Photochemistry of Vision Physiology of Vision Photochemistry of Vision Dr. Sumera Gul

Learning Objectives At the end of the lecture the students should be able to: Explain the rhodopsin-visual cycle Explain the importance of Vitamin A in vision Discuss the mechanism of excitation of Rods when rhodopsin is activated by light Discuss Light and Dark adaptation

Review of Previous Lecture

Functional Segment of Rod and Cones (1) the outer segment, (2) the inner segment, (3) the nucleus, (4) the synaptic body.

In the outer segments of the rods and cones are large numbers of discs. Each disc is actually an infolded shelf of cell membrane. There are as many as 1000 discs in each rod or cone.

Absence of melanin in Albinos When they enter a bright room Light is reflected in many directions after impinging retina Visual acuity is usually 20/100 to 20/200

Photochemistry of Vision

Light- sensitive pigments Cone pigments Color Pigments Rhodopsin Visual purple

Rhodopsin-Retinal Visual Cycle After exposure to light Retinal (retinene) and Scotopsin separate

Photochemistry of Color Vision by Cones Color pigment Retinal and Photopsin 30-300 times less sensitive than rods

Color Vision Blue sensitive pigment Red sensitive pigment Green sensitive pigment Only one pigment is present in a cone

Color Vision Red: Green : Blue 99:42:0

Color Vision Red: Green : Blue 83:83:0

Color Vision Red: Green : Blue 0:0:25

Color Blindness Protanope Deuteranope Red-green color blindness X-linked recessive 8% of all women are carriers

Light and Dark Adaptation

Light Adaptation What will happen if a person has been in bright light for hours?

Light Adaptation Large portions of the photochemicals in both the rods and the cones will have been reduced to retinal and opsins. Furthermore, much of the retinal of both the rods and the cones will have been converted into vitamin A. Low concentrations of the photosensitive chemicals in the rods and cones sensitivity of the eye to light is correspondingly reduced.

Dark Adaptation What will happen if a person remains in darkness for a long time?

Dark Adaptation The retinal and opsins in the rods and cones are converted back into the light-sensitive pigments. Vitamin A is converted back into retinal to increase light-sensitive pigments.

Dark Adaptation After remaining is bright light when a person is exposed to complete darkness Sensitivity if retina is very low initially After 1 min increases 10 fold

Dark Adaptation Dark Adaptation Curve Early portion is due to adaptation of cones (4 times rapid) Rods are slowly adapting More sensitive 100 or more rods converge on a single ganglion

Light Dark Adaptation Change in pupillary size Neural Adaptation

Questions?