The Eye

Contents

The eye

 

 

The Eye

 

The Sclera

The strong outer layer that hold the eye together. It is soft connective tissue, and the spherical shape of the eye is maintained by the pressure of the liquid inside.

The Choroid

This layer contains the blood vessels that feed every cell of the eye. It also contains the pigmented cells that make the retina appear black.

The Retina

This contains the light-sensitive photoreceptor cells and their associated neurones.

The Cornea

This is a specialised part of the cornea at the front of the eye. It is made of aligned collagen fibres and is transparent and tough.

The Iris

This is made of pigmented cells, which give eye colour, and muscle cells, which control the amount of light entering the eye.

The Lens

This is a transparent, rubbery tissue made of proteins, which crystallise to form a glass-like lens.

The Ciliary body

This supports the lens. It comprises circular muscles and radial elastic fibres called suspensory ligaments. Together theses control the shape of the lens, as described below.

The Humours

These are the names for the fluids inside the eye. The vitreous humour behind the lens is more viscous than the aqueous humour in front of the lens.

The Retina

The retina contains the photoreceptor cells and their associated interneurones and sensory neurones. They are arranged as shown in this diagram:

A surprising feature of the retina is that it is back-to-front (inverted). The photoreceptor cells are at the back of the retina, and the light has to pass through several layers of neurones to reach them. This is due to the evolutionary history of the eye, and in fact doesn’t matter very much as the neurones are small and transparent. There are two kinds of photoreceptor cells in human eyes: rods and cones, and we shall look at the difference between these shortly. These rods and cones form synapses with special interneurones called bipolar neurones, which in turn synapse with sensory neurones called ganglion cells. The axons of these ganglion cells cover the inner surface of the retina and eventually form the optic nerve (containing about a million axons) that leads to the brain.

Visual Transduction

Visual transduction is the process by which light initiates a nerve impulse. The structure of a rod cell is:

The detection of light is carried out on the membrane disks in the outer segment. These disks contain thousands of molecules of rhodopsin, the photoreceptor molecule. Rhodopsin consists of a membrane-bound protein called opsin and a covalently-bound prosthetic group called retinal. Retinal is made from vitamin A, and a dietary deficiency in this vitamin causes night-blindness (poor vision in dim light). Retinal is the light-sensitive part, and it can exists in 2 forms: a cis form and a trans form:

In the dark retinal is in the cis form, but when it absorbs a photon of light it quickly switches to the trans form. This changes its shape and therefore the shape of the opsin protein as well. This process is called bleaching. The reverse reaction (trans to cis retinal) requires an enzyme reaction and is very slow, taking a few minutes. This explains why you are initially blind when you walk from sunlight to a dark room: in the light almost all your retinal was in the trans form, and it takes some time to form enough cis retinal to respond to the light indoors.

The final result of the bleaching of the rhodopsin in a rod cell is a nerve impulse through a sensory neurone in the optic nerve to the brain. However the details of the process are complicated and unexpected. Rod cell membranes contain a special sodium channel that is controlled by rhodopsin. Rhodopsin with cis retinal opens it and rhodopsin with trans retinal closes it. This means in the dark the channel is open, allowing sodium ions to flow in and causing the rod cell to be depolarised. This in turn means that rod cells release neurotransmitter in the dark. However the synapse with the bipolar cell is an inhibitory synapse, so the neurotransmitter stops the bipolar cell making a nerve impulse. In the light everything is reversed, and the bipolar cell is depolarised and forms a nerve impulse, which is passed to the ganglion cell and to the brain. Fortunately you don’t have to remember this, but you should be able to understand it.

 

Rods and Cones

Why are there two types of photoreceptor cell? The rods and cones serve two different functions as shown in this table:

Rods

Cones

Outer segment is rod shaped Outer segment is cone shaped
109 cells per eye, distributed throughout the retina, so used for peripheral vision. 106 cells per eye, found mainly in the fovea, so can only detect images in centre of retina.
Good sensitivity – can detect a single photon of light, so are used for night vision. Poor sensitivity – need bright light, so only work in the day
Only 1 type, so only monochromatic vision. 3 types (red green and blue), so are responsible for colour vision.
Many rods usually connected to one bipolar cell, so poor visual acuity (i.e. rods are not good at resolving fine detail). Each cone usually connected to one bipolar cell, so good visual acuity (i.e. cones are used for resolving fine detail such as reading).

 

Although there are far more rods than cones, we use cones most of the time because they have fine discrimination and can resolve colours. To do this we constantly move our eyes so that images are focused on the small area of the retina called the fovea. You can only read one word of a book at a time, but your eyes move so quickly that it appears that you can see much more. the more densely-packed the cone cells, the better the visual acuity. In the fovea of human eyes there are 160 000 cones per mm2, while hawks have 1 million cones per mm2, so they really do have far better acuity.

Colour Vision

There are three different kinds of cone cell, each with a different form of opsin (they have the same retinal). These three forms of rhodopsin are sensitive to different parts of the spectrum, so there are red cones (10%), green cones (45%) and blue cones (45%). Coloured light will stimulate these three cells differently, so by comparing the nerve impulses from the three kinds of cone, the brain can detect any colour. For example:

This is called the trichromatic theory of colour vision. The role of the brain in processing visual information is complex and not well understood, but our ability to detect colours depends on lighting conditions and other features of the image.

The red, green and blue opsin proteins are made by three different genes. The green and red genes are on the X chromosome, which means that males have only one copy of these genes (i.e. they’re haploid for these genes). About 8% of males have a defect in one or other of these genes, leading to red-green colour blindness. Other forms of colour blindness are also possible, but are much rarer.

Accommodation

Accommodation refers to the ability of the eye to alter its focus so that clear images of both close and distant objects can be formed on the retina. Cameras do this by altering the distance between the lens and film, but eyes do it by altering the shape and therefore the focal length of the lens. Remember that most of the focusing is actually done by the cornea and the job of the lens to mainly to adjust the focus. The shape of the lens is controlled by the suspensory ligaments and the ciliary muscles.

The suspensory ligaments are purely passive, but the ciliary muscles are innervated with motor neurones from the autonomic nervous system, and accommodation is controlled automatically by the brain.

The Iris

The retina is extremely sensitive to light, and can be damaged by too much light. The iris constantly regulates the amount of light entering the eye so that there is enough light to stimulate the cones, but not enough to damage them. The iris is composed of two sets of muscles: circular and radial, which have opposite effects (i.e. they’re antagonistic). By contracting and relaxing these muscles the pupil can be constricted and dilated:

The iris is under the control of the autonomic nervous system and is innervated by two nerves: one from the sympathetic system and one from the parasympathetic system. Impulses from the sympathetic nerve cause pupil dilation and impulses from the parasympathetic nerve causes pupil constriction. The drug atropine inhibits the parasympathetic nerve, causing the pupil to dilate. This is useful in eye operations.

The iris is a good example of a reflex arc.

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