and ___

Answer 12

1. light intensity
2. illumination of the background.

Question 13

Why the human eye can sense any color?

Answer 13

The human eye has 3 different color sensitive receptors (each is sensitive in a range of wavelengths (blue region,red-yellow region and green region). By mixing them in various weighting factors it is possible for us to sense any color.

Question 14

Where do varying spectral sensitivities of rods and the 3 types of cones arise from?

Answer 14

From their rhodopsin molecule’s differing absorption spectra

Question 15

How is the sensation of color form?

Answer 15

It is formed in the brain after complicated quantitative analysis of the proportions of hyperpolarized cones and the earlier visual experiences?

Question 16

Why don’t we see colors in twilight?

Answer 16

We do not see colors in twilight when our sight relies purely on rod cells

Question 17

Give the titles for axes

Answer 17

• y axis → relative absorption
• x axis → wavelength (nm)