Vision (condensed)

Presbyopia.

In some eyes, especially in older people, the elasticity of the eye has decreased and the lens is no longer as convex as before.

A. Maximum accomodation.

In that situation, a maximum accomodation (= contraction of the ciliary muscle) will still not be able to move the focus to the fovea and the image remains blurry. This situation is called presbyopia.

B. Reading lenses.

These presbyopic patients can be helped by giving them reading lenses, which are convex lenses (positive or "+" lenses). They help in breaking the diverging light rays to focus the light rays on to the fovea.

In a myopic eye (=myopia), light rays from far away fall in front of the fovea. To the patient, the image looks blurred. The solution is to provide th patient with a concave ("-") lens.

Note that such a person can look sharp at images close to the eye (like reading) as this will move the focus towards the fovea. That is why these people are called "nearsighted"

In a hyperopic eye (=hyperopia), parallel light rays will fall "behind" the retina. To help these patients, a convex ("+") lens is required which will help 'break' the light rays more.

Note that these patients can (and do) help themselves by accomodating their lens. This will also move the focus onto the fovea. They do this automatically and therefore often they don't know that they have a refraction anomaly. They are therefore called "farsighted". They will often complain of headaches or tiredness as their ciliary muscle contracts all the time.

Cataract:

In some patients, the lens becomes, gradually, non transparant. There are many reasons for this to happen including metabolic diseases, congenital or old age. Because the lens becomes gradually less transparent, the patients will see the images more and more blurred. The therapy is to remove the lens and to replace it with a new artificial intra-ocular lens.

Intra-ocular pressure:

This also provides for a small pressure in the eye of about 5-10 mmHg. This keeps the eye in the shape of a ball and all its internal structures (lens etc) straight. If the pressure were too low, then the eyeball would collapse and vision becomes blurred.

Glaucoom 1:

If the pressure gets too high (=glaucoom), then another danger arises. A too high pressure (> 20 mmHg) will impede or block blood flow through the optic nerve. These vessels are crucial as they perfuse the retina.

Glaucoom 2:

If the blood perfusion is stopped, the photoreceptor cells will become ischaemic (= no blood) and die. The person will become blind.

Glaucoom 3:

An acute glaucoom (pressures 70-80 mmHg) can occur if there is a sudden obstruction of the flow to the canal of Schlemm. A chronic glaucoom (pressures 20-30 mmHg) occurs when the obstruction is limited.

The retina consists of:

the fovea: located in the center. This is a very small area (< 1 square millimeter) that contains three types of cone photoreceptors (for red, green and blue). This small area provides for sharp and colour vision. (memory trick; cone = colour)

the peripheral retina which contains only rod photoreceptors. These rods only sense black and white but are more sensitive than the cones.

There is also a blind spot, located in the inferior and nasal quadrant of the eye where the optical nerves exit the eye on their way to the brain.

Photoreceptors:

Both types of photoreceptors share the same plan:

1. At one end is a stack of "shelves" which are really infoldings of the plasma membrane. These shelves contain millions of the photo pigment molecules (such as rhodopsin).

2. The second part (or segment) contains the molecular machinery for the cell (mitochondriae etc).

3. The third part contains the nucleus

4. At the other end is the synapse that connects the receptor cell to other nervous cells in the retina.

The difference between the two cells is that in the rods, the shelves are of the same size whereas in the cones, the shelves diminish in size further away from the cell body, hence its shape and its name.

The retina has two layers:

a. the photoreceptor layer: which consists of rods (in the diagram) or cones.

b. the nervous layer: the receptor cells do not connect immediately to the brain cells. Instead they interconnect with other nerve cells. These intermediary cells already process the signals before communicating with the ganglion cells. The axon of the ganglion cells then combine to form the optic nerve.

Note that the direction of light is opposite what you would expect. The light rays have to go through the (thin) nervous layer before reaching the photoreceptors.

Lateral inhibition is a technique often used in the central nervous system.

In the retina, it is performed by the horizontal cells.

When (a group of) rods or cones gets excited, then the horizontal cells will inhibit the neighbouring synapses.

A. The ciliary muscle also controls the iris and therefore the size of the pupil. It actually consists of two muscles; one outer muscle which is oriented in the radial direction and a second inner muscle that is oriented in the circular direction.

B. Miosis: When the circular muscle contracts, the hole (= pupil) within the muscle becomes smaller. This works like a sphincter that you can see in other parts of the body (in the gut or the blood vessels for example).

This is actually a famous reflex (pupillary light reflex) that doctors often use when shining a bright light into the eye to check whether the patient is still alive. This reflex is controlled by the parasympathetic nervous system.

C. Mydriasis: is the opposite action (dilatation of the pupil) which is caused by contraction of the radial muscle. This happens in dim light allowing more light into the eye.

This contractionis controlled by the sympathetic nervous system.

The visual pathways consist of four parts:

a. the optic nerve

b.the optic chiasm

c. the optic tract

d. the optic radiation

a. The optic Nerve:

runs from the retina to the optic chiasm. It consists of nerve axons from both the temporal and the nasal retina.

b. The optic Chiasm:

In this chiasm (=crossing), the axons from the nasal retina from both eyes cross to the other side whereas the axons from the temporal retina's do not cross.

c. The optic Tract:

Therefore, the optic tract on the right side (blue) contains the axons from the right temporal retina and the nasal left retina. The left optical tract (red) contains the other axons.

d. The optic Radiation:

The axons end in the lateral geniculate body into a new series of axons that radiate towards the visual cortex.