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  • Write short notes on the following: (e) Retina

Q5.    Write short notes on the following:

        (e) Retina

Answers (1)

Retina is the innermost, neurosensory, thin layer of the eyeball. The external surface of the retina is in contact with the choroid and its inner surface is in contact with the vitreous humour. The retina is the site of image formation.

  • The external surface consists of four layers:

Pigmented layer:

  • This layer is made of single layer of cells. The cells contain dark-brown pigment.
  • Layer of photoreceptor cells:
    • It contains two types of photoreceptor cells-rods and cones.

Rods:

  • Rod cells are elongated and rod-shaped.
  • They contain a purplish-red protein pigment called rhodopsin or visual purple. Rhodopsin contains a derivative of vitamin A.
  • Rods are sensitive to dim light and provide vision in dark called twilight vision or scotopic vision.
  • Rods do not respond to colours.

Cones:

  • Cone cells are sensitive to bright light and colours. Hence, they are responsible for photopic vision or daylight vision.
  • The pigment present in the cone cells is iodopsin.
  • There are three kinds of cone cells which respond to red, green and blue light.
  • The other colours are detected by the simultaneous stimulation of more than one kind of cone cells.
  • When all the three types of cells are stimulated simultaneously, a sensation of white light is produced.
  • Cone cells are insensitive to dim light, and hence, colour cannot be recognised in the dark.

Blind spot:

  • The optic nerve leaves the brain and the retinal blood vessels enter the brain at a point where the photoreceptor cells are absent. It is called the blind spot.

Macula lutea:

  • Lateral to the blind spot is a yellowish pigmented spot called macula lutea or yellow spot.
  • It lies exactly opposite to the centre of the cornea.
  • Macula lutea has a central pit called fovea.
  • Fovea lack blood vessels and rods.
  • Fovea has only cone cells, and it is the region of most distinct vision.e cells, and it is the region of most distinct vision.
Posted by

Priyanka Kumari

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