The eye: structure, reflexes and defects

In a nutshell

The eye is a sense organ that allows us to see. It has clever reflexes that adjust for certain levels of light to protect our eyes and for near or distant objects. Sometimes our eyes don't work properly, this can lead to short or long-sightedness. These vision defects can be corrected with glasses, contact lenses, laser eye surgery or replacement lens surgery.

Anatomy of the eye

1.	Suspensory ligaments	Control the shape of the lens.
2.	Iris	The iris contains muscles that allow it to control the diameter of the pupil and therefore how much light enters the eye.
3.	Cornea	The transparent outer layer found at the front of the eye. It refracts (bends) light into the eye.
4.	Pupil	The hole in the middle of the iris.
5.	Lens	This focuses light on the retina.
6.	Ciliary muscles	These control the shape of the lens.
7.	Sclera	The tough outer supporting wall of the eye.
8.	Retina	Contains receptor cells sensitive to light intensity and colour.
9.	Optic nerve	Carries impulses from the receptors on the retina to the brain.

The iris reflex

Very bright light can damage the retina. The iris reflex adjusts for bright light to protect it.

When light receptors in the eye detect very bright light, the reflex is triggered and the pupil is made smaller. The circular muscles in the iris contract and the radial muscles relax. This reduces the amount of light that can enter the eye. In dim light, the radial muscles contract and the circular muscles relax, this makes the pupils wider.

Accommodation

Accommodation is the process where the eye changes the shape of the lens so the eye can focus light on the retina.

Near objects

Biology; Cells and control; KS4 Year 10; The eye: structure, reflexes and defects

i	The ciliary muscles contract (1.), which slackens the suspensory ligaments (2.).
ii	The lens becomes curved and fatter.
iii	This increases the amount by which it refracts light.

Distant objects

Biology; Cells and control; KS4 Year 10; The eye: structure, reflexes and defects

i	The ciliary muscles relax (1.), causing the suspensory ligaments to pull tight (2.).
ii	The lens becomes less curved and thinner.
iii	It refracts light by a smaller amount .

Vision defects

Sometimes the eye doesn't work properly. This means people can be long-sighted or short-sighted, have cataracts or be colour blind.

Long-sighted

Long-sighted people cannot focus on near objects. The medical term for this is hyperopia.

i	This occurs when the lens is the wrong shape and doesn't refract the light enough or the eyeball is too short.
ii	The images of near objects are brought into focus behind the retina.
iii	Glasses with a convex lens (curves outwards) can correct long-sightedness. This lens refracts the light rays so they can focus on the retina.

Biology; Cells and control; KS4 Year 10; The eye: structure, reflexes and defects — A shows uncorrected long-sightedness where 1. is the object and 2. is the image. B shows corrected long-sightedness where 1. is the object, 2. is the image and 3. is the lens.

Short-sighted

Short-sighted people cannot focus on distant objects. The medical term for this is myopia.

1.	This occurs when the lens is the wrong shape and refracts light too much or the eyeball is too long.
2.	The images of distant objects are brought into focus in front of the retina
3.	Glasses with a concave lens (curves inwards) can correct short-sightedness. This causes light rays to focus on the retina.

Biology; Cells and control; KS4 Year 10; The eye: structure, reflexes and defects — A shows uncorrected short-sightedness where 1. is the object and 2 is the image. B shows corrected short-sightedness where 1. is the object, 2. is the image and 3. is the lens.

Cataracts

Cataracts occur when the lens becomes cloudy, so light cannot pass through it properly. The person receives a blurred image on the retina. They can be treated by replacing the cloudy lens with an artificial lens.

Colour blindness

A normal retina contains three types of cone that detect red, green and blue light. The colour of an image is detected by how much each type of cone is stimulated.

When a person is colour blind, at least one type of cone is missing or doesn't work properly. In red-green colour blindness, either the red or green cone is missing and the person cannot distinguish between red and green. This is an inherited condition that mostly affects males.

Treatments for vision defects

There are some alternatives to wearing glasses for people with long-sightedness and short-sightedness.

Contact lenses

Contact lenses are thin lenses that sit on the surface of the eye and are shaped to make up for the fault in focusing. They are lightweight, invisible and convenient for sports so are a popular choice. The two types are hard lenses and soft lenses. Soft lenses are usually more comfortable but have an increased risk of eye infections.

Laser eye surgery

A laser is used to vaporise tissue. This changes the shape of the cornea which changes how strongly it refracts light into the eye. Slimming down the cornea makes it less powerful and improves short sight. Changing the shape of the cornea so it is more powerful will improve long sight.

This is effective as the surgeon can precisely control how much tissue the laser takes off but like all surgical procedures, there is a risk of complications such as infection or a reaction in the eye that causes your vision to be worse than before.

Replacement lens surgery

In replacement lens surgery, the natural lens of the eye is removed and an artificial lens made of clear plastic is put in its place. This may be a more effective treatment for long-sightedness for some people but this carries higher risks than laser eye surgery as it involves working directly inside the eye. One risk is possible damage to the retina, as this can lead to complete loss of sight.