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

Retina and Neural Mechanisms Choroid. 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. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms 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 mm2. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms 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 sodium-potassium pumps in the membrane. The synaptic terminal communicates with bipolar cells. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms 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. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms 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/mm2 and 30,000/mm2 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. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms Cones. Retina contains three different types of cones: blue, green and red. Concentration of cones in fovea is 147,000/mm2, 5000/mm2 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. 6/19/2019 Retina and Neural Mechanisms

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. 6/19/2019 Retina and Neural Mechanisms

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. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms Ganglion cells. There are 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. 6/19/2019 Retina and Neural Mechanisms

Retina and Neural Mechanisms 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. 6/19/2019 Retina and Neural Mechanisms

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. 6/19/2019 Retina and Neural Mechanisms