By Optometrist Nabila Jones from Optegra Eye Sciences
Eyes really are a feat of evolution. They enable us to see tiny specks and huge mountains; to see bright, vibrant colours and yet also see in the dark.
Like many organs in the body, our eyes work hard every day. They ensure we can see the world around us as numerous parts within the eye function together to bring objects in to focus and send information to the brain.
Sight is our most precious sense – have you ever wondered just how all those muscles, lenses and spaces work together to allow us to see what is in front of us? To focus an image, just like a camera?
Let us explain how those precious spheroidal-like structures work and what happens when the visual process is disrupted.
Eye make up
No, not the products you use around your eyes to enhance them, but the actual structural elements of the eye itself.
An eye is made up of several parts:
- Cornea – the clear window at the front surface of the eye, is the main ‘refractive’ surface, its is the first part of the eye that helps to focus light on the retina. If the cornea is not completely spherical this is typically referred to as astigmatism and can cause images to be blurry.
- Sclera – the tough white coloured protective part of the eye, it is composed of dense tissue covering approximately four fifths of the eyeball.
- Uvea-immediately behind the sclera is the uvea, a structure made up of the iris, ciliary body, and choroid. The choroid is a highly vascular layer that supplies blood to the outer layers of the retina. The ciliary body has a very important job of producing aqueous humour. The muscles of the ciliary body also relax or contract to change the lens of the eye helping us to focus.
- Aqueous humour – this is a fluid that nourishes the avascular lens and cornea the of the eye. It fills the spaces between the cornea and iris (anterior chamber) and the iris and lens (posterior chamber).
- Iris–a doughnut shaped coloured muscle which constricts or dilates the pupil, the black opening in the middle of the eye that controls the amount of light that enters. Iris colour is determined by genetics, with brown eyes having more of the melanin protein than blue eyes
- Crystalline Lens – a curved transparent structure made from proteins that sits behind the iris which like the cornea also focuses light rays on to the retina.
- Vitreous humour – this is a gel like substance between the lens and retina that helps the eye keep its shape.
- Retina – metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)—this receives and organises light information, translating it into electrical signals sent to the brain via the optic nerve which then creates. The brain processes these signals into images we can see.
- Macula: a yellow area on the retina at the back of the eye which surrounds the fovea.
- Fovea: forms a small indentation at the centre of the macula. When the eye is looking at an object, the part of the image that is focused on the fovea is the image most accurately registered by the brain.
- There are also six extraocular muscles that attach to the eyeball and enable it to move. They are in the eye socket and work in pairs to move the eye up, down, side to side, and to rotate it.
More than meets the eye – how do our eyes enable us to see?
Our eyes do an amazing job of capturing light from objects around us, but they don’t actually ‘see’ anything! That part is done by the visual cortex – a part of the brain which receives information from the retina.
At a very basic level, light reflects off objects into our eyes where the optic nerve processes this information and sends it to the brain.
But that gives little credit to the cornea, lens, iris, pupil, retina and all the muscles which work hard along the way. So, here’s a step-by-step guide:
- Light enters the eye through the pupil, having been focussed onto the cornea. The cornea curvature allows light to bend or ‘refract’ incoming light into the lens
- Depending on the amount of light being reflected, the iris will dilate or constrict the pupil to control the light entering. You might notice that when you step outside into bright sunlight, you are momentarily blinded until the pupil adjusts. The iris also adjusts depending on how far the objects you are looking at are, when in the far distance the iris causes the pupil to dilate and when focussing for near the pupil constricts.
- The lens also bends ‘refracts’ the light through the vitreous humour and focuses an image on to the retina. Depending on the distance of the object being viewed the lens will change shape using attached muscles, becoming fatter for close-up objects or thinner for those further away.
- The retina has a thin lining of photoreceptor cells. These cells, called rods and cones, have distinctive shapes and are sensitive to different wavelengths of light
- Rod cells let us see black, white and shades of grey – we have approximately 120 million of these, mainly distributed outside of the fovea (highly specialised region of the central retina). They also enable us to see shapes of objects and see in the dark – nocturnal animals have many rod cells. Our 7 million ‘cone’ cells mainly concentrated at the fovea of the retina allow us to see high resolution colour. There are three different types each sensitive to the wavelength of a different primary colour – red, green, or blue. In combination cones allow us to see hundreds of different colours and shades.
- The rod and cone cells react to specific wavelengths of light and trigger nerve impulses. There are no photoreceptor cells where the optic nerve connects to the eye – this is known as the ‘blind spot’
- The nerve impulses are then carried through the optic nerve through the visual pathway of the brain where they are processed and interpreted to give us sight as we know it.
Sometimes, our eyes can experience trauma, disease or get affected by age related changes, causing vision impairment. One of the most common causes of reversible visual impairment is cataracts – a clouding of the crystalline lens which obscures light entering the eye.
There are many other causes of vision impairment including, but not limited to, damage to the optic nerve, interrupting the important flow of messages being sent to the brain; pathology, trauma or aging of of the retina, affecting the perception of images or causing visual loss; and changes to the shape of or disease the cornea.
Some disease left undetected, such as glaucoma and age-related macular degeneration (AMD) can cause vision loss, early detection and correct treatment can help preserve vision!
The natural lens of the eye loses flexibility over time, this affects our ability to focus on near objects as we age and is why so many of us need reading glasses from around the age of 40 years.
As with all hard-working organs, regular maintenance is key to healthy eyes. Make sure you have your eyes tested at least every two years, as your optometrist can see right into the back of your eye to check for degenerative conditions which may have no early symptoms.
For more information on eye conditions and how they could affect you, please visit www.optegra.com.