The Eye
Optics 
[Cornea | Pupil
| Iris | Lens ]
Retina 
[ General | Peripheral
| Fovea | Optic Disk
]
Back to Main Index
The Cornea
The cornea is the first and most powerful refracting surface of the eye.
Light passes through the transparent cornea on its way to the retina. It
has a greater curvature than the rest of the eyeball and a refractive power
of approximately 42 dioptres.
Although only 0.5 cm thick in the centre, the cornea comprises 5 layers.
The junction between the cornea and the white sclera
is known as the limbus.
Back to Optics Index
The Pupil
The pupil is a dark circular hole in the middle of the eye. It is surrounded
by the sphincter of the pupil, a band of muscle in the iris about 1mm wide,
with a parasympathetic nerve supply. This can cause the pupil to grow or
shrink to control the amount of light reaching the retina. Animals which
have eyes very sensitive to light (e.g. cats) have elliptical pupils, which
can be closed completely.
Back to Optics Index
The Iris
The iris is an annular (ie shaped like a POLO) membrane dividing the aqueous
humour into the anterior chamber, nearer the cornea,
and the posterior chamber, towards the lens.
The inner portion of the iris, the pupillary zone, is separated from the
ciliary zone by the zig-zagging collarette.
The colour of eyes is determined by the amount of pigment in the iris. With
no pigment the eyes appear blue; with increasing amounts of pigment the
colour tends towards grey, brown and black.
The iris meets the white sclera at the ciliary process.
Back to Optics Index
Ciliary Process
This is an important junction where the iris
and the sclera meet. Close by is the circular
canal of Schlemm, which runs around the eye just below the limbus.
Aqueous humour is exuded from secretory cells just below the pigment epithelium
in the cauliflower-like ciliary processes. The aqueous humour drains through
the Zonnules of Zinn to the posterior chamber
and through the pupil to the anterior chamber. The fibrous Zonnules of Zinn,
which support the lens, are attached to the valleys
between the ciliary processes.
The smooth muscle of the ciliary body consists of both radial and circular
fibres. When we wish to focus on some close object we must increase the
power of our optics. This process is called accommodation.
The out of focus retinal image triggers the parasympathetic system which
contracts the ciliary muscle. The muscle moves forward and inwards; consequently
the zonnules of zinn relax, decreasing the tension in the lens capsule which
becomes more convex, increasing the lens' power.
Back to Optics Index
The Lens
The transparent, crystalline lens is biconvex. The radii of curvature of
the anterior surface is 10mm and of the posterior surface is -6mm, in the
unaccommodated state. The structure has a high refractive index and an accommodative
power of approximately 20 dioptres. It is held in place by the Zonnules
of Zinn.
Back to Optics Index
The Limbus 
The Limbus is an important junction where the clear cornea meets the sclera
and the conjunctiva. In front of this boundary of the eye lies the trabecular
mesh, a network of fine criss-crossing strands. Here also is found the circular
Canal of Schlemm which drains the aqueous into the scleral and episcleral
veins. A blockage of this canal can cause a build up of aqueous increasing
the intraocular pressure in the anterior chamber, which may result in glaucoma.
Back to Optics Index
The Sclera
The sclera is the tough, white, fibrous, outer tunic of the eyeball, covering
most of its surface.
Back to Optics Index
Zonnules of Zinn
Also known more boringly as the suspensory ligament, the Zonnules of Zinn
comprise a network of collagen fibres which connect the outer edge of the
lens with the ciliary processes. In this hammock of fine fibres lies the
lens. To the right is the margin of the vitreous humour, to the left is
the posterior chamber, which lies between the zonnules and the iris. Below
in the ciliary processes lie cells which are excreting aqueous humour, which
flows to the pupil.
Back to Optics Index
General Retina
Over most of its area,the retina is composed of five layers:
Response to light in the retina starts in photoreceptors in the receptor
layer and travels through the middle three layers to ganglion cells in the
ganglion cell layer.
A simple definition of the term "receptive
field" refers to "the set of one or more retinal receptors which
transmit impulses to a given cell in the nervous system". However,
the overlapping receptive fields of retinal ganglion cells have a complex
substructure determined by the connections between cells in the five layers
of the retina.
Back to Retina Index
Peripheral Retina
There are hardly any cones in the peripheral retina, but very many rods,
though they are far more sparse here than closer to the fovea. The rods
here are also shorter and wider than in the central retina. Receptive fields
at the periphery are very large with many rods converging onto one ganglion
cell.
Back to Retina Index
Receptor Layer
The receptor layer consists of two types of photoreceptor - rods
and cones. The human receptor layer consists of approximately 120 million
rods and 6 million cones arranged side by side like the pile of a carpet.
The distribution of these photoreceptors varies across the surface of the
retina. There are no rods at all in the fovea, and very few cones are found
at the periphery, where rods predominate.
The receptor layer is at the back of the retina; light must pass through
the intervening layers to reach the photoreceptors. The neuronal elements
of the retina are not themselves light sensitive, but transmit impulses
generated when light hits the photoreceptors.
Back To General Retina
Rods and Cones
Rods and cones (the names reflect their respective shapes) contain light
sensitive pigments. Each photoreceptor consists of an outer segment which
contains hundreds of thin plates of membrane (lamellae). The outer segment
is connected by a cilium to an inner segment which contains a nucleus. Rods
are about 500 times more sensitive to light than cones,
but cones give us colour vision.
Back To General Retina
Rods
A rod cell is narrow and cylindrical in shape, and its outer segment is
filled with approx 900 free-floating lamellae, (or rod disks) which contain
visual purple (rhodopsin). The rod disks are separated from each other and
from the outer segment membrane by cytoplasm.
The inner segment contains the cell nucleus, and from here a fibre ending
in a single end-bulb (a rod spherule) extends into the outer plexiform layer,
where it connects with the dendrites of bipolar cells.
Most nerve cells have a resting potential of about -70mV which depolarises
with stimulation to give an interior potential of +40mV. Rods however, have
an interior potential of only -30mV in darkness which hyperpolarises in
light to -60mV.
Back To General Retina
Cones
Cones are shorter, broader, and more tapered than rods. They contain no
visual purple; instead other pigments are present which make colour vision
possible. The lamellae in cones are formed from one continuous folded surface,
rather than separate disks, as is found in the rods. Cones synapse with
horizontal and bipolar cells via flattened oval end-bulbs, called cone pedicules.
The outer segments of cones contain one of three different photopigments
which can absorb light of long, medium or short wavelengths respectively,
thus providing us with colour vision.
Back To General Retina
Fovea
The fovea lies slightly below and to one side of the optic disk. It is found
in the centre of a shallow depression or pit (the macula). Only cones are
present at the fovea, which is an area approximately 0.2 mm in diameter:
all other parts of the retina including blood vessels are pushed aside.
The cones here have individual connections with the bipolar and ganglion
cells, hence the fovea gives us our most sensitive and acute vision.
Back to Retina Index
Optic Disk
This is the point at which axons leave the eyeball and join the optic nerve.
Also, arteries enter and veins leave the retina at the optic disk. There
are no photoreceptors here, hence it is known as the 'blind spot'. It is
a pinky-yellow oval, approximately 2mm in diameter, and situated in the
nasal retina.
Back to Retina Index
Ganglion Cells
Ganglion cells have concentric receptive fields of two types: on-centre
and off-centre. The type of receptive field a particular cell has can be
discovered by shining a small spot of light on the appropriate part of the
retina , and recording from the cell in question.
Ganglion cells keep up a steady spontaneous firing rate in darkness or in
diffuse light. For a cell with an on-centre/off-surround receptive field
(as in the picture), light on the centre of the field increases the cell's
firing rate. Light on the surround suppresses spontaneous firing. Light
all over the receptive field will tend to cancel itself out. Off-centre/on-surround
receptive fields show the opposite behaviour.
Rather than a simple mosaic arrangement, neighbouring ganglion cells receive
their inputs from overlapping arrays of receptors, thus a single spot of
light can stimulate very many ganglion cells simultaneously.
Not all ganglion cells look the same, some are small with skinny axons;
some are big with fat axons. The dendrites of the ganglions synapse mostly
with bipolar cells, but also with amacrine and horizontal cells.
Back To General Retina
Outer Plexiform Layer
This layer includes the synapses between the photoreceptors and the bipolar
cells (blue). It also contains the extensive dendritic fields of the
large horizontal cells (purple) which spread
parallel to the surface of the retina. Horizontal cells, like the amacrines,
can thus combine messages from adjacent receptors.
Back To General Retina
Inner Nuclear Layer
The inner nuclear layer is filled with the cell bodies and nuclei of horizontal,
bipolar and amacrine
cells. These latter, like horizontal cells, can form connections parallel
to the surface of the retina.
Back To General Retina
Inner Plexiform Layer
This layer contains the junctions between the ganglion
cells and bipolar cells. Not all the synapses
appear at the same depth in this layer; some ganglion cell dendrites seem
to end shortly after entering the layer, while others penetrate much further.
There are also synapses between amacrine cells
and ganglion cells here.
Back To General Retina
Ganglion Cell Layer
This layer contains the cell bodies and nuclei of the secondary sensory
neurones of the visual pathway, called ganglion cells.
The layer also contains some retinal blood vessels.
This is the lowest point in the visual pathway which responds with nerve
impulses (although amacrines sometimes respond in a transient fashion).
At this stage the axons are not myelinated, so transmission rates are fairly
slow.
Back To General Retina
Amacrine Cell
Amacrines have horizontally spreading processes rather than axons. They
connect the axons of bipolar cells and the dendrites of ganglion cells.
Amacrines do not connect with photoreceptors, but carry information laterally
across the inner plexiform layer. They respond primarily with transient
and depolarising potentials. However some amacrines give sustained responses
which can be hyper- or depolarising. Many transmitters may be involved with
amacrines, but GABA is probably one of the main ones.
Back To General Retina
Bipolar Cell
Like retinal ganglion cells, bipolars have concentric receptive fields.
These are of two types: on-centre and off-centre. If stimulation of the
centre of a cell's receptive field results in a positive response, and stimulation
of the surround gives a negative response, then we have an on-centre type
receptive field.
Bipolars are neurones each with one large dendrite.These synapse with either
a single cone or more usually several rods and/or cones. The bipolar axon
synapses with the dendrites of a ganglion cell. The responses of the bipolar
cell, like those of the receptors and horizontal cells, are graded potentials.
Some bipolar cells, like the receptors, hyperpolarise in response to light,
and some depolarise.
Back To General Retina
Horizontal Cell
Most vertebrate retinas contain two types of horizontal cell, one with a
short axon; one with no axon. However axonless cells have not been reported
in the primate retina. The axons of horizontal cells synapse with bipolar
cells. A horizontal cell responds to illumination of the retina with sustained
graded potentials. Horizontal cells have relatively large, homogeneous receptive
fields, and play a vital role in forming the receptive field surrounds of
retinal bipolar and ganglion cells.
Back To General Retina
Created by George Mather, University of Sussex (georgem@biols.susx.ac.uk).
Some of the images and text used in these pages were originally developed
at the Department of Psychology, York University, as part of the GRASP project.