Visual process of Eye in detail

Visual process is the series of actions that take place during visual perception. During visual process, image of an object seen by the eyes is focused on retina, resulting in production of visual perception of that object. When the image of an object in environment is focused on retina, the energy in visual spectrum is converted into electrical potentials (impulses) by rodsand cones of retina through some chemical reactions. Impulses from rods and cones reach the cerebral cortex through optic nerve and the sensation of vision is produced in cerebral cortex. Thus, process of visual sensation is explained on the basis of image formation and neural, chemical and electrical phenomena.

Visual process of Eye in detail

IMAGE FORMING MECHANISM

While looking at an object, light rays from that object are refracted and brought to a focus upon retina. Image of the object falls on the retina in an inverted position and reversed side to side. Inspite of this, the object is seen in an upright position. It is because of the role played by cerebral cortex.

Light rays are refracted by the lens and cornea.

Refractory power is measured in diopter (D). A diopter is the reciprocal of focal length expressed in meters. Focal length of cornea is 24 mm and refractory power is 42 D. Focal length of lens is 44 mm and

refractory power is 23 D.

NEURAL BASIS OF VISUAL PROCESS

Retina contains the visual receptors, which are also called light sensitive receptors, photoreceptors or electromagnetic receptors. Visual receptors are rods and cones. There are about 6 million cones and 12

million rods in the human eye. Distribution of the rods and cones varies in different areas of retina. Fovea has only cones and no rods. While proceeding from fovea towards the periphery of retina, the rods increase and the cones decrease in number. At the periphery of the retina, only rods are present and cones are absent.

STRUCTURE OF ROD CELL

Rod cells are cylindrical structures with a length of about 40 to 60 μ and a diameter of about 2 μ.

Each rod is composed of four structures:

1. Outer segment

2. Inner segment

3. Cell body

4. Synaptic terminal.

1. Outer Segment

Outer segment of rod cell is long and slender. So it gives the rod-like appearance. It is in close contact

with the pigmented epithelial cells. Outer segment of rod cell is formed by the modified cilia and it contains a pile of freely floating flat membranous disks. There are about 1,000 disks in each rod. Disks in rod cells are closed structures and contain the photosensitive pigment, the rhodopsin. Rhodopsin is synthesized in inner segments and inserted into newly formed membranous disks at the inner portion of outer segment. New disks push the older disks towards outer tip. Older disks are engulfed (by phagocytosis) from tip of the outer segment by cells of pigment epithelial layer. Thus, outer segment of rod cell is constantly renewed by the formation of new disks. Rate of formation of new disks is 3 or 4 per hour.

2. Inner Segment

Inner segment is connected to outer segment by means of modified cilium. Inner segment contains many types of organelles with large number of mitochondria.

3. Cell Body

A slender fiber called rod fiber arises from inner segment of the rod cell and passes to outer nuclear

layer through external limiting membrane. In outer nuclear layer, the enlarged portion of this fiber forms the cell body or rod granule that contains the nucleus.

4. Synaptic Terminal

A thick fiber arising from the cell body passes to outer plexiform layer and ends in a small and enlarged

synaptic terminal or body. Synaptic terminal of the rods synapses with dendrites of bipolar cells and horizontal cells. Synaptic vesicles present in the synaptic terminal contain neurotransmitter, glutamate.

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