The Eye

Optics

[Cornea | Pupil | Iris | Lens ]

Retina

[ General | Peripheral | Fovea | Optic Disk ]

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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.
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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.
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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.
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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.
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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.
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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.
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The Sclera

The sclera is the tough, white, fibrous, outer tunic of the eyeball, covering most of its surface.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.