VISION PHYSIOLOGY

Physiology of vision is a complex phenomenon which
is still poorly understood. The main mechanisms
involved in physiology of vision are :

1. Initiation of vision (Photo transduction), a function of photo receptors (rods and cones),
2. Processing and transmission of visual sensation,a function of image processing cells of retina and visual pathway, and
3. Visual perception, a function of visual cortex and related areas of cerebral cortex.

1.  PHOTO TRANSDUCTION

The rods and cones serve as
sensory nerve endings for visual sensation. Light falling upon the retina causes photochemical changes which in turn trigger a cascade of biochemical reactions that result in generation of electrical changes. Photochemical changes occuring in the rods and cones are essentially similar but the changes in rod pigment (rhodopsin or visual purple) have been studied in more detail. This whole phenomenon of conversion of light energy into nerve impulse is known as phototransduction.

 Photochemical changes
The photochemical changes include : 

• Rhodopsin bleaching. 

Rhodopsin refers to the visual pigment present in the rods – the receptors for night (scotopic) vision. Its maximum absorption spectrum is around 500 nm. Rhodopsin consists of a colourless protein called opsin coupled with a carotenoid called retinine (Vitamin A aldehyde or II-cis-retinal). Light falling on the rods converts 11-cis-retinal component of rhodopsin into all-trans-retinal through various stages . The all trans-retinal so formed is soon separated from the opsin. This process of separation is called photodecomposition and the rhodopsin is said to be bleached by the action of light. Rhodopsin regeneration. The 11-cis-retinal is regenerated from the all-trans-retinal separated from the opsin (as described above) and vitamin-A (retinal) supplied from the blood. The 11-cis-retinal then reunits with opsin in the rod outer segment to form the rhodopsin. This whole process is called rhodopsin regeneration . Thus, the bleaching of the rhodopsin occurs under the influence of light, whereas the regeneration process is independent of light, proceeding equally well in light and darkness.

•   Visual cycle. 

In the retina of living animals, under constant light stimulation, a steady state must exist
under which the rate at which the photochemicals are bleached is equal to the rate at which they are regenerated. This equilibrium between the photodecomposition and regeneration of visual pigments is referred to as visual cycle

•   Electrical changes 

The activated rhodopsin, following exposure to light, triggers a cascade of complex biochemical reactions which ultimately result in the generation of receptor potential in the photoreceptors. In this way, the light energy is converted into electrical energy which is further processed and transmitted via visual pathway

The rest of mechanisms in the upcoming posts