Question

explain how a light stimulus falling on the retina is transduced into a neural signal and transmitted to the higher centre of the brain. Include in your answer a description of the receptors and neural pathways involved.

Answer 1

Visual sensation is a special sensation _It requires receptors,sensory pathway and brain involvement.

Rods and cones are photoreceptors present in retinal layer.Visual pathway starting from retina consists of Optic nerve_optic chaisma_optic tracts_lateral geniculate bodies_optic radiations_and visual cortex(occipital lobe-area 17,18,19).Optic berve is the second cranial nerve.Axons of ganglion cells form optic nerve(second order neuron).

photoreceptors_bipolar cells_ganglion cells_optic nerve.(way of trans mission leading to visual pathway)

Main mechanisms involved in physiology of vision are_initiation of vision(phototransduction),a funcyion of photo receptors(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_rods and cones act as sensory nerve ending and receive signal.Light falling upon retina causes photochemical changes which triggers a cascade of biochemial reactions that result in generation of electical changes.This whole phenomenon of conversion of light energy into nerve impulse is known as phototransduction(includes photochemical changes and electrical changes).

Photochemical changes process are  shown in the flow chart and visual cycle given in the picture it self.To summarize_rhodopsin refers to visual pigment present in rods_receptor for night (scotopic)vidion.It consists of a colorless protein called opsin coupled with a carotenoid called retinine(vitamin A aldehyde or 11-cis retinal.Light falling upon rods convert 11-cis retinol component of rhodopsin into 11-cis retinal through various stages as shown in figure.The All trans retinal so formed is soon separated from opsin called photodecomposition,rhodopsin is said to be bleached by action of light.

The11-cis retinal is regenerated from all trans retinal separated from opsin,which again reunites with opsin to form rhodopsin(called rhodopsin regeneration).this is independent of light.

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

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

This receptor potential is transmitted by electronic conduction(that is direct flow of current and not as action potential)to other cells of retina-horrizontal celks,amacrine cells,ganglion cells. Tgese ganglion cells transmitt the visual signal by means of action potential to the neurons of lateral geniculate body and later to the primary visual cortex both as serial and parallel processing.These are perceived and interpreted in these higher brain areas.