The adult eyeball is about 2.5 cm in diameter. The eyeball is held in place by six extrinsic muscles, which allow the eye to be moved. The front surface of the eye is protected by the eyelids and the eyelashes. The reflex action of 'blinking' protects the surface of the eye. Under the eyelids is a thin transparent layer called the conjunctiva. This is kept moist by secretions from the lachrymal glands (tear glands) which lie above and to the outside of each eye. The fluid contains the enzyme lysozyme which kills bacteria. After passing over the conjunctiva, it drains from the eyes into the nasal cavity. The eyeball has a three layered structure.
Structure of the eye
Iris - regulates the amount of light entering through the pupil. Iris is a continuation of the choroid. Pigmented, colour of eye.
Wall of eye is composed of three layers:
(a) Sclera Sclerotic - outer layer, tough protects and helps maintain shape of eye. White except at front where transparent - called cornea.
(b) Choroid - middle layer, vascular, feeds retina cells. In humans, cells contain a black pigment melanin, which prevents light reflection in the eye.
(c) Retina - inner layer, light sensitive cells - cones and rods. Fovea (yellow spot) in man only cones found here. Blind spot, retinal absent - where optic nerve leaves eye. Filled by jelly-like vitreous humour containing about 99% water, some salts, and hyaluronic acid, which forms the gel.
Lachrymal glands - secrete tears which lubricate exposed surface of eye. Watery secretion helps prevent abrasion of eye’s surface by dust particles and helps combat infection of the eye. Blinking clears away debris.
Lens - biconvex, crystalline. Held in position by suspensory ligaments attached to a ring of smooth muscle called the ciliary body.
Optic nerve - fibres of sensory neurones leading from retina at back of the eye. Transmits impulses generated in the retina to the brain.
Aqueous humour - between cornea and lens - colourless watery fluid.
Vitreous humour - between lens and retina - clear gelatinous mucoprotein.
Accommodation = The process by which light is focused onto the retina. Cornea reflects light towards the lens. The lens focusing the light on to the retina.
Near object - ciliary muscles contract allowing the suspensory ligaments to slacken which allows the retina to bulge (i.e. has a shorter focal length) focusing light on to the retina.
Distant object - ciliary muscles relax and the vitreous humour pressing against the wall of the eye pulls the suspensory ligaments taut. Therefore lens thinner (i.e. longer focal length)
1) Myopia (short sight) Nearsightedness. If the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye.
- Hypermetropia (long sight) Farsightedness. If the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye - particularly those from nearby objects - will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem. As people get older, the lens becomes less elastic and can’t ‘bulge’ to focus near objects i.e. muscles become weaker. A kind of long sight.
3) Astigmatism - caused by uneven curvature of the cornea and or lens. Corrected by cylindrical lens ground to correct shape.
4) Cataracts One or both lenses often become cloudy as one ages. When a cataract seriously interferes with seeing, the cloudy lens is easily removed and a plastic one substituted. The entire process can be done in a few minutes as an outpatient under local anaesthesia.
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In human eyes and those of some other mammals, there are two types of photo-receptors called rods and cones. Rods are sensitive to different intensities of light. Cones sensitive to different wavelengths of light and enable to see things in colour (most mammals only have rods).
Pigment (melanin) absorbs light rays which would otherwise pass through retina. Prevents reflection of light to other parts of retina which would cause hazy images. Some mammals have a reflective layer called the tapetum in retina e.g. common nocturnal mammals - means light entering eye stimulates retina cells twice - increases sensitivity at low intensities.
Blind spot - where neurons project through retina into optic nerve, no photoreceptors.
Approx. 143million per eye - twilight or night vision i.e., vision in low light intensities. Lamellae contain a photosensitive pigment called rhodopsin which consists of a protein called opsin attached to retinal (a derivative of Vitamin A). When exposed to light, rhodopsin splits into opsin and retinal. This causes a change in membrane permeability, causing an electrical signal, the generator potential. Signal are transmitted to the bipolar cells and horizontal cells.. Rhodopsin has to be resynthesised to maintain the rod’s ability to respond to light. This requires energy, hence the presence of mitochondria. Rods are present in all parts of the retina except the fovea. Many rods synapse with each sensory neurone (up to 150 rods/sensory neuron). Known as retinal convergence. Therefore a very small amount of light can be utilised by rods due to this summation effect. Also means only a vague picture of the object even in the brightest of light can be seen with only rods. Greater number of rods around periphery of retina. Dark adaptation - when entering a dark room after being in bright light, takes several seconds before objects distinguished clearly. During this time pupils dilate, but of greater importance rhodopsin is reformed and the rods begin to function effectively.
- Approx. 7m/eye - daylight and colour vision. Only functional in normal daylight i.e. not at low intensities. Contain pigment iodopsin. Not easily broken down, not even at high light intensities. Most cones concentrated in fovea region (no rods). Each cone forms a synapse with very few, sometimes only one, sensory neurone. This gives great visual acuity i.e. can distinguish between objects very close together. In fovea region, cones closer together, than rods and cones in other parts of retina, i.e. most sensitive part of retina.
Trichromatic theory - thought to be 3 types of cones corresponding to the three primary colours, blue, green, red (max. absorption 450, 525, 550mm respectively) i.e., colour vision is due to the differential stimulation of 3 different cone types. Colour blindness - partial or total - 8% men - 0.4% women (sex linked).
Bipolar cells. the bipolar cells are neurones which gather information from the photoreceptors and transmit it to the next cell layer. Rod bipolar cells normally receive input from several rods. On the other hand, the cone bipolar cells are connected to just one or two cones so that good definition is preserved, called high visual acuity. Horizontal and amacrine cells. These help to co-ordinate the activity of the bipolar cells.
Ganglion cells. There are about 1 million ganglion cells and there axons travel over the surface of the retina towards the blind spot (this area contains no photosensitive pigments) where they pass, through forming the optic nerve which carries information to the brain. The information from the right eye is transmitted to the left hand side of the brain and that from the left eye is transmitted to the right hand side of the brain. The requires the optic nerves from each eye to cross over each other at a point called the optic chiasma. The information is sent to the visual cortex, at the rear of the brain, for processing.
Stereoscopic (binocular) vision
Animals with eyes set in front of their heads and directed forward have stereoscopic vision e.g. predators, e.g. owls, lions, etc., and tree dwellers e.g. apes. Each eye forms its own image of an object so that 2 sets of impulses sent to brain (normally brain correlates these so we gain a single impression of the object) - knock on head, alcohol see double. Since each eye ‘sees’ a slightly different angle of same object, the combination of these two images produces a 3-D image.
Judgement of distance
Stereoscopic vision helps animals judge distances. Most animals with eyes at sides of their head can judge distance only by the apparent size of objects and by parallel i.e. apparent movement of near objects against a background of distant objects when the head is turned from side to side.
Animals with their eyes in the sides of their head can usually see all round. Advantage for prey animals
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The Human Eye
The human eye is wrapped in three layers of tissue:
- the sclerotic coat
- This tough layer creates the "white" of the eye except in the front where it forms the transparent cornea. The cornea
- admits light to the interior of the eye and
- bends the light rays to that they can be brought to a focus.
The surface of the cornea is kept moist and dust-free by secretions from the tear glands.
- the choroid coat
- This middle layer is deeply pigmented with melanin. It reduces reflection of stray light within the eye. The choroid coat forms the iris in the front of the eye. This, too, is pigmented and is responsible for eye "colour". The size of its opening, the pupil, is variable and under the control of the autonomic nervous system. In dim light (or when danger threatens), the pupil opens wider letting more light into the eye. In bright light the pupil closes down. This not only reduces the amount of light entering the eye but also improves its image-forming ability (as does "stopping down" the iris diaphragm of a camera).
- the retina The retina is the inner layer of the eye. It contains the light receptors, the rods and cones (and thus serves as the "film" of the eye). The retina also has many interneurons that process the signals arising in the rods and cones before passing them back to the brain. (Note: the rods and cones are not at the surface of the retina but lie underneath the layer of interneurons.)
The blind spot
All the nerve impulses generated in the retina travel back to the brain by way of the axons in the optic nerve (above). At the point on the retina where the approximately 1 million axons converge on the optic nerve, there are no rods or cones. This spot, called the blind spot, is thus insensitive to light.
The lens is located just behind the iris. It is held in position by zonules extending from an encircling ring of muscle. When this ciliary muscle is
- relaxed, its diameter increases, the zonules are put under tension, and the lens is flattened;
- contracted, its diameter is reduced, the zonules relax, and the lens becomes more spherical.
These changes enable the eye to adjust its focus between far objects and near objects.
The iris and lens divide the eye into two main chambers:
- the front chamber is filled with a watery liquid, the aqueous humour.
- the rear chamber is filled with a jellylike material, the vitreous body.
Four kinds of light-sensitive receptors are found in the retina:
- 3 kinds of cones, each "tuned" to absorb light from a portion of the spectrum of visible light
- cones that absorb long-wavelength light (red)
- cones that absorb middle-wavelength light (green)
- cones that absorb short-wavelength light (blue)
This scanning electron micrograph (courtesy of Scott Mittman and David R. Copenhagen) shows rods and cones in the retina of the tiger salamander.
Each type of receptor has its own special pigment for absorbing light. Each consists of:
- a transmembrane protein called opsin coupled to
- the prosthetic group retinal. Retinal is a derivative of (which explains why night blindness is one sign of vitamin A deficiency) and is used by all four types of receptors.
The amino acid sequence of each of the four types of opsin are similar, but the differences account for their differences in absorption spectrum. The retina also contains a complex array of interneurons:
- bipolar cells and ganglion cells that together form a path from the rods and cones to the brain
- a complex array of other interneurons that form with the bipolar and ganglion cells and modify their activity.
Ganglion cells are always active. Even in the dark they generate trains of and conduct them back to the brain along the optic nerve. Vision is based on the modulation of these nerve impulses. There is not the direct relationship between visual stimulus and an action potential that is found in the senses of hearing, taste, and smell. In fact, action potentials are not even generated in the rods and cones.
Rhodopsin is the light-absorbing pigment of the rods. It is incorporated in the membranes of disks that are neatly stacked (some 2000 of them) in the outer portion of the rod. (This arrangement is reminiscent of the organisation of , another light-absorbing device.)
The outer segments of the rods contain the orderly stacks of membranes which incorporate rhodopsin. The inner portion contain many mitochondria. The two parts of the rod are connected by a stalk (arrow) that has the same structure as a cilium.
Although the disks are initially formed from the plasma membrane, they become separated from it. Thus signals generated in the disks must be transmitted by a chemical mediator (a "second messenger" called cyclic GMP () to alter the potential of the plasma membrane of the rod.
Rhodopsin consists of an opsin of 348 amino acids coupled to retinal.
The opsin has 7 segments of that pass back and forth through the lipid bilayer of the disk membrane.
Retinal consists of a system of alternating single and double bonds. In the dark, the hydrogen atoms attached to the #11 and #12 carbon atoms of retinal (red arrows) point in the same direction producing a kink in the molecule. This configuration is designated cis. When light is absorbed by retinal, the molecule straightens out forming the all- trans isomer.
This physical change in retinal triggers the following chain of events culminating in a change in the pattern of impulses sent back along the optic nerve.
- Formation of all- trans retinal activates its opsin.
- Activated rhodopsin, in turn, activates many molecules of a protein called transducin.
- Transducin activates an enzyme that breaks down cyclic GMP. (GMP is the guanine-containing cousin of .)
- The drop in cGMP closes in the plasma membrane of the rod causing an increase in its membrane potential from -40 to as much as -80 mV.
- This slows the release of a at the synapse of the rod. However, because this transmitter is inhibitory, the effect is a "double-negative" one, i.e. positive.
- Interneurons are relieved of their normal inhibition. This, in turn, relieves the inhibition of the spontaneous firing of the ganglion cells to which they are connected.
So the retina is not simply a sheet of photocells, but a tiny brain centre that carries out complex information processing before sending signals back along the optic nerve. In fact, the retina really is part of the brain and grows out from it during embryonic development.
Rod vision is acute but coarse.
Rods do not provide a sharp image for several reasons.
- adjacent rods are connected by and so share their changes in membrane potential
- several nearby rods often share a single circuit to one ganglion cell
- a single rod can send signals to several different ganglion cells.
So if only a single rod is stimulated, the brain has no way of determining exactly where on the retina it was.
However, rods are extremely sensitive to light. A single photon (the minimum unit of light) absorbed by a small cluster of adjacent rods is sufficient to send a signal to the brain. So although rods provide us with a relatively grainy, colourless image, they permit us to detect light that is over a billion times dimmer than what we see on a bright sunny day.
Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colours. Most of the 3 million cones in each retina are confined to a small region just opposite the lens called the fovea. So our sharpest and colourful images are limited to a small area of view. Because we can quickly direct our eyes to anything in view that interests us, we tend not to be aware of just how poor our peripheral vision is.
The three types of cones provide us the basis of colour vision. Cones are "tuned" to different portions of the visible spectrum.
- red absorbing cones; those that absorb best at the relatively long wavelengths peaking at 565 nm
- green absorbing cones with a peak absorption at 535 nm
- blue absorbing cones with a peak absorption at 440 nm.
Retinal is the prosthetic group for each pigment. Differences in the amino acid sequence of their opsins accounts for the differences in absorption.
The response of cones is not all-or-none. Light of a given wavelength (colour), say 500 nm (green), stimulates all three types of cones, but the green-absorbing cones will be stimulated most strongly. Like rods, the absorption of light does not trigger action potentials but modulates the membrane potential of the cones.
The term colour blindness is something of a misnomer. Very few (~1 in 105) people cannot distinguish colours at all. Most "colour-blind" people actually have abnormal colour vision such as confusing the red and green of traffic lights. As high as 8% of the males in some populations have an inherited defect in their ability to discriminate reds and greens. These defects are found almost exclusively in males because the genes that encode the red-absorbing and green-absorbing opsins are on the X chromosome.
The X chromosome normally carries a cluster of from 2 to 9 opsin genes. The minimum basis for normal red-green vision is one gene that absorbs efficiently in the red and one that absorbs well in the green (chromosome 1 in the figure). Multiple copies of these genes are also fine (2 and 3). Males with either a "green gene" or "red gene" missing are severely colour blind (4 and 5). However, if all the red genes carry mutations (this seldom seems to be the case for the green genes - nobody knows why), then they may have red-green colour blindness that ranges from mild to severe depending on the particular mutations involved (6). The rule seems to be that the more the mutations shift the pigment towards green, the more serious the defect. However, a large number of mutations doesn't always produce serious defects. Multiple mutations in a single gene may offset each other producing only mild defects. And as long as one normal copy of each gene is present, the presence of additional mutated genes seldom produce a serious problem (7).
Abnormal blue sensitivity occasionally occurs in humans but is much rarer than abnormalities in red-green vision. The gene for the blue-cone opsin is located on chromosome 7. Thus this trait shows an autosomal pattern of inheritance being found in females as often as in males.
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