All About our HUMAN EYE

There are many eye related problems and defects of the eye, the main few are briefly discussed 

below:

  Defects of the eye:

Myopia: (nearsightedness) This is a defect of vision in which far objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye’s lens too strong. Myopia can be corrected by wearing glasses/contacts with concave lenses these help to focus the image on the retina.

Hyperopia: (farsightedness) This is a defect of vision in which there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hyperopia can be corrected by wearing glasses/contacts that contain convex lenses.

Astigmatism: This defect is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.

Eye related problems:

Cataracts: A cataract is a clouding of the lens, which prevents a clear, sharp image being produced. A cataract forms because the lens is sealed in a capsule and as old cells die they get trapped in the capsule, with time this causes a clouding over of the lens. This clouding results in blurred images.

Age-related macular degeneration (ARMD)

This is a degenerative condition of the macula (the central retina). It is caused by the hardening of the arteries that nourish the retina. This deprives the retinal tissue of the nutrients and oxygen that it needs to function and causes a deterioration in central vision.

Glaucoma:   The eye produces a clear fluid (aqueous humor) that fills the space between the cornea and the iris. This fluid filters out through a complex drainage system. It is the balance between the production and drainage of this fluid that determines the eyes intraocular pressure (IOP). Glaucoma is a disease caused by increased IOP usually resulting from a malfunction in the eye’s drainage system. Increased IOP can cause irreversible damage to the optic nerve and retinal fibers and if left untreated can result in a permanent loss of vision.

 HUMAN EYE

 

Cornea: the transparent part at the front of the eye that refracts light entering the eye onto the lens.

Lens: a transparent structure behind the pupil that refracts incoming light and focuses it onto the retina. The lens is able to change shape in order to improve the focus.

Iris: This is the coloured part of the eye that controls the amount of light that enters the eye, it is able to contact and dilate in order to control the size of the pupil depending on the light intensity.

Sclera: the outer white part of the eye that protects the inner structures.

Retina: this is the light sensitive layer inside the eye that contains light sensitive photoreceptive cells called rods and cones. These cells change light into sight by converting light into electrical impulses. These electrical messages are sent from the retina to the brain and interpreted as images.

Optic nerve: this leaves the eye at the optic disk and transfers all the visual information to the brain.

Conjunctiva: a transparent vascular membrane that lines the inside of the eyelids and extends over the front of the white part of the eye (the sclera).

Aqueous humor: this fluid circulates the front part of the eye, it provides nourishment and helps maintain the eye pressure.

Vitreous humor: the clear gel in the centre of the eye that helps the eye to maintain its spherical shape.

RODS & CONES 

Rod cells are able to function in dim light but do not perceive colour. 

Rod cells are able to work in low light intensity because the cell can respond to a single photon of light (more sensitive). The brain requires fewer than 10 such responses to perceive the sensation of a flash of light. 

Rod cells however are unable to distinguish colour therefore despite being more sensitive than cones to light they enable us to see at night but only in black and white.

A rod cell has an elongated structure with the outer segment specialized for photoreception. (See diagram on right) It is this segment that contains the many discs which are membrane enclosed sacks densely packed with photoreceptor molecules.

The photoreceptive molecule is rhodopsin which consists of the protein opsin linked to 11-cis retinal  a prosthetic group.

A human retina contains about 125 million rod cells

The number of rods and cones vary over the surface of the retina

Cone cells function in bright light and are responsible for colour vision. A human retina contains approximately 6million cone cells. They have a similar structure to rod cells but their outer segment which contains the photoreceptive pigments is cone shaped.  

Cones require a reasonable amount of light to be stimulated so only function during the day. This is why we have colour vision but only during the daytime when the light intensity is high enough to stimulate the cone receptors. 

Like rods, cones contain an opsin and the chromophore 11-cis-retinal, but the opsins differ from rhodopsin so that each cone is responsive to one of three colors: red, blue or green, named by the region of the visual spectrum at which they absorb. (560,426,530nm respectively)  Cones are spread throughout the retina but are especially concentrated in a central area

The three cone receptors are homologs of rhodopsin. Like rhodopsin they are members of the 7TM receptor family and they utilize 11-cis retinal as their chromosphere.