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What is sight? Free essay! Download now

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What is sight?

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Downloads to date: N/A | Words: 1000 | Submitted: 06-Jun-2010
Spelling accuracy: N/A | Number of pages: | Filetype: Word .doc


What is sight?


How the retina works?
The retina is sensitive to light in the wavelength of 380 – 760 nm. The retina contains 2 different cells that are responsible for capturing the light intensity, the rod and cones. There are approximately 6 million cone cells contained with in the retina, they are mostly dense with in the fovea this is the area in which is mostly accurate. Cones function best in bright light and allow us to appreciate colour.

There are approximately 120 million rod cells in the retina, they are spread evenly throughout the retina and function well in dim lighting. The rods are responsible for night vision. The type of visual pigment molecule in rods is known as rdodpin. This molecule is made up of a protein (opsin) and a light absorbing compound derived from vitamin A, called retinal. If we do not have enough vitamin A in our blood, then there is a chance of night blindness occurring.

Retina at work
1. In the dark sodium ions flow out through outer channels in the out segment of the retina.
2. At the same time, the plasma membrane of the inner segment reverses this movement of ions- sodium ions are continuously pumped out at this point. Here ATP formed in the mitochondria of the inner segment.
3. A concentration gradient is maintained between outer and inner segment, down which sodium ions flow.
4. The influx of sodium ions at the inner segment slightly depolarises the cell, and the potential difference across the plasma membrane is about -40mV.
5. The rod cell releases a neurotransmitter substance (glutamate) that binds to the bipolar cells and prevents its depolarisation.
6. No action potential in generated in the optic nerve that synapses with that bipolar cell.

When light falls on the retina, it causes rhodopsin to break down into retinal and opsin this is known as bleaching. Opsin therefore becomes an enzyme that activates reactions to close the cation channels of the outer segment, and so the flow of sodium ions is blocked. The inner segment continuous to pump out sodium ions, this causes the interior of the rod to become more negative (hyperpolarisation). In this condition no neurotransmitter is released by the rod cell and the bipolar cell becomes depolarised.

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