Week 1 Class Notes PDF

STRUCTURES EXTERNAL TO THE EYE THE ORBITAL CONNECTIVE TISSUE NETWORK LIDS AND LASHES CONJUNCTIVA THE ORBITAL CONNECTIVE TISSUE NETWORK The dense connective tissue network of the orbit which lines, covers and separates orbital structures. It also serves to anchor soft tissue structures and compartme...

STRUCTURES EXTERNAL TO THE EYE THE ORBITAL CONNECTIVE TISSUE NETWORK LIDS AND LASHES CONJUNCTIVA THE ORBITAL CONNECTIVE TISSUE NETWORK The dense connective tissue network of the orbit which lines, covers and separates orbital structures. It also serves to anchor soft tissue structures and compartmentalises areas of the eye. While the connective tissue network is continuous, it is described according to its position and function. a. Periorbita (Orbital fascia) - outermost b. Orbital septum (Palpebral fascia) c. Tenon’s capsule (Fascia bulbi) - innermost Periorbita is a thin, tough, connective tissue lining of all structures within the orbit, including the bony fossa itself. Membrane which serves as an attachment site for muscles, tendons and ligaments. It also represents a support structure for the blood supply of the orbital bones. Tenon’s capsule (fascia bulbi) is a thin membrane of elastic and collagen fibres which envelops the globe from the limbus to the optic nerve. Composed of collagen fibrils arranged irregularly in different orientations, forming a three-dimensional network that provides tissue resistance to stress. It has been found to be less elastic and thicker in individuals with infantile esotropia which may be stress induced alterations due to prolonged deviation of the eye (Shauly et al, 1992). It is closely bound to the sclera with fine fibres, and anteriorly attached to the sclera round the limbus. The following structures pass through Tenon’s capsule: the optic nerve, ciliary nerves, ciliary arteries, vortex veins and tendons of the extraocular muscles. The capsule is thicker beneath the eye and thus acts as a suspensory (hammock-like) supporting band which counteracts the gravitational displacement of the globe. Here it is known as the suspensory ligament of Lockwood. The capsule also provides sheaths of connective tissue around the extraocular muscles from where they pass through. 1 The suspensory ligament of Lockwood is referred to a hammock-like structure which is a dense connective tissue that runs from its attachment on the lacrimal bone at the medial orbital wall to the zygomatic bone on the lateral wall. Tenon’s capsule, sheaths of 2 extraocular muscles and the inferior lid aponeurosis makes up this ligament. This ligament is of significance since it is able to afford some support to the globe, especially if the bones in the floor of the orbit are absent. Orbital septum (Palpebral fascia) connects the bony frame of the orbit with the eyelid. It is regarded as the framework of the eyelid and separates the lid from the orbital cavity. The orbital septum is of particular significance as it provides a barrier function to the spread of disease beyond the eyelid into the orbit. 2 LIDS & LASHES STRUCTURE The lids are considered to be motile modifications of the skin, underlying muscle and associated tissues. It is covered anteriorly by skin and posteriorly by mucous membrane. The aperture between the lids is known as the palpebral aperture. In the open position, the lid margins meet at an angle which encloses an inner and outer canthus. The outer canthus makes an angle of between 30 to 40, but the inner one is not so distinct. source: homepage.mac.com/.../images/eye-and-orbit.gif 3 The inner canthus contains a small pink pad of tissue known as the caruncle which is actually an undeveloped section of the lower lid. It contains sweat glands and sebaceous glands with their openings being in about 15 to 20 tiny hair follicles. These serve to trap impurities in the tears. Next to the caruncle, can be found the plica semilunaris, which is a small crescent of folded tissue, representing the vestige of a third eyelid which is present in other mammals. As it is more vascular than the rest of the conjunctiva, it is light red. Source: www.indiana.edu/~pietsch/liveeye33edited.jpg The eyebrow is regarded as the upper limit of the upper lid, demarcating it from the forehead. The superior palpebral furrow or sulcus is found at this point. The brow is a mobile elevation of skin covered with hairs, and functions to divert moisture from the palpebral aperture to a certain extent, serve to warn against approaching danger from above, and act in important facial expressions. The free margin (intermarginal strip) of the lid is relatively flat and about 2mm wide and 30mm long, sharp edged posteriorly and rounded over anteriorly. From the anterior edge emerges the lashes (ciliary portion) except in that part encircling the caruncle and plica (lacrimal portion). The puncta, about 0.3mm in diameter, are to be found at the junction of the lacrimal and ciliary portions. The Meibomian gland orifices are to be found along the margin of the ciliary portion, and the skin terminates just in front of these openings, and is replaced by the palpebral conjunctiva which lines the inner surface of the lid. (source: www.bartleby.net) The margins of the eyelids are normally closely applied to the globe and pathologic states can cause the eyelid margins to lose normal contact with the globe and to turn either in toward the globe (entropion) or away from the globe (ectropion – common consequence of Bell’s palsy). In senile ectropia, the lower lid may sag outward as the orbicularis loses tone. Tears can then spill over the lid margin. 4 The lid is composed of six or seven layers of tissue. The skin of the lids is the thinnest in the body and highly elastic. It therefore folds easily and permits rapid opening and closing of the palpebral fissure. In each lid there is a wide fibrous tarsal plate which follows the curvature of the eyeball and is firmly attached to the medial and lateral palpebral ligaments and to the orbital septum. The tarsus consists of dense fibrous tissue and contains no cartilage. There are about two to three rows of lashes, totaling to up to 100 to 150 in the upper lid and half that number in the lower lid, which serve to screen the half-closed eyes against foreign bodies and to divert moisture. The lashes on the upper lid curve outward and upward, and that on the lower lid curve outward and downward. Their lifespan is about 3- 5 months, after which the lash falls out and a new one grows in its place. If a cilium is pulled out, the new one replacing it reaches full size in about 2 months. Each lash base has a modified pair of sebaceous glands (glands of Zeiss) which open into the hair follicle by short, wide ducts. Infection of these glands result in a hordeolum, and excessive secretion of the glands produce marginal blepharitis. The follicles can penetrate as far back as the tarsal plate. Each follicle is surrounded by a nerve plexus with a very low threshold of excitation and thus touching a cilium is sufficient to excite the nerve plexus and produce a reflex blink. The skin of the lids is loosely attached to the underlying muscle by thin areolar tissue which is easily pulled or pushed away thereby creating a potential cavity which can be readily distended by blood from a haemorrhage (black eye), as escape of lymph (oedematous or puffy eye), or air from a fractured sinus wall. FUNCTION: Primarily protection by: the screening and sensing action of the cilia (eyelashes) against small foreign bodies. secretions of the glands of the eyelids, including lubrication of the eye together with the lacrimal system. During closure of the eye, the capillary network beneath the lids is a source of nourishment to the cornea. the movement of the eyelids protection against excessive radiation GLANDS OF THE LIDS Meibomian Glands (sebaceous glands) They were first discovered by Meibomius in 1666. They are believed to be modified sebaceous glands which have lost their cilia in the course of evolution. They are located in the tarsal plate. The meibomian secretion is produced by cubical glandular epithelium and is thought of functioning as a precorneal tear film to prevent evaporation of the tear film, restrain tear overflow at the lid margin and prevent adhesion of the lids. There are approx 25 glands in the upper lid and 20 shorter ones in the lower lid. There is a possibility of blockage of these glands, which raises lumps on the inside of the lids and can be a source of corneal irritation or pressure. 5 (source: www.bartleby.net) Glands of Zeiss (sebaceous glands) and Moll (sweat glands) Both these glands are associated with the cilia and are usually paired and attached to each hair follicle opening into it. The glands of Moll resemble small sweat glands The eyelids also contain the accessory lacrimal gland tissue of Krause and Wolfring. 6 Source: anatomy.iupui.edu/.../lecture.f04/Eyef04/fov.jpg Sagittal section through the upper eyelid. (After Waldeyer.) a. Skin. b. Orbicularis oculi. b’. Marginal fasciculus of Orbicularis (ciliary bundle). c. Levator palpebræ. d. Conjunctiva. e. Tarsus. f. Tarsal gland. g. Sebaceous gland. h. Eyelashes. i. Small hairs of skin. Sweat glands. k. Posterior tarsal glands. (source: www.bartleby.net) THE LOWER LID The lower lid is a reduced version of the upper lid, and has no distinct boundary at the cheek. The lower tarsal plate is smaller and more rectangular. It has no separate muscle corresponding to the levator and is less mobile, rising only slightly on closure of the lids. The lower lid is however, thicker. The skin of the lower lid sags more readily than the other. The upper lid in the open position overlaps the corneal margin, the lower not. The upper lid is more difficult to evert. The capsulopalpebral fascia in the lower lid serves to retract in on downgaze. 7 BLOOD SUPPLY The bloods supply to the lids is principally from branches of the ophthalmic artery. Medially, the area is served by two branches of the medial palpebral artery which anastomose with the lateral palpebral artery, which is an offshoot of the lacrimal artery. Veins of the eyelids are bigger and more numerous than arteries and are of two sorts. Divisions of the ophthalmic vein receive blood from the conjunctiva and pretarsal drainage and goes to the angular vein and the superficial temporal vein. Blood vessels of the eyelids, front view. 1, supraorbital artery and vein; 2, nasal artery; 3, angular artery, the terminal branch of 4, the facial artery; 5, suborbital artery; 6, anterior branch of the superficial temporal artery, 6 malar branch of the transverse artery of the face; 7, lacrimal artery; 8, superior palpebral artery with 8’, its external arch; 9, anastomoses of the superior palpebral with the superficial temporal and lacrimal; 10, inferior palpebral artery; 11, facial vein; 12, angular vein; 13, branch of the superficial temporal vein. LYMPHATIC SUPPLY Two systems of the lymphatic vessels drain the lids. The lower set drains the lower lid and nose to reach the submandibular nodes. A second set drains the upper lid into the parotid nodes. 8 NERVE SUPPLY The nerve supply of the lids is from the trigeminal nerve (Vth), with the upper lid being supplied by the ophthalmic division and the lower lid is innervated by the maxillary division. The motor nerve supply of the orbicularis oculi, is from the Facial nerve (7th). The sympathetic system is also involved innervating the involuntary palpebral muscles (Muller’s muscle), and interruption causes ptosis, whilst excitation at the level of the cervical system causes staring. Secretory nerves supply the Meibomian glands. The oculomotor nerve (3 rd) supplies the levator palpebral. EYELID MOVEMENTS Elevation of the upper lid is achieved by means of a contraction of the levator palpebral superioris (innervated by the 3 rd cranial nerve) and Muller’s muscle (supplied by the sympathetic nervous system) which is smooth muscle fibres from under surface of the levator. In extreme upward gaze, the frontalis muscle aids the levator in further elevating the lid. The levator muscles of the two upper lids behave as yoke muscles in accordance with Hering’s law. Thus when the levator on one side is weak, eg. in unilateral myasthenia gravis, the lid on the unaffected side may be retracted in an unconscious attempt to elevate the ptotic lid. If the eye with the ptosis is covered, relieving the effort needed to elevate the lid, the retraction on the normal side disappears. The upper lid follows the globe on voluntary upward gaze, however, in reflex blinking, the globe and the lid move in opposite directions. The globe turns upward as the eyelids close and downward when they open. If the levator is paralyzed, the Muller’s fibres don’t have any firm origin from which to pull and their contraction become ineffective thereby resulting in an incomplete ptosis. However, if the sympathetic nerve supply is paralyzed, only a slight droop of the lid results, as seen in Horner’s syndrome. The eyelids are closed by the action of the orbicularis oculi muscle (innervated by the 7 th cranial nerve). This muscle is adherent to the skin and is divided into an orbital, preseptal and pretarsal portion. The orbital portion is primarily involved in forcible closure, whilst the preseptal and pretarsal muscles participate in the lacrimal pump. There are three different types of eyelid closure viz blinking (reflex and spontaneous), voluntary winking and blepharospasm. 9 Reflex blinking can occur in response to tactile, optic or auditory stimuli. Spontaneous blinking occurs at frequent intervals during waking hours and without any obvious stimuli. Each person may have an individual rate of blinking. Spontaneous blinking does not occur or is infrequent during the first few months of life. The blink rate continues in person who is blind and therefore doesn’t depend on retinal stimulation. During blinking there is a narrowing of the palpebral aperture in a zipper-like action from the lateral canthus toward the medial canthus which aids the displacement of the tear film toward the lacrimal puncta. During blinking the lower lid remains stationary. Most persons blink about 15 times /minute and the duration of a full blink is approximately 0.3 to 0.4 seconds. The average period between blinks is 2.8 sec in men and just under 4 sec on women. Voluntary winking is produced by the simultaneous contraction of the palpebral and orbital portions of the orbicularis muscle. Blepharospasm is a frequent accompaniment of inflammatory diseases of the anterior segment. Bell’s Phenomenon is a protective action that brings the cornea up under the covering eyelid and away from impending danger. It is however, not present in 10% of individuals and therefore is not necessarily a sign of disease. 10 Jaw Winking / Marcus Gunn is the opening and shutting of the eye on one side during the act of chewing. It is due to the pterygoid muscle being abnormally linked neurologically with the levator muscle. THE PALPEBRAL APERTURE The normal palpebral aperture is about 27-30mm long and in the primary position 8- 11mm wide, and is almond shaped. In children, the fissures are not so long but are relatively wider, and in infants the fissures may be nearly circular. The edge of the upper lid covers the upper limbus from 10 to 2 0’clock position, and the edge of the lower lid usually lies les than 1mm below the lower limbus. If the upper limbus is completely exposed but bilaterally symmetric, it may have little significance. Unequal apertures however have pathologic significance. The width of the aperture can also reflect the psychologic state of the individual. The width also depends on the combined tonus of the levator, Muller’s muscle and the orbicularis eg. when fatigued the levator loses its tonus causing the lids to feel heavy. Widening of the apertures caused by retraction is seen in thyrotoxicosis. It is called Dalrymple’s sign and can be caused by either excessive tonus of the Muller’s muscle or excessive tonus of the levator. 11 THE CONJUNCTIVA The conjunctiva is a membranous junction between the lids and the globe of the eye which forms a barrier between the orbit and the outside. It is described as a mucous membrane. The conjunctiva lines the inner surface of the lids and is reflected to lie against the surface of the sclera, with which it fuses at the corneo-scleral junction or a little further into the cornea proper. Medially the conjunctiva folds into the plica semilunaris. It has a palpebral part, a fornix and a bulbar portion. The palpebral portion is that lining the lids. The conjunctiva where the inner part of the eyelids and the eyeball meet, the palpebral conjunctiva is reflected at the superior fornix and the inferior fornix to become the bulbar conjunctiva. The conjunctival fornix forms a folded linear sac which prevents stretching when the eye moves. The bulbar conjunctiva lines the globe, is thin and transparent and loosely attached to the tissue beneath except at the limbus in a 3mm wide zone. The palisades of Vogt are found in the limbal conjunctiva and can be seen as radial ridges about 0.5mm wide and 1-2mm long. They appear as light elevations, often with pigment in the furrows and are most distinct at the lower limbal areas. HISTOLOGY Histologically, the conjunctiva comprises of: an epithelium containing goblet cells which secrete mucous fluid the substantia propria, which is a connective tissue structure containing nerves and blood vessels and lymph vessels. The main structures are the surface epithelium and underlying connective tissue (stroma). At the margin there are many- layered, non-keratinized squamous epithelium resistant to “wear and tear”. The epithelium in the different portions serve as excellent traps for debris and assists in moving the trapped debris nasally with blinks for collection by the hairs of the caruncle. The stroma has a superficial adenoid layer containing lymphocytes, and is thickest at the fornix, thinner elsewhere but absent in the marginal and tarsal zones. The stroma can be divided into an adenoid layer and a fibrous layer. The adenoid layer contains lymphoid tissue and is some area may contain “follicle-like” structures. The adenoid layer does not develop until after the first 2 to 3 months of life which explains why inclusion conjunctivitis of the newborn is papillary in nature rather than follicular. The fibrous layer is composed of connective tissue that attaches to the tarsal plate which explains the appearance of the papillary reaction in inflammations of the conjunctiva. 12 The goblet cells are able to maintain a sufficient supply of lacrimal fluid even if the lacrimal glands fail. Failure of all of the goblet cells, eg in xerosis, leads to drying of the conjunctiva. The mucus produced is also responsible for the proper dispersion of the precorneal tearfilm. The accessory lacrimal glands (Krause and Wolfring– since their secretion resembles that of the lacrimal gland) are located in the stroma. The glands of Krause are found in each fornix (about 42) and approx 6-8 in the lower fornix. The glands of Wolfring are larger than those of Krause. There are up to five found near the upper region of the tarsus and fewer below the lower lid. Since there is no nervous control of these glands, they maintain a constant level of tears. CONJUNCTIVAL BLOOD VESSELS The blood supply arises from the palpebral arterial arcades which produce the posterior palpebral arteries, reaching backwards via the fornices to the limbus. Here they anastomose with the anterior ciliary arteries, and produce a deep plexus of vessels in the scleral tissues. The veins closely accompany the arteries, emptying into the venous network of the lids. The conjunctiva is rich in lymphatics, which also drain through the vascular arcades of the eyelids. When there is an inflammation of the cornea or conjunctiva, the conjunctival vessels dilate, making the tissues appear red. NERVE SUPPLY Sensory innervation for the bulbar conjunctiva is from the long ciliary nerves which are branches of the nasociliary nerve. The upper fornix and palpebral conjunctiva are served by the frontal and trochlear divisions of the ophthalmic nerve, while the lacrimal nerve covers the region of the outer canthus. The conjunctiva of the lower eyelid is innervated by the infraorbital nerve. CONJUNCTIVAL BACTERIA From the first week of life the conjunctiva carries bacteria, usually equal between both eyes but subject to variation in individuals. These bacteria resemble those common to the skin. However, due to its relatively low temperature due to exposure, evaporation of lacrimal fluid and moderate blood supply, these bacteria do not readily propagate themselves. Furthermore, the tears are not a good culture medium and contain a bacteriostatic enzyme, lysozyme. 13 REFERENCES: Spooner, JD. (1957) Ocular Anatomy. The Hatton Press Ltd: London. Moses RA, Hart WM. (1987) Adler’s Physiology of the eye – clinical application. The CV Mosby Company: St Louis. Miller SJH. (1990) Parson’s diseases of the eye (18th edition). Churchill Livingstone: Edinburgh. Saude T. (1993) Ocular Anatomy and Physiology. Blackwell Scientific Publications: London. Shauly Y, Miller B, Lichtig C, Modan M and Meyer E. (1992) Tenon’s capsule: ultrastructure of collagen fibrils in normals and infantile esotropia. Investigative Ophthalmology & Visual Science 33: 651-656. Riordan-Eva P and Whitcher JP. (2004) Vaughan & Asbury’s General Ophthalmology (16th edition) McGraw Hill: Boston. 14  THE LACRIMAL APPARATUS The lacrimal system of the eye is involved in tear production and drainage. Secondarily, it is also involved in ensuring that adequate tear distribution is maintained over the cornea. LACRIMAL GLAND The lacrimal gland is found at the upper, temporal aspect of the orbit. It weighs about 1g and is a flat acinous gland. The tendon of the levator muscle almost divides it into an upper 2/3rd or orbital part (attached to the orbital roof) and a lower palpebral part. Histologically, the gland comprises of a mass of lobules each one consisting of a further mass of small tubular acini with a single layered pyramidal epithelium wall. The secretory product of these cells loose their granules after secretion, converges on the ducts to enter the conjunctival sac. It comprises two types of secretory cells, one secretes mucus and the other a serous product. In the liquid there is lysozyme and other proteins. The tubules contain contractile myoepithelial cells which assist in expressing the secretions, which arise in the acinous cells and are collected into a system of ducts. The final product has a proteinous content, with bactericidal property supplied by lysozyme and immunoglobulin. The gland, together with its accessory glands, also contributes the aqueous layer of the tear film. Source: anatomy.iupui.edu/.../lecture.f04/Eyef04/fov.jpg 1 The gland obtains its blood supply from the lacrimal artery and is drained by the lacrimal vein into the ophthalmic vein. The accessory lacrimal glands (Krause and Wolfring) number about 60, and are located in the conjunctiva of the two eyelids and are responsible for the basal secretion. They produce 5% of the aqueous component of the tears. Innervation of the lacrimal gland Parasympathetic innervation comes from the salivary nucleus via the intermediate nerve to the geniculate ganglion and then through the major petrosal nerve. These pre- ganglionic fibres end in the pterygopalatine ganglion. From this ganglion, the fibres follow the zygomatic nerve laterally and forwards in the orbit via the lacrimal nerve to the lacrimal gland. Here there is contact by the parasympathetic fibres, with the secretory cells and the cells of the outgoing ducts. The pontine secretory centre is under the influence of the brain and responds to mechanical irritation, emotional disturbances, smell or taste. The sympathetic innervation from the secretion centre moves along to the superior cervical ganglion. Post-ganglionic fibres then follow the internal carotid artery to the major petrosal nerve and from here together with the parasympathetic fibres, reaches the gland. The sympathetic fibres end at the blood vessels and thus have a regulatory effect on the gland’s blood supply. Source: en.wikipedia.org/wiki/Lacrimal_gland 2 The normal flow of tears is under sympathetic control chiefly by a regulation of the gland’s blood supply. Strong light and air pollution tend to increase lacrimation. Reflex lacrimation arises from corneal or conjunctival irritation, coughing, sneezing, taste or smell. Some individuals experience a marked flow of tears during a meal called ‘crocodile tears’. They arise from a faulty nervous connection which diverts impulses to the salivary glands and to the lacrimal glands as well. DRAINAGE OF TEARS The lacrimal puncta are small round openings about 0.3mm in diameter leading to the canaliculi or tear canals. The puncta are to be found on each papilla located 5mm lateral to the inner canthus. Each papilla rise about 1mm above the lid margin and are directed backwards, just dipping into the lacrimal lake formed by the tears near the inner canthus. The tear canals are about 10mm long after a sharp bend. Their initial 2mm before the sharp bend run vertically upwards in the top lid and downwards in the lower lid. The two passages then bend sharply into the lacrimal sac. The lacrimal sac occupies the lacrimal fossa, and is about 12mm long. Its lumen is stretched open on account of the thin surrounding layer of connective tissue called the lacrimal fascia. The lacrimal sac has fibrous attachments with Horner’s muscle and contraction of this muscle dilates the sac. The lacrimal sac merges with the nasolacrimal duct and empties into the inferior nasal meatus. The whole passage is about 15mm long moving downwards and laterally. The mucous membrane of the region which opens into the nose produces a fold, the plica lacrimalis, or Hasner’s valve, which overhangs the area where the nasal passage begins. This valve stops air from moving upwards when the nose is blown. KINETICS OF THE TEARS The tears form a thin film over the cornea and conjunctiva. At the ciliary margin of the lower lid, and to some extent along the upper lid, there is a tear meniscus / prism which is normally about 0.2mm. The tear fluid comes out of the exit canals of the main and accessory lacrimal glands, glowing across the eye to reach the marginal tear prisms of both lids. At each blink, the tears prism is moved upwards and downwards across the conjunctiva and cornea. A continuous stream also comes from the temporal side towards the puncta. On closing the eye, the tears are swept towards the inner canthus. Each punctum is kept open by strong fibrous coating and uses capillary action to take in liquid. Fibres of the lacrimal portion of the orbicularis make closure movements of the lower lid, pressing on 3 the ampulla and shortening the canaliculi which encourages the flow of liquid to the lacrimal sac. There is a reduction of orbicularis tonus as the eyes open. Then the larimal sac flattens and the ampulla expands, producing suction to bring in fresh liquid. THE TEAR FILM Source: www.theschepens.org/images/tear_film.gif The tear film is composed of 3 layers: i. superficial lipid layer (outer layer) This layer is produced by the meibomian glands in the lid margins. It is composed mainly of cholesterol, fatty acids and phospholipids. Its functions include: a. stabilizes the tear film. b. Reduces evaporation of the aqueous layer. c. Increases surface tension and therefore assists in the vertical stability of the tear film preventing them from overflowing the lower lid margin. d. Lubricates the eyelids as they pass the surface of the globe. Reasons for lipid layer deficiencies include: a. contact lens wear b. lid problems eg. meibomian glands blocked due to electrolysis c. Bell’s palsy – lagophthalmos. ii. Aqueous layer (middle layer) 4 This layer is about 7mm thick and is produced by the main lacrimal gland and accessory glands. Its forms a major portion of the tear film. It contains salts, proteins (primarily albumin), glucose, lactate, water soluble molecules. Its functions include: a. supplies atmospheric oxygen to the corneal epithelium. b. Has antibacterial substances eg. lactoferrin and lysozyme therefore the dry eye is more susceptible to infection. c. Provides a smooth optic surface by abolishing any irregularities in the cornea. d. Washes away debris from the conjunctiva and cornea. Reasons for aqueous layer deficiencies (keratoconjunctivitis sicca) include: a. a decrease in aqueous production by the lacrimal gland. b. Trauma to the lacrimal gland c. Related to medication / environment. iii. Mucin layer (inner layer) This is the innermost layer adjacent to the epithelium. It is secreted by the goblet cells in the conjunctiva and by crypts of Henle and glands of Manz. It spreads over the cornea by a wiping action of the lids. Its functions include: a. converts corneal epithelium to a hydrophilic surface therefore allowing the tear film to adhere to the epithelium and allowing the cornea to be wetted by the aqueous tears. (mucin which is a glycoprotein, is absorbed onto the cell membranes of the epithelial cells and anchored by the microvilli). Source: eyelearn.med.utoronto.ca/.../images/14Tear11.jpg 5 Reasons for mucin layer deficiencies: a. goblet cells destroyed especially in trachoma. b. Vitamin A deficiency (hypovitaminosis A) which decreases the stability of mucin and leads to xerophthalmia. The average pH of tears is 7.35 but can range from 5.20 – 8.35. Source: webeye.ophth.uiowa.edu/.../doxy-1.gif THE MECHANISM OF BLINKING The blinking mechanism is responsible for the maintenance of corneal and conjunctival moisture by the tears, as well as, for the drainage of tears. Closure of the lids is as a result of contraction of the palpebral portion of the orbicularis, and opening the palpebral aperture is by means of the levator muscle. Blinking can be voluntary or involuntary, and involuntary blinking can further be spontaneous or reflex. Spontaneous blinks arise in the blink centre in the basal ganglia. The frequency of blinking during waking hours is between 15 and 20 times a minute, however, variations occur according to the state of attention or activity of the individual, and on the environmental conditions. Reflex blinking is the result of a direct stimulus which can include flashes of light, mechanical contact with the cornea, conjunctiva or eyelids; as well as strong sudden sounds. Voluntary blinking takes place when a deliberate closure of the palpebral aperture takes place, and can be controlled. Closing of the eyelids is accompanied by a movement of the eye (Bell’s phenomenon) which is a co-ordinated reflex involving the nuclei of the facial and oculomotor nerves. As the eye close, there is a decrease of tonus in the levator muscle and the electrical activity ceases which causes an immediate increase in activity in the superior rectus and decrease in inferior rectus muscle activity. Consequently the eye moves up and out. 6 REFERENCES: 1. Spooner, JD. (1957) Ocular Anatomy. The Hatton Press Ltd: London. 2. Moses RA, Hart WM. (1987) Adler’s Physiology of the eye – clinical application. The CV Mosby Company: St Louis. 3. Miller SJH. (1990) Parson’s diseases of the eye (18th edition). Churchill Livingstone: Edinburgh. 4. Saude T. (1993) Ocular Anatomy and Physiology. Blackwell Scientific Publications: London. 5. Shauly Y, Miller B, Lichtig C, Modan M and Meyer E. (1992) Tenon’s capsule: ultrastructure of collagen fibrils in normals and infantile esotropia. Investigative Ophthalmology & Visual Science 33: 651-656. 6. Riordan-Eva P and Whitcher JP. (2004) Vaughan & Asbury’s General Ophthalmology (16th edition) McGraw Hill: Boston. 7