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We all have some understanding of the importance of the eye. We rely on sight every day. Studying the eye can help us understand the basis of eye problems such as conjunctivitis, colour blindness, and cataracts. It’s a fascinating topic to learn more about.

 How our eyes work

Diagram: Ruth Lawson
  • Light enters the eye through the pupil (part of the iris) and is focused onto the fovea (focal point) through a combination effort between the cornea and the lens.
  • The cornea acts as an ‘outer lens’ and bends incoming light onto the lens. The lens then refocuses this light onto the first layer of cells in the retina as an upside-down image.
  • Cells in the retina convert the light into a series of electrical impulses that are sent via the optic nerve to the brain. The brain adds all these impulses signals together to re-form the upside-down image and makes sense of it by turning it up the right way.

 

Focus on Chemistry

Chemistry is a crucial part of the third step, where the retinal image is broken down into neural signals that are sent to the brain.

The retina is made up of several layers of cells: a layer of ganglion cells (which make up the optic nerve), a layer of neural cells and a layer of rods and cones.

Light must first travel to the rods and cones at the back of the retina, where some really interesting chemistry takes place to break the image into electrical impulses. These impulses are then transmitted to the neural cells, then to the optic nerve and the brain. This process is called phototransduction.

Chemistry in Rods and Cones

Picture: Wikipedia Commons

There are some 120 million rods and 6 million cones in the retina. Rods are responsible for night vision, sensitive motion detection and peripheral vision. Cones are responsible for colour vision.

A light sensitive pigment called rhodopsin is found in the rods and cones.

Rhodopsin is made up of one of several types of opsin proteins and a small molecule called retinal, chemically bound together. The different types of opsin proteins are designed to detect, among other things, different wavelengths of light and so allow us to see in colour.

 

Chemistry in the Retina

Light hitting a molecule of rhodopsin causes isomerisation (a change in shape) of the pigment as it gains energy.

This energy-rich rhodopsin molecule then transfers energy to neural cells, which causes them to start an electrical impulse that travels along the optic nerve to the brain to paint a very small part of a large picture.

This process occurs in the millions of rod and cone cells in the retina, resulting in information about an overall image being sent to the brain.

Meanwhile, the excited rhodopsin molecule has lost its extra energy and splits into its two parts of opsin and chemically changed retinal. A complicated chemical process then regenerates the original retinal molecule and joins it back to the opsin protein to make rhodopsin again.

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