Physiology of vision
Physiology of vision is a complex phenomenon which is still poorly understood. The main mechanisms involved in physiology of vision are :
* Initiation of vision (Phototransduction), a function of photoreceptors (rods and cones),
* Processing and transmission of visual sensation, a function of image processing cells of retina and visual pathway,
* Visual perception, a function of visual cortex and related areas of cerebral cortex.
PHOTOTRANSDUCTION
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 stage. 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.
PROCESSING AND TRANSMISSION OF VISUAL IMPULSE
The receptor potential generated in the
photoreceptors is transmitted by electrotonic conduction (i.e., direct flow of electric current, and not as action potential) to other cells of the retina viz. horizontal cells, amacrine cells, and ganglion cells.
However, the ganglion cells transmit the visual signals by means of action potential to the neurons of lateral geniculate body and the later to the primary visual cortex.
The phenomenon of processing of visual impulse is very complicated. It is now clear that visual image is deciphered and analyzed in both serial and parallel fashion.
Serial processing.
The successive cells in the visual pathway starting from the photoreceptors to the cells of lateral geniculate body are involved in increasingly complex analysis of image.
This is called sequential or serial processing of visual information. Parallel processing. Two kinds of cells can be distinguished in the visual pathway starting from the ganglion cells of retina including neurons of the lateral geniculate body, striate cortex, and extrastriate cortex.
These are large cells (magno or M cells) and small cells (parvo or P cells). There are strikinging differences between the sensitivity of M and P cells to stimulus features.
The visual pathway is now being considered to be made of two lanes: one made of the large cells is called magnocellular pathway and the other of small cells is called parvocellular pathway.
These can be compared to two-lanes of a road. The M pathway and P pathway are involved in the parallel processing of the image i.e., analysis of different features of the image.
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