Human Coordination » Eye Structure and Function

What you'll learn this session

Study time: 30 minutes

The structure of the human eye and how each part functions
How the eye adapts to different light conditions
The process of accommodation for focusing on near and distant objects
Common eye defects and how they can be corrected
How the brain processes visual information

Structure of the Human Eye

The eye is an amazing sensory organ that allows us to see the world around us. It works like a sophisticated camera, capturing light and converting it into electrical signals that our brain can understand.

Key Definitions:

Cornea: Transparent front part of the eye that helps focus light.
Iris: Coloured part of the eye that controls the size of the pupil.
Pupil: The dark opening in the centre of the iris that allows light to enter.
Lens: Transparent structure behind the pupil that focuses light onto the retina.
Retina: Light-sensitive layer at the back of the eye containing photoreceptors.
Optic nerve: Transmits visual information from the retina to the brain.

👀 External Eye Structure

The cornea is the transparent front part of the eye. It protects the eye and bends (refracts) light as it enters. Behind the cornea is a fluid-filled space called the anterior chamber containing aqueous humour, which helps maintain the eye's shape.

The iris is the coloured part of your eye. It contains muscles that control the size of the pupil, which is the dark opening in the centre. The pupil changes size to control how much light enters the eye - getting smaller in bright light and larger in dim light.

🔎 Internal Eye Structure

Behind the pupil sits the lens, which is transparent and flexible. It changes shape to focus light onto the retina at the back of the eye. The space between the lens and retina is filled with a jelly-like substance called vitreous humour, which helps maintain the eye's shape.

The retina contains two types of photoreceptor cells: rods (for dim light and peripheral vision) and cones (for colour vision and detail). Visual information is sent to the brain via the optic nerve.

How the Eye Adapts to Light Conditions

Our eyes need to work in a wide range of light conditions, from bright sunlight to near darkness. Two key mechanisms help us adapt:

Pupil Reflex

The iris controls how much light enters the eye by changing the size of the pupil:

☀ Bright Light Response

In bright conditions, circular muscles in the iris contract, making the pupil smaller (constriction). This reduces the amount of light entering the eye, preventing damage to the retina and improving focus.

🌙 Dim Light Response

In dim conditions, radial muscles in the iris contract, making the pupil larger (dilation). This allows more light to enter the eye, helping us see in low light. This is why your pupils look bigger in the dark.

Dark Adaptation

When we move from a bright to a dark environment, our eyes undergo several changes:

Pupils dilate to let in more light
The retina becomes more sensitive to light
The eye shifts from using mostly cones to using mostly rods
Rhodopsin (a light-sensitive pigment in rods) regenerates

Full dark adaptation can take up to 30 minutes, which is why it takes time for your eyes to adjust when you enter a dark cinema.

Accommodation - Focusing Near and Far

Accommodation is the process by which the eye changes focus from distant to near objects. This is achieved by changing the shape of the lens.

🗺 Distant Vision

When looking at distant objects (more than 6 metres away):

Ciliary muscles relax
Suspensory ligaments tighten
Lens becomes thinner and less curved
Light rays focus correctly on the retina

📖 Near Vision

When looking at close objects:

Ciliary muscles contract
Suspensory ligaments loosen
Lens becomes fatter and more curved
This increases the lens's focusing power

💻 Digital Eye Strain

Looking at screens for long periods can cause:

Fatigue of ciliary muscles
Reduced blinking (dry eyes)
Headaches and blurred vision
The 20-20-20 rule helps: Every 20 minutes, look at something 20 feet away for 20 seconds

Common Eye Defects

Several common vision problems occur when light doesn't focus correctly on the retina:

👓 Short-sightedness (Myopia)

Problem: Distant objects appear blurry because light focuses in front of the retina.

Causes: Eyeball too long or lens too curved.

Correction: Concave (diverging) lenses that spread light rays before they enter the eye, allowing them to focus correctly on the retina.

📕 Long-sightedness (Hyperopia)

Problem: Near objects appear blurry because light would focus behind the retina.

Causes: Eyeball too short or lens too flat.

Correction: Convex (converging) lenses that bring light rays together more quickly, allowing them to focus correctly on the retina.

Case Study: Colour Blindness

Colour blindness affects approximately 1 in 12 men and 1 in 200 women worldwide. The most common form is red-green colour blindness, where people have difficulty distinguishing between red and green colours.

This condition is usually inherited and occurs when one or more types of cone cells in the retina don't work properly. It's more common in males because the genes for the most common forms of colour blindness are on the X chromosome. Males have only one X chromosome, so if that chromosome carries the colour blindness gene, they will be colour blind. Females have two X chromosomes, so they need the gene on both to be colour blind.

While there's no cure for inherited colour blindness, special glasses and contact lenses can help some people distinguish colours better. Most people with colour blindness adapt well and can lead normal lives, though certain careers that require precise colour discrimination may be challenging.

How We Process Visual Information

Seeing isn't just about the eyes - it's also about how our brain interprets the signals it receives:

Light enters the eye through the cornea and pupil
The lens focuses light onto the retina
Photoreceptors (rods and cones) convert light into electrical signals
These signals travel through the optic nerve to the brain
The visual cortex in the occipital lobe processes these signals
The brain interprets the signals as images we can understand

Interestingly, the image formed on your retina is actually upside-down, but your brain automatically flips it the right way up! Your brain also fills in your blind spot (where the optic nerve leaves the eye and there are no photoreceptors) so you don't notice a gap in your vision.

🎨 Binocular Vision

Having two eyes gives us several advantages:

Depth perception: Each eye sees a slightly different image, which the brain combines to create 3D vision
Wider field of view: Two eyes cover more area than one
Better vision in low light: Two eyes capture more light than one

💡 Optical Illusions

Optical illusions show us how our brain interprets visual information. They work by:

Exploiting assumptions our brain makes about the world
Creating conflicts between different visual processing systems
Demonstrating that what we "see" is actually our brain's interpretation of reality

🧠 Test Your Knowledge!