Eye Anatomy and Physiology PDF

The special senses CHAPTER 8 shape and about 2.5 em in diameter. The space between the Sight and the eye eye and the orbital cavity is occupied by adipose tissue. The...

The special senses CHAPTER 8 shape and about 2.5 em in diameter. The space between the Sight and the eye eye and the orbital cavity is occupied by adipose tissue. The bony walls of the orbit and the fat within it protect the eye Learning outcomes from injury. Structurally, the two eyes are separate but, unlike the After studying this section, you should be able to: ears, some of their activities are coordinated so that they describe the gross sbucture of the eye ru:mnally function as a pair. It is possible to see with only one eye (monocular vision) but three-dimensional vision is describe the route taken by nerve impulses from impaired when only one eye is used, especially in relation to the retina to the cerebrum the judgement of speed and distance. explain how light entering the eye is focused on the retina state the functions of the extraocular eye muscles Structure explain the functions of the accessory organs of Internally, the eye is divided into two c::hambers, the the eye. anterior and the posterior c::hambers, with the lens of the eye, the dliary body and the suspensory ligaments (Fig. 8.8) separating them. The anterior chamber is 6lled with a The eye is the organ of sight. It is situated in the orbital cavity, clear, watery fluid called aqueous humour, and the posterior a bony socket built into the bones of the face and supplied by chamber is filled with a jelly-like substance called vitreous the optic: nerve (2nd cranial nerve). It is almost spherical in humour (vitreous body). -\.;._""'"'"'-~.-- Poste~or chamber (contain!~ vltl'l'lOUII humour) Medial rectus muscle - Postlricr ~ Arrtllrl~ I cavity l B chamber --- Anterior cavity F"tgure 8.8 Transverse section of the right eye. Conjuncllva Cornea 213  SECTION 2 Communication There are three layers of tissue in the walls of the eye: the outer fibrous layer: sclera and cornea the middle vascular layer or uveal tract: the choroid, ciliary body and iris the inner nervous tissue layer: the retina. Sclera and cornea Tile sclera. or white of the eye, forms the outermost layer of the posterior and lateral aspects of the eyeball and is continuous anteriorly with the cornea. It is a firm fibrous membrane that maintains the shape of the eye and gives attachment to the extrinsic muscles of the eye (see Table 8.1, Cornea p. 221). Anteriorly the sclera continues as a clear transparent Pupil epithelial membrane, the cornea. Light rays pass through the cornea to reach the retina. Tile cornea is convex anteriorly and is involved in refracting (bending) light rays to focus them on the retina. Iris, with circular and rallating Choroid IITIOOth muscle ftbras Tile choroid (Fig. 8.9; see also Fig. 8.8) lines the posterior five-sixths of the inner surface of the sclera. It is very rich in blood vessels and is deep chocolate brown in colour. Light Suspensory enters the eye through the pupiL stimulates the sensory ligaments receptors in the retina (see below) and is then absorbed by the choroid. Ciliary body Figure 8.9 The choroid, ciliary body and Iris. VIewed from the front. The ciliary body is lhe anterior continuation of the choroid, consisting of ciliary muscle (circular smooth muscle fibres) The iris is supplied by parasympathetic and sympathetic and secretory epithelial cells. The lens is att.adted to the nerves. Parasympathetic stimulation constricts the pupil and ciliary body by radiating suspensory ligaments, like the sympathetic stimulation dilates it (see Figs 7.45 and 7.44,. spokes of a wheel (see Fig. 8.10). Contraction and relaxation respectively). of the ciliary muscle fibres, which are attached to these The colour of the iris is genetically determined and ligaments, determine the size and thickness of the lens. depends on the number of pigment cells present. Albinos The epithelial cells secrete aqueous humour, which have no pigment cells and people with blue eyes have fewer circulates through the anterior chamber to nourish its than those with brown eyes. structures. The ciliary body is supplied by parasympathetic branches Lens of the oculomotor nerve (3rd cranial nerve). Stimulation causes contraction of the ciliary muscle and accommodation The lens (Fig. 8.10) is a highly elastic circular biconvex body, of the eye (p. 218). lying immediately behind the pupil. It consists of fibres enclosed within a capsule and is suspended from the ciliary body by the suspensory ligament. Its thickness is controlled Iris by the ciliary muscle through the suspensory ligament. The The iris is the visible coloured ring at the front of the eye lens bends (refracts) light rays reflected into the eye from and extends anteriorly from the ciliary body, lying behind objects in the visual field. It is the only structure in the the cornea and in front of the lens. It divides the anterior eye that can vary its thickness, and therefore its refractory chamber of the eye into anterior and posterior cavities, which power, to focus light rays on the retina. contain aqueous humour secreted by the ciliary body. The When the ciliary muscle contracts, it releases its pull on iris is composed of pigment cells and two layers of smooth the lens, increasing its thickness. The nearer the object being muscle fibres, one circular and the other radiating (see Fig. viewed, the thicker the lens becomes to allow focusing (see 8.9). In the centre is an aperture called the pupil Fig. 8.18). 214  The special senses CHAPTER 8 Retina centralis, consisting only of cones. Towards the anterior part of the retina there are fewer cones than rods. The retina is the innermost lining of the eye (see Fig. 8.8). It About 0.5 em to the nasal side of the macula lutea all the is an extremely delicate structure composed of several layers retinal nerve fibres converge to form the optic nerve. The of nerve cell bodies and their axons, lying on a pigmented small area of retina where the optic nerve leaves the eye is layer of epithelial cells. The light-sensitive layer consists 1he optic disc or blind spot It has no lightwsensitive cells. of sensory receptor cells, rods and cones, which contain photosensitive pigments that convert light rays into nerve Blood supply to the eye impulses. The retina lines about three-quarters of the eyeball and is Arterial supply is from the ciliary arteries and the central t:hickest at the back. It thins out anteri.arly to end just behind retinal artery. These are branches of the opht:halmk artery, a the ciliary body. Near the centre of the posterior part is the branch of the internal carotid artery. macula lutes. or yellow spot (Figs 8.11A and 8.12). In the Venous drainage is by a number of veins, including the centre of the yellow spot is a little depression called the fovea central retinal vein, which eventually empty into a deep venous smus. The central retinal artery and vein are encased in the optic nerve, which enters the eye at the optic disc (see Fig. 8.8). --~~- Ciliary body - - """\"'rH-- - Ciliary muscle...- - -tr- t - - - Lens -'N c..,---- Retirw. ~rL---- Choroid '7"=~---- Sdara Figura 8.12 The retina. VIewed through the pupil with an Figure 8.10 The lens and suspensory ligaments viewed from the ophthalmoscope. (Paul Barker/Science Photo Ubrary. Reproduced front. The iris has been removed. with permission.) Maa.Jia lutea Conaaonly Rods ConHhapad Rod-shaped andconaa nerve cell nerve eel A B c Figure 8.11 The retina. (A} Magnified section. (B) Ught..sensltlve nerve cells: rods and cones. (C) Coloured scanning electron micrograph of rods (gf'96fl) and cones (blue). (C, Omlkron/Sclence Photo Ubrary. Reproduced with permission.) 215  SECTION 2 Communication Interior of the eye The anWi.or chamber of the eye, ie. the space between the cornea and the lens, is incompletely divided into anterior and posterior cavities by the iris (see Fig. 8.8). Both cavities contain a clear aqueous fluid, aqueous humour, secreted into the posterior cavity by the ciliary glands. It circulates in front of the lens, through the pupil into the anterior cavity, and returns to the venous circulation through the scleral venous Optic chlaam1 sinus (canal of Schlemm) in the angle between the iris and cornea (see Fig. 8.8). The intraocular pressure remains fairly constant between 1.3 and 2.6 kPa (10 and 20 mmHg), as production and drainage rates of aqueous humour are equal. An increase in this pressure causes glaucoma (p. 228). Aqueous humour supplies nutrients and removes w~s from the transparent structures in the front of the eye that have no blood supply, ie. the cornea, lens and lens capsule. Behind the lens and filling the posterior chamber of the eyeball is the vitreous body. This is a soft, colourless, transparent, jelly-like substance composed of 99% water, mineral salts and mu.coprotein. It maintains sufficient intraocular pressure to support the retina against the choroid and prevent the eyeball from collapsing. The eye keeps its shape because of the intraocular pressure exerted by the vitreous body and the aqueous humour. Optic neNes {second cranial neNes) The b."bres of the optic nerve (Fig. 8.13) originate in the retina Vlaual 1ra1 In ccdpllallobe of cerabrum and converge to form the optic nerve about 0.5 em to the nasal side of the macula lutea at the optic disc. The nerve Figure 8.13 The optic nerves and their pathways. pierces the choroid and sclera before passing backwards and medially through the orbital cavity. It then passes through of the cerebrum (see Fig. 7.20). Other neurones originating in the optic foramen of the sphenoid bone, backwards and the lateral geniculate bodies transmit impulses hom the eyes medially to meet its counterpart from the other eye at the to the cerebellum, where, together with impulses from the optic chiasma. semicircular canals of the inner ears and from the skeletal muscles and joints, th~ contribute to the maintenance of Optic chiasma posture and balance. 8.2 This is situated immediately in front of and above the pituitary gland, which lies in the hypophyseal fossa of the sphenoid bone (see Fig. 9.2). In the optic chiasma the nerve Physiology of sight fibres of the optic nerve from the nasal side of each retina Light waves travel at 300,000,000 metres per second (m/s), cross over to the opposite side. The fibres from the temporal or about 186,000 miles per second. This is much greater than side do not cross but continue backwards on the same side. the speed of sound in air (about 340 m/s) and explains why This crossing over provides both cerebral hemispheres with we see lightning before hearing thunder. sensory input from each eye. Light is reflected into the eyes by objects within the field of vision. White light is a combination of all the colours of the Optic tracts visual spectrum (rainbow), i.e. red, orange, yellow, green, These are the pathways of the optic nerves, posterior to the blue, indigo and violet This is demonstrated by passing optic chiasma (see Fig. 8.13). Each tract consists of the nasal white light through a glass prism that refracts (bends) the fibres from the retina of one eye and the temporal fibres rays of the different colours to a greater or lesser extent, from the retina of the other. The optic tracts pass backwards depending on their wavelengths (Fig. 8.14). Red light has the to synapse with nerve cells of the lateral geniculate bodies longest wavelength and violet the shortest. of the thalamus. From there the nerve fibres proceed This range of colour is the spectrum of visible light. In a backwards and medially as the optic radiations, to terminate rainbow, white light from the sun is broken up by raindrops, in the visual area of the cerebral cortEx in the occipital. lobes which act as prisms and reflectors. 216  The special senses CHAPTER 8 Refraction of the light rays Beam rA white light Red When light rays pass from a medium of one density to a medium of a different density, they are refracted. Fig. 8.14 Orange shows how a glass prism separates a beam of white light Yellow into its constituent colows, by refracting each wavelength to a different degree. In the eye the biconvex lens refracts and Green focuses light rays (Fig. 8.16). This principle is used to focus Blue light on the retina. Before reaching the retina, light rays pass successively through the conjunctiva. cornea. aqueous fluid, Indigo lens and vitreous body. These structures are all denser than Violet air and, with the exception of the lens, they have a constant refractory power, similar to that of water. This means that although they all refract light rays entering the eye, their Figure 8.14 Refraction. White light is broken into the colours of the ability to do this is fixed and so cannot be adjusted to assist visible spectrum when it passes through a glass prism. in focusing. The electromagnetic spectrum The electromagnetic spectrum is broad but only a small part is visible to the human eye (Fig. 8.15). Beyond the long end are infrared waves (heat), microwaves and radio waves. Beyond the short end are ultraviolet (UV) rays, Xwrays and gamma rays. UV light is not normally visible because it is absorbed by a yellow pigment in the lens. Following removal Ughtrays of the lens (cataract extraction), it is usually replaced with an artificial one to prevent long-term damage to the retina from UV light rays. A specific colour is perceived when only one wavelength is reflected by the object and all the others are absorbed, e.g, an object appears red when it reflects only red light. Objects A appear white when they reflect all wavelengths of light into the eye, and black when they absorb all the light hitting them and so reflect nothing. In order to achieve clear vision, light reflected from objects within the visual field is focused on to the retina of each eye. The proc:esses involved in producing a clear visual image are: refraction of light rays change in the size of the pupils accommodation (adjustment of the lens for near vision; see below). Ught rays focused on to reUna B (at mawla lutaa) Each of these processes is explained individually in the next.sections, but effective vision is dependent on all three Figura 8.16 Refraction of light rays passing through a biconvex operating in a coordinated manner. lena. (A) A glass lens. (B) The lens In the f'1>/8. The spectrum of visille light LONG WAVELENGTH SHORT WAVELENGTH Ultraviolet RaciD 'IWMII Mlci'DWIMIS X-rays G.nma 111)'1 rays Figura 8.15 The electromagnetic spectrum. 217  SECTION 2 Communication If the pupils were dilated in bright light. too much light would enter the eye and damage the sensitive retina. In dim light. if the pupils were constricted, insufficient light would enter the eye to activate the light-sensitive pigments in the rods and cones, which stimulate the nerve endings in the retina, enabling vision. The iris consists of one layer of circular and one of mdiating smooth muscle fibres. Contraction of the circular :&"bres constricts the pupil, and contraction of the mdiating :&"bres dilates it The size of the pupil is controlled by the autonomic nervous system; sympathetic stimulation dilates the pupils and parasympathetic stimulation constricts them. Accommodation Near vision Figure 8.17 Section of the eye showing the focusing of light rays In order to focus on near objects, ie. within about 6 metres, on the rvtina. Diagrammatic representation of light rays reaching the retina showing inverted image. accommodation is required and the eye must make the following adjustments: constriction of the pupils Focusing of an image on the retina convergence Light rays reflected from an object are refracted by the lens change in the refractory power of the lens. when they enter the eye, as shown in Fig. 8.16, although the image on the retina is actually upside down (Fig. 8.17). The brain adapts to this early in life so that objects are perceived Constriction of the pupils 'the right way up'. This assists accOII\IIIDdation by reducing the width of the beam of light entering the eye so that it passes through the Abnormal refraction within the eye is corrected using central curved part of the lens (see Fig. 8.17). biconvex or biconcave lenses (p. 230). convergence {movement at the eyeballs) Lens Light rays from nearby objects enter the two eyes at different The lens is a biconvex elastic transparent body suspended angles and for clear vision they must stimulate corresponding behind the iris from the ciliary body by the suspensory areas of the two retinae. Extrinsic muscles move the eyes and ligament (see Fig. 8.10) and is the only structure in the eye to obtain a clear image they also rotate them so that they able to change its refractive power. Light mys entering the converge on the object viewed. This coordinated muscle eye need to be refracted to focus them on the retina. Light activity is under autonomic control When there is voluntary from distant objects needs least refraction and, as the object movement of the eyes, both eyes move and convergence is comes closer, the amount of refraction needed increases. To maintained. The nearer an object is to the eyes, the greater focus light rays from neat objects on the retina, the refractory the eye rotation needed to achieve convergence; fut example, power of the lens must be increased - by accommodation. focusing near the tip of one's nose gives the appearance of To do this, the ciliary muscle contmcts, moving the ciliary being 'cross-eyed'. If convergence is not complete, the eyes body inwards towards the lens. This lessens the pull on are focused on different objects or on different points of the the suspensory ligaments and allows the lens to bulge, same object There are then two images sent to the brain and increasing its convexity and focusing light rays on the retina this can lead to double vision, or diplopia. If convergence is (Fig. 8.18B). not possible, the brain tends to ignore the impulses received To focus light mys from distant objects on the retina, from the divergent eye (see Sql.rlnt, p. 229). the circular ciliary muscle relaxes, increasing its pull on the suspensory ligaments. This makes the lens thinner and Changing the refractory power al the lens focuses light mys from distant objects on the retina (Fig. Oumges in the thickness of the lens are made to focus light 8.18A). on the retina. The amount of adjustment depends on the distance of the object from the eyes, i.e. the lens is thicker for near vision and at its thinnest when focusing on objects more Size of the pupils than 6 metres away (see Fig. 8.18). Looking at near objects Pupil size contributes to clear vision by controlling the 'tires' the eyes more quickly, owing to the continuous use amount of light entering the eye. In bright light the pupils of the ciliary muscle. The lens loses its elasticity and stiffens are constricted and in dim light they are dilated (Fig. 8.19). with age, a condition known as presbyopia (p. 225). 218  The special senses CHAPTER 8 Distant vision Ciliary muade In the ciliary body relaxed Cliary muscle in the cilary body contracted A _ -- Suspensory ligaments slack ' Lens bulges B Figure 8.18 Accommodation: acUon of 111e ciliary muscle on 1he shape of 111e lens. (A) Distant vision. (B) Near vision. Smoolh muada ftbrae of lrts pigments in these cells and they generate nerve impulses, which are conducted to the occipital lobes of the cerebrum Cira.ll Radial Radial fibnls contract via the optic nerves (see Fig. 8.13). Cones The cones are sensitive to light and colour; bright light is required to activate them and give sharp, dear colour vision. Cones are concentrated at the macula lutea, which is the spot on the retina where light rays fall from an object in the direct Blight light Nor11111lllght Dim light field of vision. This means that whatever is being directly observed is seen in detail, brightly coloured and in sharp Figure 8.19 Changes in pupil size in response to intensity of light. focus. Cone numbers fall sharply and rod numbers rise on the retina peripheral to the macula lutea. Distant vision Rods Objects more than 6 metres away from the eyes are focused The rods are much more light-sensitive than the cones on the retina without adjustment of the lens or convergence (see Fig. 8.11), so they are used when light levels are low. of the eyes. Stimulation of rods leads to monochromic (black and whitE) vision. Rods outnumber cones in the retina by about 16 : 1 and are much more numerous towards the periphery. Functions of the retina The retina is the light-sensitive (photosensitive) part of Rhodopsin& the eye. The light-sensitive nerve cells are the rods and This is a family of light-sensitive pigments, found in both cones, and their distribution in the retina is shown in Fig. rods and cones, which are broken down (bleached) when 8.11A light rays cause chemical changes in light-sensitive they absorb light hitting the cell. Degradation of the 219  SECTION 2 Communication rhodopsin molecule generates an action potential Following bleaching, the rhodopsin molecule has to be reassembled before it is functional again. There is only one type of rhodopsin in rods, absorbing at a single wavelength, which is why rods give monochromatic vision. Cones, however, have one of three different rhodopslns, absorbing at three different wavelengths and giving rise to so-