Welcome to the making of a movie called Sight. But our hero here is not an actor or a director…it is the humble movie screen. Hard to digest? A look at how the retina works will convince you otherwise…
The eye is made up of a number of components of which the retina is like a movie screen which shows you the picture you are seeing.
The retina (derived from the Latin word rete or net) is a sensory tissue that lines the inside of the eyeball. The light rays that travel from the object are received by the iris (the coloured part of the eye) and the lens. However, until they fall on the retina, which acts as a screen, they are just that: Rays of light. Unless our hero steps in to do his job, the brain would not be able to make head or tail of this. It is the retina that converts light into a language the brain understands: that of electrical impulses.
When we are foetuses, our hero, the retina develops as an outgrowth of the central nervous tissue. Thus it has sensory receptors as well as neurons and internal circuits. Thus, it is literally, a bit of our brain that has journeyed out to have a look at the world!
Covering an area of 1,100mm2, the retina is essentially transparent. The choroid which contains blood vessels and is located below the retina, gives it its reddish hue. (The reason of the red eye in photographs). It consists of millions of photoreceptor cells called rods and cones. Rods enable us to see in dim light while cones allow for daylight, colour and sharp vision.
The 120 million rods and 6 million cones are distributed very smartly in the retina: The centre of the retina which has the greatest resolving power of the eye is called the macula. It is about the size of a ballpoint and is responsible for central vision. It has a depression in the centre called the fovea which houses most of the cone cells. The outer portion of the retina has the rod cells which help in night vision and peripheral vision.
In the centre of the retina is the optic nerve along with the blood vessels that supply nutrition to the retina. The region where the optic nerves come together to exit the eye does not contain any photoreceptor cells. This is called the ‘Blind Spot’. To find your blind spot:
Close your left eye. Place your head 20 inches away from the monitor. Look at the dot with your right eye. Slowly move your head closer while looking at the dot. At a certain distance, the + will disappear when the + falls on your retina. Yes, our hero is a bit of a trickster too!
The retina is divided into various layers which can be simplified as the nerve fibre layer, ganglion cell (nerve cells that continue to become the optic nerve) layer, photoreceptor cell layer and retinal pigment epithelium cell layer from inside outwards. Bipolar cells are nerve cells which connect the photoreceptor cells to the ganglion cells.
The Retina converts light into neural signals by 4 basic steps:
a. Photoreception: Light passes through the inner layers of the retina to reach the photoreceptor cells, the rods and cones. An image is produced by the patterned excitation of the rods and cones.
b. Transmission to bipolar cells: When light falls on the receptor, it sends a proportional response to the bipolar cells which are in contact with photoreceptor cells and relay the impulse further.
c. Transmission to Ganglion cells: Ganglion cells receive impulses from the bipolar cells.
d. Transmission along optic nerve: The ganglion cells converge to form the optic nerve which carries the signal to the brain. Although there are more than 130 million retinal receptors, there are only about 1.2 million nerve cell fibres (called axons) in the optic nerve. This proves that a large amount of pre-processing is done within the retina. The retina manages to do this by compressing the image to fit the limited capacity of the optic nerve by a process called spatial encoding. Smart, isn’t it?
Within the optic nerve, one group of nerve cells from each eye crosses over to join the opposite optic nerve (called the optic chiasma). This helps each half of our brain get information from both the eyes.
…And this how, you ‘see’ things so effortlessly while the retina does so much in so little a time! Wouldn’t you agree that the retina, the humble movie screen is the true unsung hero of this movie?