| Conjunctiva | Is a thin protective covering of epithelial cells. It protects the cornea against damage by friction (tears from the tear glands help this process by lubricating the surface of the conjunctiva) |
| Cornea | Is the transparent, curved front of the eye which helps to converge the light rays which enter the eye |
| Sclera | Is an opaque, fibrous, protective outer structure. It is soft connective tissue, and the spherical shape of the eye is maintained by the pressure of the liquid inside. It provides attachment surfaces for eye muscles |
| Choroid | Has a network of blood vessels to supply nutrients to the cells and remove waste products. It is pigmented that makes the retina appear black, thus preventing reflection of light within the eyeball. |
| Ciliary body | Has suspensory ligaments that hold the lens in place. It secretes the aqueous humour, and contains ciliary muscles that enable the lens to change shape, during accommodation (focusing on near and distant objects) |
| Iris |
Is a
pigmented muscular structure consisting of an inner ring of circular
muscle and an outer layer of radial muscle.
Its function is to help control the amount of light entering the
eye so that: |
| Pupil | Is a hole in the middle of the iris where light is allowed to continue its passage. In bright light it is constricted and in dim light it is dilated |
| Lens | Is a transparent, flexible, curved structure. Its function is to focus incoming light rays onto the retina using its refractive properties |
| Retina | Is a layer of sensory neurones, the key structures being photoreceptors (rod and cone cells) which respond to light. Contains relay neurones and sensory neurones that pass impulses along the optic nerve to the part of the brain that controls vision |
| Fovea (yellow spot) | A part of the retina that is directly opposite the pupil and contains only cone cells. It is responsible for good visual acuity (good resolution) |
| Blind spot | Is where the bundle of sensory fibres form the optic nerve; it contains no light-sensitive receptors |
| Vitreous humour | Is a transparent, jelly-like mass located behind the lens. It acts as a ‘suspension’ for the lens so that the delicate lens is not damaged. It helps to maintain the shape of the posterior chamber of the eyeball |
| Aqueous humour | Helps to maintain the shape of the anterior chamber of the eyeball |
In
vertebrates vision begins with light entering the pupil.
The cornea and the lens focus and invert the light signal and project it
to the back of the eye where the retina is located.
The retina consists of several layers of alternating cells and processes
that convert the light signal into a neural signal,
The
retina contains the photoreceptor cells and their associated
interneurones and sensory neurones. They are arranged as shown in the following
diagrams:
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 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.
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 acuity (i.e. rods are not good at resolving fine detail). | Each cone usually connected to one bipolar cell, so good acuity (i.e. cones are used for resolving fine detail such as reading). |
Visual Acuity
Visual acuity the amount of detail that can be seen. The cones are responsible for high visual acuity (high resolution). 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. Spatially, much more clarity is perceived in cones than for the rods. This is because one cone cell synapses to one bipolar cell which in turn synapses onto one ganglion cell as the information is relayed to the visual cortex. 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.
Convergence
From the diagram above, you will notice that many rods can synapse onto one bipolar cell. A ray of light reaching one rod may not be enough to stimulate an action potential along a nerve pathway. Several rods link to one bipolar cell so that enough transmitter molecules at reach the threshold level. This depolarisation results in an action potential in the bipolar cell. This is summation, as a result of rod cell teamwork!
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:
Red light g stimulates red cones mainly
Yellow light g stimulates red + green cones roughly equally
Cyan light g stimulates blue and green cones roughly equally
White light g stimulates all 3 cones equally
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 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.
Light rays from a distant object are almost parallel so do not need much refraction to focus onto the retina. The lens therefore needs to be thin and “weak” (i.e. have a long focal length). To do this the ciliary muscles relax, making a wider ring and allowing the suspensory ligaments (which are under tension from the pressure of the vitreous humour) to pull the lens out, making it thinner.
Light rays from close objects are likely to be diverging, so need more refraction to focus them onto the retina. The lens therefore needs to be thick and “strong” (i.e. have a short focal length). To do this the ciliary muscles contract, making a smaller ring and taking the tension off the suspensory ligaments, which allows the lens to revert to its smaller, fatter shape.
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 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.
Last updated 22/04/2004