Describe the function of the retina (basic)
Key Terms
The pupil reflex (dim light)
Radial muscles of iris contracted
Circular muscles of iris relaxed
Dilated pupil (more light enters eye)
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The pupil reflex (bright light)
Radial muscles of iris relaxed
Circular muscles of iris contracted
Contracted pupil (less light enters eye)
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| Term | Definition |
|---|---|
Describe the function of the retina (basic) | Contains the light receptors |
Describe the function of the Optic nerve. | Carries impulses between the eye and the brain |
Describe the function of the Sclera. | Tough white outer layer of the eye - it helps protect the eye from injury |
The retina (detailed) | Light passes through the eyeball to the retina. There are two main types of light receptors called rods and cones. Rods are more sensitive to light than cones so they are useful for seeing in dim light. There are three different types of cone cells which produce colour vision. |
The pupil reflex (dim light) |
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The pupil reflex (bright light) |
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Accommodation to focus on a near object. | The lens becomes thicker which allows the light rays to refract (bend) more strongly. |
Accommodation to focus on a distant object. | The lens is pulled thin which allows the light rays to refract slightly. |
What happen if the position of the object is near? (in terms of structure) | Ciliary muscles - contract
Suspensory ligaments - slacken/loosen
Muscle tension on lens - low
Lens shape - fat/thicker
Refraction of light rays - Light is refracted strongly |
What happens if the position of the object is distant? (in terms of structure) | The ciliary muscles relax - relax Suspensory ligaments - stretched/tighten Muscle tension on lens - high Lens shape - thin Refraction of light rays - Light is only refracted slightly |
Correcting vision defects | Two common defects of the eyes are myopia (short-sightedness) and hyperopia (long-sightedness). In both cases, rays of light do not focus on the retina so a clear image is not formed.
These two defects are treated with spectacle lenses, which refract (bend) the light rays so that they do focus on the retina. |
myopia | short sighted-ness |
Short sight | Someone who is short-sighted can see near objects clearly, but cannot focus properly on distant objects.
Short sight is caused by one of the following:
the eyeball being elongated - so that the distance between the lens and the retina is too great.
the lens being too thick and curved - so that light is focused in front of the retina.
Short-sightedness can be corrected by placing a concave lens in front of the eye. |
Hyperopia | Long sighted-ness |
Long sight | Someone who is long-sighted can see distant objects clearly, but they cannot focus properly on near objects.
Long-sightedness is caused by one of the following:
the eyeball being too short - so the distance between the lens and retina is too small
a loss of elasticity in the lens - meaning it cannot become thick enough to focus (which is often age-related)
As a result, the lens focuses light behind the retina instead of onto it. Long-sightedness is corrected by putting a convex lens in front of the eye. |
Alternatives to wearing spectacle lenses. | New technologies have provided alternatives to wearing spectacle lenses: hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye. Contact lenses work by being in ‘contact’ with your eye. They float on the surface of the cornea. They work like spectacle lenses, by focusing and refracting the light. Also:
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What is a cataract and how can it be treated? | A cataract is a cloudiness in the lens, caused by a build up of protein inside the lens. Cataracts can be treated by removing the lens and replacing with a plastic lens. |
What type of cells start electrical impulses along nerves? | Receptor |
What part of a nerve cell insulates the electrical signal? | Myelin sheath |
What neurones carry signals to effectors? | Motor neurones |
Where do electrical signals go in a reflex arc? | Spinal cord |
What affect do stimulants like caffeine have on your reaction times? | Stimulants like caffeine decrease your reaction times. (Decreasing reaction times means faster reactions. Increasing reaction times means slower reactions.) |
What part of the brain controls balance and coordination of movement? | Cerebellum. |