SYLLABUS REFERENCE

CSEC HSB

[B6.9] relate the internal structures of the eye to their functions;
[B6.10] explain how images are formed in the eye;
[B6.11] explain accommodation in the eye;
[B6.12] discuss the causes of, and corrective measures for eye defects and diseases.

CSEC Biology

[B7.9] relate the structure of the human eye to its functions as a sense organ;
[B7.10] explain accommodation; sight defects and the corrections of each;

The Eye is a Sense Organ That Contains Light Receptors

EXTERNAL STRUCTURE OF THE EYE

Eyelid

Protects the eye
Blinking keeps surface of eye and conjunctiva moist

Conjunctiva

Thin, transparent epithelium covering the eyeball
Secretes mucus to lubricate, clean and maintain moisture

Iris

Pigmented. This is where eye colour is located.
Has radial and circular muscles which regulate the amount of light entering the eye through the pupil.

Pupil

Round hole at the centre of the iris, through which light enters

Sclera

Tough, opaque, non-elastic layer on the outside of the eyeball.
Helps to maintain shape of eyeball.

INTERNAL STRUCTURE OF THE EYE

Cornea

Transparent, curved layer bends light to converge at lens.

Lens

Soft, transparent and elastic
Focuses light on the retina

Ciliary body/muscle

Muscle which help the lens alter its shape after focusing

Aqueous Humour

Transparent water liquid in front of the iris and pupil
Medium for diffusion of oxygen and nutrients
Refracts light and maintains shape of eyeball

Vitreous Humour

Transparent jelly, in the space between lens and retina
Refracts light and maintains shape of eyeball

Retina

Light-sensitive layer of the eye
Has photoreceptor cells (rods and cones)

Yellow Spot / Fovea Centralis

High cone density for night vision

Blind Spot

Where nerve fibres connecting rods and cones leave the eye
Has no photoreceptor cells

Choroid

Middle layer with black pigments
Prevents internal reflection of light

Optic Nerve

Made of sensory nerve fibres
Transmit information from eye to brain

HOW DOES THE EYE WORK?

Accommodation

Images are focused onto the retina by the adjustments made to the eye.
Ciliary muscles contract & relax to focus on near and distant objects.
This contraction & relaxation changes the lens curvature.
Change in curvature in turn changes the distance of focus (focal length) of the lens.

Focusing on Near Objects

Ciliary muscles contract
Tension in suspensory ligament reduces
Lens becomes more convex
Pupil size reduced

Focusing on Distant Objects

Ciliary muscles relax
Tension in suspensory ligaments increases
Lens become more concave
Pupil size increased

Response to Light

The pupil controls the amount of light entering the eye
Light enters as a stimulus to regulate the size of the pupil
Both eyes adjust the same way to the light conditions.

EYE DISORDERS

Myopia (nearsightedness) and hypertropia (farsightedness) can be fixed using lenses, placed in eye glasses.

Nearsightedness and its Correction

Farsightedness and its Correction

Astigmatism

This is an imperfection in the curvature of your eye's cornea or lens.

With astigmatism, the eyeball has an egg or oval shape, instead of a rounded shape. As a result, vision for BOTH near and far objects is blurry and distorted.

Glaucoma

This is a disease that damages the eye's optic nerve. It usually happens when fluid builds up in the front part of the eye. That extra fluid increases the pressure in one's eye, damaging the optic nerve.

Glaucoma is a leading cause of blindness for people over 60 years old. But blindness from glaucoma can often be prevented with early treatment.

Glaucoma is usually controlled with eyedrop medicine.

Used every day, these eye drops lower eye pressure.

Some do this by reducing the amount of aqueous fluid the eye makes.

Cataract

A cataract is a clouding of the normally clear lens of the eye.

For people who have cataracts, seeing through cloudy lenses is a bit like looking through a frosty or fogged-up window.

Most cataracts develop when aging or injury changes the tissue that makes up the eye's lens.

Proteins and fibers in the lens begin to break down, causing vision to become hazy or cloudy.

At first, stronger lighting and eyeglasses can help a patient deal with cataracts. But if impaired vision interferes with one's usual activities, one might need cataract surgery. Fortunately, cataract surgery is generally a safe, effective procedure.

How Diabetes Affects the Eyes

Both Type 1 and Type 2 diabetes causes high blood sugar concentrations.

High concentrations of sugar in the blood damages the arteries that carry oxygenated blood from the heart to the various organs & tissues in the body. This includes the tiny blood vessels (arterioles and capillaries) that provide nutrients and oxygen to the eye, and especially the retina. Also, parts of the eye itself is exposed to high concentration of sugar.

The result is a range of diabetic eye issues, which include the following:

Blurred vision
Diabetic retinopathy
Glaucoma
Macular oedema
Cataracts

Blurred Vision

This is usually one of the first diabetic eye symptoms and issues.

Exposure to high sugar concentrations causes swelling of the lens. This, in turn, produces blurry vision.

The good news is that this condition can be reversed, simply by controlling blood sugar levels. Once blood sugar levels are successfully controlled, it can take up to six weeks for the lens (and one's vision) to return to normal.

If the blurry vision is intense enough to interfere with a person's ability to function, then one may need a change to eyeglass prescription. This would be a temporary change until vision returns to normal.

Diabetic Retinopathy

This is a diabetic eye disease.

This occurs when blood vessels in the retina swell, leak fluid or bleed due to damage caused by high blood sugar concentrations. Other abnormalities include growth of extra blood vessels in the eye, and closing off of existing blood vessels to the retina.

Early states may be symptomless. However, as the disease progresses one can start to experience the following symptoms:

Blurred vision
Vision that becomes blurry, then suddenly returns to clear
Sudden appearance of or considerable increase in eye floaters
Poor night vision
Blank spots or dark shadows in one's field of vision

If left untreated, this disease can eventually lead to permanent blindness.

Macular Oedema

This can be a consequence of diabetes and diabetic retinopathy.

This is the collection of fluid - from leaking blood vessels - in the macula (aka yellow body). This is the part of the retina to produces the most detailed vision.

Initial symptoms is painless blurry vision. It can lead to permanent loss of visual acuity (aka visual sharpness).

EYE STRUCTURE & FUNCTION [CSEC HSB]