Coordination and Response in Plants and Animals The Structure and Function of the Eye

Receptors, Effectors and the Central Nervous System Syllabus Objectives Relate the structure of the eye in man to its function as a sense organ Explain sight defects and their corrections

Introduction The human eye is a highly specialized sense organ It detects the stimulus of light It is protected by a bony socket (orbit) in the skull Tear glands under the upper eyelids secrete tears Which moistens Lubricates Protects the eye as blinking occurs The eye is made up of 3 layers: Sclera/sclerotic coat Choroid layer Retina

Sclera/sclerotic coat Tough outer white layer Protects the eye Continues over the front of the eye as transparent structure cornea (helps refract light rays which enter the eye) The conjunctiva is a thin transparent membrane Lines the eyelids and the exposed part of the front of the eyeball

Choroid layer Pigmented middle payer Prevents reflection of light in the eye Contains numerous blood vessels which supply nutrients to the tissues in the eye Forms ciliary body and the iris at the front of the eye Ciliary body is connected to the lens by suspensory ligaments

Choroid layer cont’d The ligaments and the ciliary muscles work together to adjust the shape of the lens The lens a structure that is elastic transparent and Biconvex The lens bends light rays to a point of focus on the retina

Choroid layer– the iris and pupil In front of the lens is the iris The coloured part of the eye It is the circular disc of muscles (circular & radial muscles) It surrounds a central hole – the pupil The iris controls the amount of light entering the eye by adjusting the size of the pupil

Choroid layer cont’d In front of the lens there is a transparent liquid – aqueous humour Behind the lens is a transparent jelly-like material – vitreous humour The aqueous and vitreous humours help maintain the shape of the eyeball

Retina The inner layer contains light sensitive cells Rods (sensitive to dim light. Allows black and white vision) Cones (sensitive to bright light. Gives colour vision) The fovea or yellow spot is directly behind the pupil Only contains cones

Retina – rods and cones There are more rods towards the outer edges of the retina Rods and cones send impulses to the brain along the optic nerve There are no light sensitive cells at the point where the optic nerve leaves the eye – blind spot If light rays fall on this spot no impulses will be sent to the brain

Functioning of the Eye - Introduction http://www.youtube.com/watch?v=JadaWSDxBYk&eurl=http://video.google.com/videosearch?q=how+eye+works&hl=en&emb=0&aq=f&feature=player_embedded The eye is the sense organ which receives the stimulus of light Light rays from objects enter the eye and are refracted (bent) by the cornea These light rays pass through the aqueous humour, the lens and the vitreous humour where further refraction occurs The light rays then fall on the retina where an image is formed The rods and cones then sends information to the brain along the optic nerve The brain interprets what is seen

The Functioning of the Eye - Accommodation Light rays from an object spread out in all directions Rays from distant objects are almost parallel to each other Rays from near objects are more spread out For an object to be seen clearly and in detail Rays must be bent (refracted) to be focused exactly on the retina Light rays from near objects therefore need more refraction than rays from distant objects

The Functioning of the Eye - Accommodation The eye changes the degree of refraction by changing the shape of the lens This results in the production of clear images of both near and distant objects The shapes of the lens is altered by the contraction and relaxation of the ciliary muscles The amount of bending caused by the cornea, aqueous and vitreous humours is fixed The ability of the eye to form sharp images on the retina of near and distant objects is called accommodation!

The Functioning of the Eye - Accommodation When viewing distant objects the lens gets thinner (less convex) Relaxation of the ciliary muscles causes increased tension on the suspensory ligament The suspensory ligaments now pull on the lens making it long and thin A thin lens is less refractive and therefore focus distant objects

The Functioning of the Eye - Accommodation When viewing near objects the lens gets thicker (more convex) Contraction of the ciliary muscles causes a release of tension on the suspensory ligaments Because the lens is elastic, it becomes short and fat This increases the refractive power of the lens and allows near objects to be focussed

The Control of Light Entering The Eye The amount of light that enters the eye is determined by the size of the pupil The diameter of the pupil is in turn controlled by the muscles in the iris The iris has muscles arranged in 2 layers: Circular and Radial muscles These 2 sets of muscles are antagonistic in action

The Control of Light Entering The Eye – In bright light The diameter of the pupil decreases This prevents damage to the retina The circular muscles of the iris and the radial muscles relax Allowing only a small amount of light to enter the eye

The Control of Light Entering The Eye – In dim light The radial muscles of the iris contract and the circular muscles relax The diameter of the pupil increases More light enters the eye Allowing a brighter image to be formed in dim light

Sight defects and their corrections – normal vision A normal eye can accommodate to objects at different distances by changing the shape of the lens and its refractive power

Sight defects and their corrections – short sight The eyeball is longer than normal (from back to front) or the cornea too curved Light from a near object is clearly focused on the retina Rays from a distant object are focused in front of the retina and produce a blurred image A concave lens which converge the rays from a distant object and bring them to a point of focus on the retina

Short sight correction

Sight defects and their corrections – long sight This occurs when the eyeball is shorter (from back to front) than normal or the lens is too fat Light from a distant object is clearly focused on the retina However, the image from a near object is focused behind the retina and produces a blurred image A convex lens converges the light rays and causes the image from a near object to be focused on the retina

Long sight correction

Aging and sight defects With age the lens become hardened and less elastic Ability to accommodate for near vision (reading) decreases Reading glasses with convex lens corrects this condition

Cataracts This is the most common cause of blindness It results when normal transparent lens becomes opaque This prevents light from passing through and reduces vision It can be surgically treated by removing the damaged lens and replacing it with artificial lens implanted into the eye or using spectacles or contact lens

Glaucoma The aqueous humour in front of the lens is usually drained away from the eye so that it does not build it When the flow of aqueous humour becomes blocked, too much of it collects in the eye and the pressure increases within the eyeball The high pressure causes damage to the optic nerve and normal vision is reduced, eventually leading to blindness in most cases This condition can be controlled with the use of eye drops and or tablets which help to lower the pressure Surgical treatment and lasers can also be used to drain the fluid from the affected eye