Cone cell structure and function

STRUCTURE OF CONE CELL

Cone cell is the visual receptor with length of 35 μ to 40 μ and a diameter of about 5 μ. Generally, the cone cell is flask shaped. Shape and length of the cone vary in different parts of the retina. Cones in the fovea are long, narrow and almost similar to rods. Near the periphery of retina, cones are short and broad. Like rods, cones are also formed by four parts:

Cone cell structure and function


1. Outer segment

2. Inner segment

3. Cell body

4. Synaptic terminal.

1. Outer Segment

Outer segment is small and conical. It does not contain separate membranous disks as in rods. In cone, the infoldings of cell membrane form saccules, which are the counterparts of rod disks. Photopigment of cone is synthesized in the inner segment and incorporated into the folding of surface membrane forming saccule. Renewal of outer segment of cone is a slow process and it differs from that in rods.

It occurs at many sites of the outer segment of cone.

2. Inner Segment

In cones also, the inner segment is connected to outer segment by a modified cilium as in the case of rods. Though various types of organelles are present in this segment, the number of mitochondria is more.

3. Cell Body

Cone fiber arising from inner segment is thick and it enters the inner nuclear layer through external limiting membrane. In the inner nuclear layer, cone fiber forms the cell body or cone granule that possesses nucleus.

4. Synaptic Terminal

Fiber from cell body of cone leaves the inner nuclear layer and enters outer flexiform layer. Here, it ends

in the form of an enlarged synaptic terminal or body. Synaptic vesicle present in the synaptic terminal of cone cell also possesses the neurotransmitter, glutamate.

FUNCTIONS OF RODS AND CONES

Functions of Rods

Rods are very sensitive to light and have a low threshold. So, the rods are responsible for dim light vision or night vision or scotopic vision. But, rods do not take part in resolving the details and boundaries of objects (visual acuity) or the color of the objects (color vision). Vision by rod is black, white or in the combination of black and white namely, grey. Therefore, the colored objects appear faded or greyish in twilight.

Functions of Cones

Cones have high threshold for light stimulus. So, the cones are sensitive only to bright light. Therefore, cone cells are called receptors of bright light vision or daylight vision or photopic vision. Cones are also responsible for acuity of vision and the color vision.

Achromatic Interval

When an object is placed in front of a person in a dark room, he cannot see any object. When there is

slight illumination, the person can see the objects but without color. It is because, at this level, only rods are stimulated. When, the illumination is increased, the threshold for cones is reached. Now, the person can see the objects in finer details and in color. Interval between the threshold for rods and cones, i.e. interval from when an object is first seen and the time when that object is seen with color is called achromatic interval.

CHEMICAL BASIS OF VISUAL PROCESS

Photosensitive pigments present in rods and cones are concerned with chemical basis of visual process.

Chemical reactions involved in these pigments lead to the development of electrical activity in retina and

generation of impulses (action potentials), which are transmitted through optic nerve. Photochemical changes in the visual receptor cells are called Wald visual cycle.

RHODOPSIN

Rhodopsin or visual purple is the photosensitive pigment of rod cells. It is present in membranous disks

located in outer segment of rod cells.

Chemistry of Rhodopsin

Rhodopsin is a conjugated protein with a molecular weight of 40,000. It is made up of a protein called

opsin and a chromophore. Opsin present in rhodopsin is known as scotopsin. Chromophore is a chemical substance that develops color in the cell. Chromophore present in the rod cells is called retinal. Retinal is the aldehyde of vitamin A or retinol. Retinal is derived from food sources and it is not

synthesized in the body. It is derived from carotenoid substances like β-carotene present in carrots.

Retinal is present in the form of 11-cis retinal known as retinine 1. Retinine 1 is present in human eyes. It

is different from retinine 2 that is present in the eyes of some animals. Significance of 11-cis form of retinal is that, only in this form it combines with scotopsin to synthesize rhodopsin.

Photochemical Changes in Rhodopsin – Wald Visual Cycle

When retina is isolated and examined in dark, the rods appear in red because of rhodopsin. During exposure to light, rhodopsin is bleached and the color becomes yellow. When rhodopsin absorbs the light that falls on retina, it is split into retinine and the protein called opsin through various intermediate photochemical reactions. Following changes occur due to absorption of light energy by rhodopsin:

1. First, rhodopsin is decomposed into bathorhodopsin that is very unstable

2. Bathorhodopsin is converted into lumirhodopsin

3. Lumirhodopsin decays into metarhodopsin I

4. Metarhodopsin I is changed to metarhodopsin II

5. Metarhodopsin II is split into scotopsin and all-trans retinal

6. All-trans retinal is converted into all-trans retinol (vitamin A) by the enzyme dehydrogenase in the presence of reduced nicotinamide adenine dinucleotide (NADH2).

Metarhodopsin is usually called activated rhodopsin since it is responsible for development of receptor

potential in rod cells.

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