Eyesight 101

This is a simple introduction to the structure of a human Eye.

Vision is an immensely complicated process, but this is a simplified description.  It is meant to help you understand Leber Hereditary Optic Neuropathy.

 

Structure of the Human Eye

Diagram of structure of the human eye

 

The Sclera

Most of the eyeball is covered by a tough outer coat – we call this the “white” of the eye but the medical term is the Sclera.

The Cornea

This is the curved transparent window at the front of the eye.  It is the first point where light coming into the eye gets focused.

The Iris

The Iris sits behind the Cornea and controls how much light enters the eye by expanding and contracting the size of the hole in the center.  When we say someone has blue eyes, green eyes or brown eyes, we are talking about the color of their Irises.

The Pupil

This is simply the hole in the center of the Iris that lets light into the eye.  It appears black or very dark in humans because there is a lot of light outside and hardly any light is reflected back out.

The Lens

This is a soft transparent disc hanging behind the Iris.  Light comes in through the Cornea, passes through the Pupil and gets fine-tuned by changes in the shape of the Lens to form a sharp image.  The Lens is held in shape by muscular Ciliary Bodies so that we can change focus from near to far and back again by unconsciously changing the shape of the lenses in our eyes.

Focusing a clear image

To see things clearly, light has to enter through the curved Cornea, pass though the Pupil and be focused sharply onto the Retina at the back of the eye by the Lens.

Light from a lamp bouncing off a picture into the eye

 

In many people the shape of the eyeball, cornea and lens don’t fit well together, and the image is not always focused exactly onto the retina.  Some people can’t focus on things that are far away – they are near-sighted or short-sighted. Other people can’t focus on things that are too close, they are far-sighted or long-sighted. Most people grow long-sighted as they get older.

These problems in focusing can be corrected by eyeglasses or contact lenses to adjust the way light is projected onto the retina and get a nice sharply defined image.

Many people take a more drastic step and have eye surgery to change the shape of the cornea so it is more or less curved.  This is often called “laser eye surgery”.  The surgeon makes a lot of small cuts around the Cornea and remodels the shape of the Cornea so that it heals up focusing in a new place.

The principle is a little bit like cutting out darts and re-stitching to change the fit of a dress or coat.

If it works (and sometimes it doesn’t!) this kind of corrective eye surgery means the person no longer has to wear eyeglasses.

Problems that distort or blur the image

Sometimes the Cornea gets damaged by injury or illness (such as Measles ).  It is no longer transparent, and might have scars or other distortions.  These problems can’t be corrected by eyeglasses, because they are not about focusing the image, something is blocking or distorting the image coming into the eye.

This can sometimes be corrected by surgery such as a Corneal Transplant, completely replacing the scarred Cornea with one from a Donor.

Sometimes the Lens stops being transparent and becomes cloudy.  This is called a Cataract.

Cataracts are a very common cause of blindness in developing countries.   Even in developed countries cataracts are common in older people.

The treatment for a Cataract is usually to wait until the Lens is very cloudy and vision is blurred, then do a routine operation to remove the cloudy lens.

The Choroid

This is a layer of connective tissue and blood vessels inside the Sclera, supporting and supplying the Retina.

The Retina

This is the inner lining at the back of the eye that does all of the work turning light rays into nerve impulses and sending them to the Brain.

Diagrams of the Eye and layers of the Retina

Retina – Pigmented Layer

The outermost layer of the Retina is a Pigmented Layer or Pigmented Epithelium.  This (together with the Choroid layer of blood vessels and connective tissue) provides nutrients to the Retina and helps the Rods and Cones restore the pigment they need to detect light.  it also contains a lot of Melanin dark pigment   to absorb any “stray” light rays and stops them bouncing around inside the eyeball.

 

Retina – The PhotoReceptor Epithelium (PRE)

This is sometimes just called the “Rods and Cones” layer or the “light-sensitive layer”.

These are the two kinds of cells sensitive to light.   Rods and Cones are a kind of nerve cell (Neuron) that contains a special light-sensitive pigment.  When it is exposed to a light ray, the pigment reacts and the cell generates a nerve impulse.

Rods

Each eye has a bout 120 million Rod cells and they are spread all over the Retina. Each individual Rod cell is very sensitive to light, but cannot tell the wavelength or color of the light.  Rod cells let us see in dim light, but only in black and white.   That’s why all  “cats are gray at night”.  In dim light we rely more on our Rod cells to make best use of the light available.

Rods are not spread evenly across the Retina, they are more densely packed in an area at the back of the eye called the Macula or “Fovea Centralis”.   An image focused on that area will hit more Rod cells (have better resolution) and so we will see it in more detail.

That is why we get a more detailed picture of something if we look directly at it. We are pointing our eyes so that the light rays are focused on our Maculas, and we are using more Rod and Cone Cells to see it.

Cones

Each eye only has 6 or 7 million Cone cells, and they are concentrated in the center at the back of the eye.  There are many more Cone Cells in the Macula, and they are more densely packed.

Cone Cells are less sensitive to light, that is they need a stonger light before they generate a nerve signal.

There are three different kinds of Cone Cell. Each kind is sensitive to only one color.

  • S-Cone cells are sensitive to Short wavelength or Blue light.
  • M-Cones are sensitive to Medium wavelength or Green light.
  • L-Cones are sensitive to Long wavelength or Red light.

Note: Very Short wavelength light is called Ultra-Violet or UV light.  We can’t see UV light directly, but some animals can.  We notice UV light from the Sun because it causes sunburn and eye damage if we get too much of it.  Very Long wavelength light is called Infra-Red.  We can’t see Infra-Red light but can detect a strong Infra-Red light ray as Heat.

Like Rods, Cones are packed more densely in the center of our field of vision.  We have far fewer Cones around the edges of the Retina. That means we are much poorer at judging colours when using peripheral vision.

We see many more colors than just Red, Green and Blue. The Cone Cells are “wired” together by the Bipolar Layer and the network of connections.  This means that the signals from several Cone cells are combined inside the Retina,  This combination of Red, Green and Blue Cones, Bipolar Cells and Retinal Ganglion Cells work together to generate nerve impulses for Color Vision.

Retina – The Bipolar Layer

Bipolar Cells are another special kind of nerve cell or Neuron. Put Very, Very simply, the layer of Bipolar Cells receives nerve signals from Rods and Cones when light hits the Retina. There is complicated “wiring” of nerve cell connections between the light sensitive Rods and Cones, the Bipolar Cells and the Retinal Ganglion Cells.

This “Wiring” and control by the Bipolar Layer gives us a lot of special functions without having to think about them. Our eyes detect and signal the Brain about movement, flashing lights, changes in contrast, changes in color, even some important patterns like human faces.

This means we can detect and react to predators, prey or other humans much more quickly without having to consciously think about them.

Retina – Retinal Ganglion Cells (RGCs)

The nerve signals processed by the Bipolar Cells are sent to a third kind of nerve cell called a Retinal Ganglion Cell.

Again, there are more Retinal Ganglion Cells clustered around the Macula, they are smaller and more densely packed there.  That means we have better resolution,  get much more detail about shape, color, movement,  everything, from an image focused there.

The job of each Retinal Ganglion Cell is to collect the important signal information and fire a nerve impulse along one of its “limbs” or “axons” called the Retinal Nerve Fiber”.   Each Retinal Ganglion Cell grows one very long Retinal Nerve Fiber.   This carries the output nerve signal out of the eye to the Brain. Each Retinal Nerve Fiber is an Axon “growing” from the innermost side of the Retinal Ganglion Cell, furthest away from  the Bipolar Layer and PhotoReceptor Layer.

Retina – The Retinal Nerve Fiber Layer (RNFL)

This is just about the innermost layer of the Retina. There are between around 770, 000  and 1.200,000  Retinal Nerve Fibers, coming from all over the Retina, congregating at the Optic Disc.

The Retinal Nerve Fiber Layer (RNFL) is measured by doctors using a tool called Optical Coherence Tomography or OCT. It sometimes seems to grow thicker when someone is first affected by LHON because of swelling fibers, but grows significantly thinner over the next year or so as cells go dormant or even die.

The Optic Disc

The Optic Disc is where the hundreds of thousands of Retinal Nerve Fibers get bundled together to form the Optic Nerve.  It is sometimes called the Optic Nerve Head.

Normally it is a pinkish disc about 2 millimeters across.

When someone is first affected by LHON, a doctor will often see swelling of the Retinal Nerve Fiber Layer and Optic Disc.  After the Acute period the swelling is reduced, the Optic Disc becomes pale in color.

This picture shows how a light ray bounces off an object into the eye and hits the Retina.  The PhotoReceptor Rods and Cones, Bipolary Layer and Retinal Ganglion Cells turn the image into a set of nerve signals.  The nerve signals are sent out by the Retinal Ganglion Cells along the Retinal Nerve Fibers (Optic Nerve) to the Brain.

An image on the Retina is converted into a set of impulses and sent along the Optic Nerve to the Brain.

 

Impact of LHON

Leber Hereditary Optic Neuropathy affects eyesight because it damages the Optic Nerve. That is why doctors can see changes in the Optic Disc

The Retinal Ganglion Cells and their axons the Retinal Nerve Fibers get chemically “stressed”.  Doctors think that they are struggling to get enough ATP “fuel” and building up damaging Reactive Oxygen Sopecies (ROS) because of the faulty gene in their mitochondria.

No-one knows exactly why this starts happening, that is exactly what triggers the symptoms and turns a LHON Carrier into a LHON Affected person.

Doctors think that the “stressed” Retinal Ganglion Cells go dormant or even die.  Their Retinal Nerve Fibers shrink or die, and so the Optic Nerve is physically damaged.

Although no-one knows for sure, it seems to be the smaller Retinal Ganglion Cells with thinner Retinal Nerve Fibers that are more easily damaged by LHON.  Unfortunately, these are the closely-packed nerve cells at the Macula, where our best detail and color vision takes place.

The Cornea and Lens are still focusing an image correctly onto the Retina.  The Rods and Cones may even still be detecting the light. LHON has damaged the way the eye processes the signal and sends it to the Brain.

That is why ordinary eyeglasses can’t help someone affected by LHON.

Although the central, detailed vision is gone,  someone affected by LHON usually still has some peripheral vision “around the edges”.

Part of learning to live with LHON is re-learning how to see, just using peripheral vision.

Peripheral vision is a lot less detailed and is not so good at detecting color.

Often someone affected by LHON can see enough to walk around, but can’t see well in dim light, can’t judge color and can’t see detail like faces or print text.

A magnifier can help, because a small image is blown up to cover more of the peripheral retina.  Although that part of the retina has less closely packed rods and cones, the magnified image hits more of them.

 

 

 

 

 

This page was last updated August 23 2015

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