VPB 121: Veterinary Physiology - II (2+1)

BIOCHEMISTRY OF RODS AND CONES

• Photosensitive pigment – rhodopsin / visual purple : Present in rods
• Made up of proteins viz opsins
• In Rods it is called scotopsin which consist of an aldehyde of vitamin A called retinal.
• As light strikes, Rhodopsin is transformend from CIS to TRANS and by hydrolysis, gets splits into scotopsin and retinal.
• Retinal is enzymatically changed into retinol (alcholic form) and is stored as retinyl ester in the pigment cell
• Stability of rhodopsin and its functions depends on the presence of CIS form of vitamin A. Formation of Rhodopsin is continuous, in the presence of Isomerase, retinal joins with scotopsin to form rhodopsin.
• During night/darkness- rhodopsin accumulates in rods, and have maximum sensitivity to light, this is known as dark adaptation. When exposed to light concentration of Rhodopsin decreases in rods and so rods become insensitive to light and the vision is caused by cone stimulation known as light adaptation. A vitaminosis causes night blindness viz nictalopia.
• Cones: Colour sensitive photoreceptor responsive for day vision photopigment in cones are IODOPSIN consists of opsin and RETINAL IODOPSIN varies from SCOTOSIN, in their protein part.
• Cone opsins are of 3 types
  • Blue cone (430 nm)
  • Green cone (535 nm)
  • Red cone (575 nm)
• Difference cones respond to different wavelength. The primary colours are specific with cone. Various colour sensation are made possible by the combination of different cones. Colour blindness endup is due to lack of different cones.
• Domestic animals has more rods and birds have more cones.
• ELECTRO RETINOGRAM – ERG : Recording of electrical changes in the retina by flashing the light. Has 3 waves a,b and c;
•  a wave : originates from rods and cones.
•  b wave: from glial cells, Amacrine and bipolar cells.
•  c wave: from Pigment epithelium.