Eye Retina and Neural Mechanisms Prof K Sivapalan

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Eye: Retina and Neural Mechanisms. Prof. K. Sivapalan

Eye: Retina and Neural Mechanisms. Prof. K. Sivapalan

Retina. • Retina is organized in 10 layers recognizable by microscopy between the choroid

Retina. • Retina is organized in 10 layers recognizable by microscopy between the choroid and the vitrious body. • The pigment epithelium containing melanin [black] is next to choroid. • It absorbs light that escapes the receptors. – Albinos can’t form clear image because of reflection at the wall. • Next are the receptors and cell bodies. • They synapse with the bipolar cells where horizontal cells also synapse. • Amacrine cells are seen to synapse with bipolar cells and ganglion cells. • Ganglion cell is second order neuron. 6/12/2021 Retina and Neural Mechanisms Choroid. 2

Retina ctd. • Retina extends from optic nerve to ciliary body. Not on optic

Retina ctd. • Retina extends from optic nerve to ciliary body. Not on optic disc- blind spot. • Macula is a yellowish area, lateral to the optic disc, with high proportion of cones. • Fovea centralis is at the center of Macula, at the visual axis with no blood vessels and minimal other cells- thin area, ≈1 mm 2. 6/12/2021 Retina and Neural Mechanisms 3

Receptors. • Rods and cones have rod and cone like outer segment. • The

Receptors. • Rods and cones have rod and cone like outer segment. • The disks are invaginated membrane embedded with light sensitive pigments. • Inner segment has nucleus, mitochondria and sodiumpotassium pumps in the membrane. • The synaptic terminal communicates with bipolar cells. 6/12/2021 Retina and Neural Mechanisms 4

Receptor Mechanism. • The photosensitive compound is made up of a protein [opsin] and

Receptor Mechanism. • The photosensitive compound is made up of a protein [opsin] and retinine [11 cis retinal]. • In rods, the protein is rhodopsin and in cones similar but three different cone pigments. • Light changes the 11 cis retinal into all trans retinal which dissociates from opsin. • This results in closure of sodium channels through GMP mechanism and results in hyper polarization which is felt in the synaptic terminals also. • Hyper polarization reduces release of transmitter [glutamate]. This hyper polarizes the bipolar cell. • The ganglion cells fire action potentials to be conducted centrally. • An enzyme, Isomerase, converts all trans retinal into 11 cis retinal which combines with opsin to form rhodopsin. 6/12/2021 Retina and Neural Mechanisms 5

Rods. • Rhodopsin in rods is very sensitive to light- even one photon can

Rods. • Rhodopsin in rods is very sensitive to light- even one photon can activate rods. • Rods are, therefore, useful for vision in dark and the pigment is bleached in bright light which inactivates rods. • There are no rods in fovea. In 10° area its concentration is 160, 000/mm 2 and 30, 000/mm 2 in the periphery of the retina. Total is about 120 million. • If you look at some thing directly in dark, you can’t see it because the image falls on cones which are not stimulated at dim light. 6/12/2021 Retina and Neural Mechanisms 6

Cones. • Retina contains three different types of cones: blue, green and red. •

Cones. • Retina contains three different types of cones: blue, green and red. • Concentration of cones in fovea is 147, 000/mm 2, 5000/mm 2 in 10° area and none in periphery. Total is about 6 Million. • They are less sensitive compared to rods and used for color vision and vision in lighted area. • No convergence in fovea: one receptor - one bipolar cell - one ganglion cell. 6/12/2021 Retina and Neural Mechanisms 7

Sensitivity of Receptors and Color. • Wave length of visual light ranges from 397

Sensitivity of Receptors and Color. • Wave length of visual light ranges from 397 to 723 nm. • Rods respond to light of wave length around 410 to 590 nm, maximal 505 nm. • They do not respond to red light. • Blue, green and red cones respond to different ranges of wave lengths, maximally 445, 535 and 570 nm. • Different wave lengths give rise to different levels of stimulation in the cones. • Perception of color depends on integration of the stimulation from the three cones. Rods don’t contribute to color vision. 6/12/2021 Retina and Neural Mechanisms 8

Role of Horizontal and Amacrine Cells • Three images are formed in retina: light

Role of Horizontal and Amacrine Cells • Three images are formed in retina: light on receptors, in bipolar cells and in ganglion cells. • The image in bipolar cells is altered by horizontal cells and image in ganglion cells by Amacrine cells. • These cells are inhibitory in nature. • Probably most potent stimuli pass on and the surrounding areas inhibited making demarcation easy. 6/12/2021 Retina and Neural Mechanisms 9

Ganglion cells. • There about 120 million rods and 6 million cones in one

Ganglion cells. • There about 120 million rods and 6 million cones in one eye but only 1. 2 million ganglion cells. • The convergence is one is to one in the central fovea and it increases towards periphery. • 40 % of the ganglion cells are W cells. They receive signals from rods which are sensitive to movements in the field of vision and vision in dark. • 55 % are X cells. They have small receptor fields, transmit visual images, color vision and texture. • 5 % are Y cells. They have large receptor fields, pick up signals from wider areas of the retina, respond to rapid change in image and intensity. • Probably the connections of Y cells focus the vision on events in the field. 6/12/2021 Retina and Neural Mechanisms 10

Central connection. • Axons of ganglion cells form optic nerve. • Fibers from nasal

Central connection. • Axons of ganglion cells form optic nerve. • Fibers from nasal half of the retina, divided across the centre of the fovia, cross at the optic chiasm. • Fibers from temporal half do not cross. Visual fields are represented in the opposite cortex. • Optic tracts synapse in lateral geniculate bodies. • Axons go to primary visual cortex. • Number of fibers in optic radiation is double the number of that in optic tract. – divergence. • Collaterals from optic chiasm go to hypothalamus and from optic tract to superior colliculus of mid brain and mediate visual reflexes. 6/12/2021 Retina and Neural Mechanisms 11

Cortical Representation. • Primary visual cortex is on either side of the calcarine fissure.

Cortical Representation. • Primary visual cortex is on either side of the calcarine fissure. • Macula is represented in the occipital pole and the rest of the retina anterior to it. • Processing motion, recognition of objects, color vision and other aspects take place in association areas and other parts of the cortex. 6/12/2021 Retina and Neural Mechanisms 12