The eye is the organ of sight. It gathers light from the environment and forms an image on the nerve cells of the retina. The image is then transformed into nerve impulses which are interpreted by the brain. All the senses have a highly specialized receptors enabling them to respond to the appropriate stimuli.
Chemoreceptors: detect chemical stimuli
Photoreceptor: detect light stimuli
Mechanoreceptors: detect mechanical changes
Externo receptors: located on the outside of the body
Proprioreceptors: Located within the body's muscles, joints and bones
The special senses are closely linked to the nervous system both structurally and functionally and depend on the nervous system for conscious interpretation of the environmental changes that they detect.
The specific receptors of the eye are located in the rods and cones that make up the retina. They are considered externo receptors and photoreceptor which are sensitive to light intensity
Rods: Enable light when there is poor vision through the formation of silhouettes and outlines - most useful at twilight
Cones: Enable close detailed vision when there is an abundance of light, Enable colour vision
Structure and Function
Cornea: Refracts light - important in focussing onto the retina
Sclera: Maintains the shape of the eye and protects the eye, Also serves as site of muscle attachment
Iris: Controls the amount of light passing through the pupil
Ciliary Body: Changes the shape of the lens and secretes aqueous humour
Choroid: Absorbs light, Contains blood vessels for eye structures
Retina: Absorbs light and stores Vitamin A, Receives light and forms an image for transmission to the brain
Lens: Refracts light for accommodation
Anterior Cavity: Maintains the shape of the eye and refracts light through its aqueous humour
Posterior Cavity: Maintains shape of the eye and refracts light through its vitreous humour
Aqueous Humour: Fills anterior cavity helping to maintain the shape of the eye, Refracts light, Maintains intraocular pressure
Vitreous Humour: Fills posterior cavity and maintains intraocular pressure, lends shape to eye and keep retina firmly pressed against the choroid, refracts light
THE PHYSIOLOGY OF VISION
Light passes through the cornea
The pupil varies in size depending on the distance of the object
The light passes through the aqueous humour to the lens for light focussing (accommodation)
The lens is flatter for distance objects and and becomes more convex for closer objects
The light then focusses the rays on the retina
Light energy stimulates of the rods and cones in the retina
Chemical reaction takes place in the visual receptors
Nerve impulses are transported over the optic nerve to the visual cortex of the brain for interpretation. Note; There are no visual receptors to be found at the region of the retina called the optic disk - blind spot
The image is received inverted
The brain makes the image upright in the cortex of the occipital lobe of the brain
Strabismus: Also known as cross eyes - the eyes do not converge together and the person sees two images instead of one.
Myopia or Near sightedness: The image forms in front of the retina. This is due to elongation of the eyeball or due to an inability of the lends to adjust sufficiently during accommodation
Hyperopia or Far sightedness: The image forms behind the retina and is blurred because the eyeball is too short or the lens is too flat to permit close vision due to loss of elasticity with age
Astigmatism: Irregular curve of the lens or the cornea causing an inability to separate two closely placed points.
Colour blindness: Inability of cones to react to certain colours of the spectrum. This inability is a sex linked genetic disorder from females to male children
The eye in diabetes
Risks associated with diabetes can be reduced by over 50% by keeping blood sugar levels under control. Other studies indicate that over 70% of diabetics will suffer from some form of eye disease and 30% will ultimately suffer serious damage. The earlier and longer that a person has diabetes the greater the risk.
Blurred Vision: Fluid seeps into the lens of the eye causing the lens to swell impairing its ability to focus. This may be relieved when the blood sugars normalize and is seen in the early stages of diabetes.
Glaucoma: Increased pressure in the eye due to impaired fluid exchange This condition is common in many adults past the age of forty. The risk in diabetes is increased. Eye drops are used to enhance proper fluid exchange.
Cataracts: Cloudiness of the lens. Very common with advancing age but afflicts diabetics at a much earlier age. poor control speeds up the process of developing cataracts
Retinopathy, non proliferative: High levels of blood sugar cause damage to the blood vessels in the retina until they leak fluid causing the retina to swell
Macular edema: Collection of fluid in the central part of the retina causing blurred vision
Retinopathy, Proliferative, Microaneurysms: The growth of abnormal blood vessels over the surface of the retina. Rupture of the blood vessels causing hemorrhage into the vitreous humour. The passage of light to the retina is blocked causing loss of vision/blindness
Detached retina: Scar tissue within the retina causing it to pull and detach. Bleeding behind the retina causing it to detach.
CARE OF THE EYE
Maintain blood sugar levels within as close to normal levels as possible
Have a complete eye check up annually
Control high blood pressure - damage to large blood vessels affects the heart and arteries - damage to fine vessels affects eyes and kidneys
Laser treatment when appropriate to reduce the risk of loss of vision