ElGhazawy Special Sense PDF

Special sense 3 R D Y E A R  ElGhazawy | Special Sense Anatomy 1  ElGhazawy | Special Sense EYE Eye is one of the organs of special sense which carries visual information...

Special sense 3 R D Y E A R  ElGhazawy | Special Sense Anatomy 1  ElGhazawy | Special Sense EYE Eye is one of the organs of special sense which carries visual information & provides us with the sense of sight. It is kept within a bony cavity in the skull (orbital cavity). Protective mechanisms of the eye: 1. Posterior ⅔: Protected by eye orbit 2. Anterior ⅓: By eye lids 3. Tear film: Moistens & protects cornea & conjunctiva The Bony orbit Definition: It is a four-sided pyramid with the apex pointing posteriorly. Boundaries: It has apex, base, four sides: roof, floor, lateral and medial walls. (1) Apex: Lies at the posteromedial end of the orbital cavity. It is near the medial end of the superior orbital fissure. (2) Base: It is the orbital opening. (3) Roof: Is mainly formed by the orbital plate of the frontal bone (separating the orbit from the anterior cranial fossa). (4) Lateral Wall: Is mainly formed by the orbital surface of greater wing of sphenoid. (5) Floor: The orbital surface of the maxilla (separating the orbit from the maxillary sinus). (6) Medial Wall: Lacrimal Bone, orbital plate of the ethmoid bone (separating the orbit from the ethmoidal air sinuses). 2  ElGhazawy | Special Sense Portas and special features of the bony orbit - The roof: Lacrimal fossa for the lacrimal gland at its anterolateral angle. Trochlear fossa for the trochlea of the superior oblique muscle at its anteromedial angle. Optic canal close to the posterior end of the roof. - The lateral wall: Superior orbital fissure, between the lateral wall and the roof posteriorly Inferior orbital fissure, between the lateral wall and the floor posteriorly. - The floor: Infraorbital groove and canal which end on the surface of the skull at the infraorbital foramen. Lacrimal groove for the lacrimal sac. - The medial wall: anterior and posterior ethmoidal foramina at its junction with the roof.. 3  ElGhazawy | Special Sense Anatomy of the Extraocular Muscles There are 7 extraocular muscles: o one acts on the upper eyelid: levator palpebrae superioris o and the remaining six (4 recti + 2 oblique) act on the eyeball. Levator Palpebrae Superioris (L.P.S.) Attachments: Arises from the posterior part of the roof of the orbit. Ends in an aponeurosis which inserts into the: The skin of the upper eyelid. The superior tarsus (this part of L.P.S. is formed of smooth, involuntary muscle fibers called superior tarsal muscle). The superior conjunctival fornix. N. Supply The striated part of the muscle is supplied by the oculomotor N. The smooth part is supplied by the cervical sympathetic derived from the carotid nerve plexus. Actions The muscle is the elevator of the upper eyelid. Clinical Anatomy: Injury to the cervical sympathetic nerve to the head produces Horner’s syndrome: ptosis, miosis, anhidrosis and enophthalmos. 4  ElGhazawy | Special Sense The Four Recti Muscles These are: superior, inferior, medial and lateral recti muscles which are disposed around the eyeball according to their names. Attachments and General Features: 1. All arise from the common tendinous ring o which is attached around the optic canal and the medial part of the superior orbital fissure. 2. All are inserted by expanded tendons o into the sclera 6 - 7 mm behind the corneoscleral junction, anterior to the coronal equator of the eyeball. 3. All form a muscular cone around the optic N. and the posterior part of eyeball. o The apex of the cone is represented by the origin of the muscles at the tendinous ring and its base is represented by the insertion of the muscles into the sclera. 5  ElGhazawy | Special Sense The Two Oblique Muscles Superior Oblique: Attachments: Arises from the roof of the orbit superomedial to the optic canal. It ends in a tendon which passes through a fibrotendinous trochlea attached to the trochlear fossa of the frontal bone; the tendon passes posterolaterally to insert into the sclera posterior to the coronal equator of the eyeball. Inferior Oblique: Attachments: Arises from the floor of the orbit lateral to the nasolacrimal groove. It ascends posterolaterally to insert into the lateral part of the sclera posterior to the coronal equator of the eyeball Nerve Supply of the Extraocular Muscles This could be remembered by the famous equation (LR6,SO4,O3) which means that: lateral rectus is supplied by the abducent N (LR6), superior oblique by trochlear. N (SO4) and other extraocular muscles including (L.P.Superioris) are supplied by oculomotor. N (O3). 6  ElGhazawy | Special Sense Actions of the Extraocular Muscles A. Actions of the Individual Muscles on the Eyeball : 1) Medial rectus : Adduction. 2) Lateral rectus : Abduction. 3) Superior rectus : Elevation, adduction, intorsion. 4) Inferior rectus : Depression, adduction, extorsion. 5) Superior oblique : Depression, abduction, intorsion. 6) Inferior oblique : Elevation, abduction, extorsion. B. Compound Actions of the Six Extraocular Muscles: 1) Adduction : Medial rectus (assisted by superior and inferior recti). 2) Abduction : Lateral rectus (assisted by superior and inferior oblique muscles). 3) Elevation : Superior rectus + inferior oblique. 4) Depression : Inferior rectus + superior oblique. (reading and downstairs) 5) Intorsion : Superior rectus + superior oblique. 6) Extorsion : Inferior rectus + inferior oblique. Muscles act Abducted eye Mid position Adducted eye (L. rectus, S.O + I.O) (Neutral) (M. rectus, S.R + I.R) Elevation Superior rectus Superior rectus + Inferior oblique inferior oblique Depression Inferior rectus Inferior rectus + Superior oblique superior oblique Intorsion Superior rectus + superior oblique Extorsion Inferior rectus + inferior oblique 7  ElGhazawy | Special Sense Nerves within the Orbit Key facts: 1. Nerves within the orbit could be classified into motor, sensory and autonomic as follows: Motor Nerves Sensory Nerves Autonomic Nerves General sensation Vision Sympathetic to Parasympathetic to 3 cranial nerves supply 3 branches of Optic N. - Superior tarsal Ciliary muscle extra ocular muscles: ophthalmic N. muscle (of L.P.S.). (oculomotor N.) Oculomotor Lacrimal - Dilator pupillae Constrictor pupillae Trochlear Frontal - Blood vessels of (oculomotor N.) Abducent Nasociliary the eyeball. Lacrimal gland (facial N.) 2. All the above-mentioned motor and sensory nerves enter the orbit via the superior orbital fissure (SOF) except the optic N. which enters the orbit (with the ophthalmic A.) through the optic canal. 3 nerves (lacrimal, frontal, trochlear nerves) o pass through SOF outside the common tendinous ring, so they could be seen outside the muscular cone formed by the 4 recti. 3 nerves (the superior and inferior divisions of the oculomotor N. with the nasociliary N. in-between and the abducent N.) o pass through the SOF within the common tendinous ring, so they could be seen within the muscular cone formed by the 4 recti. The Optic N. (and the ophthalmic A.) o enters the orbit via the optic canal within the common tendinous ring. Both structures could be seen within the muscular cone. 4. The arrangement of all nerves entering SOF from above downwards could be remembered by the famous statement: Live Free To See No Insult At All (lacrimal, Frontal, Trochlear, Superior division of oculomotor, Nasociliary, Inferior division of oculomotor, Abducent) 8  ElGhazawy | Special Sense The Motor nerves in the Orbit Include the oculomotor, trochlear, abducent nerves: 1) The Oculomotor Nerve a. Intracerebral origin: The oculomotor. N has two nuclei: motor nucleus and a parasympathetic nucleus called Edinger-Westphal nucleus; both lie in the upper midbrain. b. Intracranial Course: It pierces the dural roof of the cavernous sinus to run in its lateral. At the anterior part of the sinus, it divides into superior and inferior divisions, which enter the orbit via the SOF within the common tendinous ring. c. Intraorbital distribution: The superior division supplies the superior rectus and L.P. superioris muscles. The inferior division supplies medial rectus, inferior rectus, and inferior oblique. The nerve to the inferior oblique gives a branch to the ciliary ganglion. This branch is the parasympathetic root to the ganglion in which it relays to be distributed to constrictor pupillae + ciliary muscle. The oculomotor N. supplies: 5 extraocular muscles (all the extraocular muscles except superior oblique and lateral rectus). 2 intraocular muscles: constrictor pupillae and ciliary muscle. 9  ElGhazawy | Special Sense 10  ElGhazawy | Special Sense 2) The Trochlear Nerve a. Intracerebral origin: The nucleus of the trochlear N. lies in the lower midbrain. It passes dorsally to decussate with the opposite one to emerge from the dorsal aspect of the midbrain. The trochlear N. has 3 unique features: o It is the most slender of the cranial nerves. o It decussates with the opposite one, so that each trochlear nucleus supplies the contralateral superior oblique muscle. o It is the only cranial nerve which emerges from the dorsal aspect of the brain stem. b. Intracranial Course: The nerve pierces the dural roof of the cavernous sinus to run in its lateral wall, enters the orbit via SOF outside (above) the common tendinous ring to supply the superior oblique muscle. 11  ElGhazawy | Special Sense 3) The Abducent Nerve a. Intracerebral origin: The nucleus of the abducent N. lies in the lower part of the pons. b. Intracranial course: It pierces dura below the dorsum sellae. It crosses the apex of the petrous temporal bone and runs within the cavernous sinus inferolateral to I.C.A. and enters the orbit via SOF within the tendinous ring to supply the lateral rectus muscle. 12  ElGhazawy | Special Sense The Sensory Nerves in the Orbit Include the ophthalmic and optic nerves. 1) The Ophthalmic Nerve (V1) Definition: it is the first division of the trigeminal N. It is purely sensory. Intracranial Course: The nerve runs in the lateral wall of the cavernous sinus. At the end of the sinus, it divides into 3 branches: o the lacrimal, frontal, and nasociliary. The Lacrimal N.: It enters the orbit via the SOF outside the tendinous ring. It runs in the lateral wall of the orbit above the lateral rectus to reach the lacrimal gland, supplying it. Distribution: Sensory fibers to the lacrimal gland. Palpebral branch to the upper eyelid. It conducts postganglionic parasympathetic nerve fibers from the sphenopalatine ganglion to the lacrimal gland. 13  ElGhazawy | Special Sense The Frontal N. It enters the orbit via the SOF outside the tendinous ring It runs anteriorly above L.P. superioris muscle immediately below the roof of the orbit. It divides into supratrochlear and supraorbital nerves. The supratrochlear N.: o escapes above the trochlea → skin of the forehead and upper eyelid. The supraorbital N.: o escapes through the supraorbital foramen → skin of the forehead up to the vertex of the head. 14  ElGhazawy | Special Sense The Nasociliary N. It enters the orbit via SOF within the tendinous ring between the two divisions of the oculomotor N. (so it is seen within the muscular cone). It escapes from the muscular cone to reach the medial wall of the orbit where it becomes the anterior ethmoidal N. Branches and Distribution: 1. A communicating sensory branch to the ciliary ganglion o (sensory and vasomotor) 2. Two long ciliary nerves: o they pierce the sclera to supply the ciliary body, iris and the cornea. o usually carry postganglionic sympathetic nerve fibers to dilator pupillae ms. 3. The infratrochlear N. o leaves the orbit below the trochlea o to supply the upper eyelid and the skin over the nasal bone. 4. Posterior ethmoidal N. o passes through the posterior ethmoidal foramen o to supply the posterior ethmoidal and sphenoidal air sinuses. 5. Anterior ethmoidal N: o It is the continuation of the nasociliary N. It passes through the anterior ethmoidal foramen and appears in the cranial cavity where it runs on the upper surface of the cribriform plate of ethmoid. o It descends through a slit lateral to the crista galli to enter the nasal cavity where it gives internal nasal branches and escapes as external nasal N. (below the lateral nasal bone). o The external nasal N. supplies the skin over the middle of the nose down to its tip.(note, that the anterior ethmoidal nerve runs in three cavities: orbital, cranial, nasal cavities) 15  ElGhazawy | Special Sense 2) The optic nerve Definition: it is the second cranial nerve which is concerned with vision. Special features: Actually, the optic nerve is a tract, as it develops from the diencephalon of brain Within the orbit, it is enveloped in 3 meninges: dura, arachnoid and pia enclosing extensions of the subdural and subarachnoid spaces as far as the back of the eyeball. This explains papilloedema encountered in cases of the increased intracranial pressure, due to compression of the central vein of the retina. It has no neurolemmal sheath; it cannot regenerate after injury. Course and Relations: Optic N. represents the axons of the ganglion cell layer of the retina. It pierces the sclera about 3 mm medial to the posterior pole of the eyeball. It is about 4 cm long and runs posteromedially from the eyeball to optic chiasma. It has 3 parts: (1) Intraorbital part: o This part is about 25 mm long. o About 12 mm behind the eyeball, the optic N. is pierced by the central artery and vein of the retina (2) Intracanalicular part: o It runs in the optic canal. o It is about 5 mm long (3) Intracranial part: o (10 mm long), It lies in the middle cranial fossa. 16  ElGhazawy | Special Sense Autonomic Nerves in the Orbit (1) The Sympathetic Innervation Preganglionic sympathetic fibers arise from T1, T2, T3 segments of the spinal cord, relay in the superior cervical ganglion of the sympathetic chain → carotid sympathetic plexus around ICA. In the cavernous sinus, the carotid plexus gives : Some fibers to oculomotor N. → its superior division→ superior tarsal muscle of L.P. superioris muscle. Some fibers to the nasociliary nerve → its communicating branch to the ciliary ganglion → short ciliary nerves of the ganglion → blood vessels of the eyeball (vasomotor). Some fibers to the nasociliary N. → its long ciliary nerves → dilator pupillae muscle. (2) The Parasympathetic a- Parasympathetic supply to the constrictor pupillae and ciliary muscle: They are supplied by the oculomotor N. and the ganglion concerned is the ciliary ganglion 17  ElGhazawy | Special Sense The Ciliary Ganglion Definition parasympathetic ganglion which is functionally linked to oculomotor N. Site: It lies within the cone of muscles lateral to the optic nerve. Roots: Parasympathetic root: Is derived from the nerve to the inferior oblique muscle, the whole pathway is as follows: Edinger-Westphal nucleus in the upper midbrain → oculomotor N. → inferior division → nerve to the inferior oblique → relay in the ciliary ganglion → short ciliary nerves → ciliary muscle and constrictor pupillae. (it is the ganglion of accomodation reflex). Sympathetic root: contains postganglionic fibers derived from the carotid plexus. It reaches the ganglion along communicating sensory branch of nasociliary N. Sensory root: Is the communicating branch of the nasociliary N. N.B.: Only the parasympathetic root synapse in the ganglion. a- Efferent branches from the ganglion: The ganglion gives rise to 10-12 short ciliary nerves which pierce the sclera beside the optic N., they carry 3 functional types of fibers: Postganglionic parasympathetic → ciliary muscle and constrictor pupillae. Postganglionic sympathetic: vasomotor Sensory fibers: carry sensation from the cornea, ciliary body and the iris. b- The parasympathetic supply to the lacrimal gland: Is supplied by the facial N. (pterygopalatine ganglion). 18  ElGhazawy | Special Sense Blood vessels in the Orbit The ophthalmic artery Definition: It is a branch of the 4th part of the ICA medial to the anterior clinoid process. It enters the orbit via the optic canal ends near the medial end of the upper eyelid by dividing into o supratrochlear o dorsal nasal arteries. Branches: 1. Central A. of the retina: 1st branch of the ophthalmic A. It runs below optic N. within its dural sheath. About 12 mm behind the eyeball artery pierces the optic N. to run (in company with the central retinal vein) within substance of the nerve to retina. It pierces the sclera and divides into superior and inferior branches. Each divides into nasal and temporal branches, the 4 branches so formed supply the corresponding quadrants of the retina. The central A. of the retina has the following special features: o It is the only blood supply to the retina. o It is an end artery; thrombosis of the artery leads to blindness. 2. Muscular branches to the extraocular muscles 3. Long posterior and short posterior ciliary arteries: → choroid and iris. 4. Lacrimal A.: → Lacrimal gland 5. Post. ethmoidal A.: → Ethmoidal air sinuses and nasal cavity. 6. Ant. ethmoidal A.: Runs with corresponding N.& have same course & distribution 7. Supraorbital A.: Accompany the corresponding N. 8. Supratrochlear A.: Accompany the corresponding nerve. 9. Dorsal nasal A.: Supplies dorsum of the nose. 19  ElGhazawy | Special Sense Ophthalmic Veins Includes a superior and an inferior vein, both are valveless and connect the facial V. to the intracranial venous sinuses. So they are considered as emissary veins. 1. The superior ophthalmic V.: Begins in the orbital roof, passes within the tendinous ring and ends in the cavernous sinus. It receives tributaries corresponding to the branches of the ophthalmic A. 2. The inferior ophthalmic V.: Begins from a venous plexus in the orbital floor. It runs posteriorly below the tendinous ring. It divides into 2 divisions; one joins the cavernous sinus and the other ends in the pterygoid plexus (see the infratemporal fossa). 20  ElGhazawy | Special Sense Lacrimal apparatus This system comprises: secretory unit (the lacrimal gland) and drainage system (lacrimal canaliculi, lacrimal sac, and nasolacrimal duct). 1) The Lacrimal Gland : A compound tubule-acinar serous gland that secrets tears It is formed of 2 parts: large orbital and small palpebral parts, both are continuous around the lateral border of the aponeurosis of the levator palpebrae superioris muscle. A. The orbital part: It forms ⅔ of the gland and lies in the lacrimal fossa in the anterolateral angle of the roof of the orbit, above aponeurosis of the L.P. superioris muscle. B. The palpebral part: It forms ⅓ of the gland and lies below the aponeurosis of L.P. superioris, projecting into the lateral part of the upper eyelid. Ducts of the lacrimal gland: About 12 ducts emerge from the orbital part, passing through the palpebral part to open into the lateral part of the superior conjunctival fornix. Innervation and blood supply: Sensory nerve supply: Lacrimal branch of ophthalmic N. Parasympathetic secret motor supply: From the facial N. (pterygopalatine ganglion). Blood supply: By lacrimal branch of the ophthalmic A. 21  ElGhazawy | Special Sense 2) Lacrimal Canaliculi Lacrimal canaliculi are fine canals, one in each eyelid; convey the tears to lacrimal sac. Each is about 10 mm long and begins at the lacrimal punctum and runs medially in the medial part of the margin of the eyelid to join the lacrimal sac. 3) The Lacrimal Sac It is about 1 × 0.5 cm Lies in the lacrimal groove in the anteromedial angle of the floor of the orbit, behind the medial palpebral ligament. It is a blind sac which receives the lacrimal canaliculi and is continuous below with the nasolacrimal duct. 4) The Nasolacrimal Duct It is about 2 × 0.5 cm. It begins from the lower end of the lacrimal sac and opens inferiorly in the anterior part of the inferior meatus of the nose 22  ElGhazawy | Special Sense Functional aspects of tears: Composition: Isotonic solution. pH: 7.4 Refractive index: 1.33 It contains: NaCl, NaHCO3, glucose, protein & lysozyme. NB: Precorneal tear film, which covers cornea & conjunctiva. It consists of: o Lipid (1%): Secreted by Meibomian glands o Water (more than 90%) secreted by lacrimal glands. o Mucin (the rest) secreted by conjunctiva. Secreted by lacrimal glands, follow to lacrimal puncta then lacrimal duct. Functions: 1- Maintains optical uniform corneal surface (by filling up inequality) 2- Corneal nutrition by dissolving oxygen for it and prevents dryness of the cornea 3- Cleaning: Remove cells, debris & foreign bodies. 4- Protective: antibacterial agent. It contains lysozyme, lactoferrin, IgA, IgM, IgG. 5- Lubricate: Facilitates lid movements over cornea. 6- Healing: Supplies corneal wounds with WBCs. 23  ElGhazawy | Special Sense Histology 24  ElGhazawy | Special Sense EYE Globe The wall of the eyeball consists of three layers which are from outside inward: I- External fibrous supporting layer [cornea & sclera]. II- Middle vascular layer [iris, ciliary body & choroid]. III- Inner nervous layer [retina]. 25  ElGhazawy | Special Sense External fibrous layer 1-The cornea The cornea is formed of five layers which are from outside inward: 1- Non keratinized stratified squamous epithelium: It is formed of about 5-6 layers. a) The basal layer is formed of cubical or columnar cells, with numerous mitotic figures, resting on a straight basement membrane. Epithelium has a high regenerative capacity, when injured, as long as the basement membrane is intact. b) Intermediate layer which consists of 3-4 layers of polyhedral cells with numerous free unmyelinated nerve ending in between. c) The top layer is formed of squamous cells with minute apical microvilli which retain a tear film over the corneal surface. 2- Bowman's membrane: It is a non-cellular transparent homogenous layer. Formed of collagen fibrils in matrix. It is a protective barrier resistant to trauma & bacteria. It cannot regenerate, once destroyed, it causes corneal opacity. 3- Substantia propria: The main bulk of the cornea (about 90% of its thickness). It consists of about 200-250 lamellae of parallel collagen fibrils. Separated by rows of flattened fibroblasts (corneal corpuscles). The collagen fibrils & the corneal corpuscles are embedded in a matrix which has the same refractive index as the fibrils & cells. 4- Descemet's membrane: It is another homogenous non-cellular layer It represents the basement membrane, of the next layer 5- Endothelium (Descemet's endothelium): It is a single layer of endothelial cells. It is responsible for: a) Formation of Descemet's membrane b) Continuous withdrawal of water from corneal stroma to keep it transparent. 26  ElGhazawy | Special Sense Factors responsible for corneal transparency: 1- Cornea is avascular. 2- Regular arrangement of lamellae of substantia propria. 3- Cells, lamellae &matrix of substantia propria have the same refractive index. 4- Minimal hydration of the cornea. 5- The surface epithelium is nonkeratinized. 6- The free nerve ending are unmyelinated. Clinical Notes: Cornea has a high regenerative power [numerous mitotic figures]. Injury of the Bowman's membrane results in corneal opacity. Corneal transplantation usually succeeds because cornea is avascular. Cornea receives its nutrition by diffusion from the aqueous humor also from the scleral capillaries. Cornea obtains its oxygen directly from air. Microvilli keep the cornea wet (tear film) so avoid corneal ulceration. 27  ElGhazawy | Special Sense 2- Sclera It is formed of irregularly arranged collagenous bundles with flattened fibrocytes in between them. All of them are embedded in a matrix with a high water content. All these characters are the causes for the opacity of the sclera. Changes At Corneo-Scleral Junction (LIMBUS) 1- The corneal epithelium o becomes continuous with that of the conjunctiva. 2- Bowman's membrane o stops at the limbus &is replaced by subconjunctival C.T. 3- The collagen lamellae o become irregular, at this point there is a circular canal surrounding the limbus, called canal of schlemm. 4- Desemet’s membrane o breaks into fine trabeculae called pectinate ligaments that enclose between them the spaces of Fontana. 5- Descemet's endothelium o lines the spaces of Fontana & it is reflected on the anterior surface of the iris. 28  ElGhazawy | Special Sense Middle vascular layer This layer consists of three parts which are from backward forward: 1- Choroid. 2- Ciliary body. 3- Iris. 1- Choroid It is a pigmented vascular layer which lies between the retina & sclera. The choroid is formed of four layers which from, out inward, are: 1- Supra-choroid: Formed of lamellae of elastic fibers rich in melanocytes. 2- The vessel layer: Contains tortuous choroidal artery & choroidal veins. 3- Choriocapillaris: A single layer of fenestrated capillaries to nourish the outer third of the retina. 4- Bruch's membrane: It is acellular membrane formed of five layers. The outermost & innermost correspond to the basement membranes of the capillary endothelium & retinal pigment epithelium. The middle three layers consist of two collagen layers with a layer of elastin in between them. Clinical Notes: Bruch's membrane acts as retinal barrier since it limits the entrances of macromolecules from fenestrated capillaries of choroid to outer part of retina. 29  ElGhazawy | Special Sense 2- Ciliary body It consists of vascular connective tissue containing pigment cells& ciliary muscles, which is covered by epithelium. The epithelium covering ciliary body is formed of two layers of cubical epithelium the inner layer is pigmented while the outer is not. Ciliary muscles: They are smooth muscle fibers, which are arranged in three directions, meridian, radial & circular. Ciliary processes: They are present on the medial side of the Ciliary body. They are thin finger like projections covered with the ciliary epithelium & contain numerous capillary loops from which the aqueous humor is secreted. Functions: 1- Formation of aqueous humor. 2- Accommodation for near vision (circular fibers of Ciliary muscle) 30  ElGhazawy | Special Sense 3- Iris Anterior surface of the iris is covered by endothelium which is continuous with that covering the posterior surface of the cornea Posterior surface of the iris is covered with two layers of pigmented epithelium which are continuous with those over the Ciliary body. Stroma of the iris is formed mainly of loose vascular pigmented C.T. that contains the constrictor pupillae & dilator pupillae muscles. Functions of the iris: 1- It allows passage of light only through pupil. 2- It controls the pupillary size & so the amount of light. 3- It shares in the formation of aqueous humor. 4- It is responsible for the color of the eye. 31  ElGhazawy | Special Sense Refractive media of the eye Definition: are transparent media through which the light passes until it reaches to retina. They are: 1- Cornea. 2- Aqueous humor. 3- Vitreous. 4- Lens. 1- Cornea See before. 2- Aqueous humor See physiology 3- Vitreous body It is a transparent jelly like substance. It fills the large posterior compartment behind the lens & Ciliary body known as vitreous cavity. It is a mucoid C.T. rich in hyaluronic acid, collagen fibers &a protein called vitrein. Functions: a) Transmission of light b) It supports the lens anteriorly &the retina posteriorly. 32  ElGhazawy | Special Sense 4- Lens Lens Capsule: An elastic basement membrane that supports the lens epithelium. Lens epithelium: It is a layer of cuboidal epithelium located immediately beneath the capsule & only covers the anterior surface of the lens. Lens fibers: They form the main bulk of the lens. They are elongated transparent structures. The lens grows throughout life by addition of new fibers to the periphery of the lens substance. The lens is suspended in its place by means of suspensory ligament that connects it with the Ciliary body. Causes of lens transparency 1. Avascular & has no nerve supply. 2. Lens fibers are uniformly arranged & densely packed. 3. Various lens constituents have the same refractive indices. Cataract: - Def.: It is loss of lens transparency - Causes: 1- Decrease glutathione (antioxidant) in lens: o So , increase O2 radicals → increase membrane ionic & water permeability 2- Degeneration & coagulation of proteins. 3- Calcification: o It is common in old age, diabetic & excess exposure to ultraviolet rays - Treatment: Surgical removal & replacement Prespiobia: This means loss of elasticity of the lens & this occurs in old age. It results in lose of accommodation for near vision. 33  ElGhazawy | Special Sense Inner nervous layer (Retina) The retina contains three basic cell types: neurons, neuroglial supporting cells& pigment cells. Histologically, the retina is formed of ten layers which are 1- The pigmented epithelium It is composed of a single layer of cuboidal epithelial cells with rounded basal nuclei resting on Bruch's membrane. The inner part of the cells is rich in melanin pigments. By the E.M. the cells are rich in r RE, s ER, lysosomes &basal mitochondria. The cells carry microvilli that interdigitate with the rods & cones. Functions: 1- Transport of nutrition. 3- Phagocytosis of old discs. 2- Storage of vitamin A 4- Absorption of excess light. 5- Share in formation of Bruch's membrane. Clinical Notes: Retinal detachment occurs at the line between pigmented epithelium layer& other nine layers of retina as this layer is firmly adherent to Bruch's membrane & choroid. 2- The rods &cones layer is made of dendritic processes of rod &cone cells which are photosensitive. These processes are highly modified cilia. 3- The outer limiting membrane It is formed of a junctional complex between Muller's cells & the photoreceptors. 4-The outer nuclear layer It contains nuclei of the rod &cone cells (1st order neurons). 5- The outer reticular layer This layer is formed by synapse between axons of rod & cone cells with dendrites of bipolar cells & horizontal cells. 34  ElGhazawy | Special Sense 6- The inner nuclear layer It contains the cell bodies of four types of cells: a- The bipolar nerve cells (2nd order neuron) which make synapse with either the rod or cone cells. b- Horizontal cells: They are present in the outer part of this layer. They connect the rod & cone cells laterally. c- Amacrine cells: They are present in the inner part of this layer. They connect bipolar nerve cell with several ganglion cells. d- Muller's cells: Long supportive & nutritive glial cells. The bodies of these cells are present in the inner nuclear layer. They have long cytoplasmic processes. The outer processes extend to outer limiting membrane. Their inner processes form the inner limiting membrane. 7- The inner reticular layer This layer contains the synapse between the axons of bipolar nerve cells & the dendrites of ganglion cells. Also amacrine cells synapse with them. 8- The ganglion cell layer It contains few large nerve cells (3rd order neurons). 9- The optic nerve fibers layer This layer contains the un-myelinated axons of the ganglion cells which have no neurilemal sheath (transparent, cannot regenerate). The axons bend at a right angle & come out as optic nerve. 10- Inner limiting membrane It is formed by the tight junction between the terminal ends of the processes of Muller's cells. 35  ElGhazawy | Special Sense Rod & Cone cells These are the first order neuron in the visual pathway. Each cell consists of a dendritic process, cell body (nuclear region) & central process (axon). The dendritic process is formed of two segments; outer and inner. Rods cones Number 70-120 million about 7 million Site peripheral part of retina central part of the retina Outer Cylindrical in shape. Cone. segment Contains flattened discs containing visual Contains flattened discs containing pigment= rhodopsin. the visual pigment = iodopsin. These discs are continuously phagocytosed by They are constant i.e. they are not the pigmented cells. phagocytosed by the pigmented cells. Inner Connected by a short cilium to the outer segment. segment Site of protein formation so it is rich in rER, Golgi apparatus & mitochondria. Cell body Contains the nucleus. Sends an inner fiber (axon) that synapses with the dendrites of the bipolar nerve cells. Functions Responsible for vision in dim light (by night). Responsible for colored vision& for high visual acuity. They are the only receptor present in fovea centralis. CLINICAL Deficiency in vitamin A results in deficiency of Lack of photopigments within the NOTE rhodopsin so the person suffers from night cones result in color blindness. blindness. 36  ElGhazawy | Special Sense ACCESSSORY STRUCTURES OF EYE (Ocular adnexa) 1- The conjunctiva It is a transparent mucous membrane that covers the sclera (bulbar conjunctiva) then reflected as conjunctival fornix to line the eye lid (palpebral conjunctiva). It consists of epithelium & underlying connective tissue. The epithelium is stratified columnar epithelium with goblet cells that changes to stratified squamous epithelium at the corneo-scleral junction with no goblet cells Functions: 1. Protection of the eye by secretion of goblet cells 2. Drainage of the aqueous humor by subconjunctival vein. 37  ElGhazawy | Special Sense 2- Eye lid It consists of: 1. Thin skin devoid of fat & has fine hairs. The lid margin o carries 3-4 rows of eye lashes o in between them there are sebaceous glands called Zeis glands & apocrine sweat glands called Moll's glands. 2. Striated muscles: Orbicularis occuli that close the eye & levator palpebrae which elevates the eye lid 3. Tarsal plate: It is a thick plate of connective tissue that contains a modified sebaceous gland called meibomian gland. 4. Palpebral conjunctiva. CLINICAL NOTES: Inflammation of Zeis or Moll's glands leads to sty. Inflammation of Meibomian glands leads to calazion 38  ElGhazawy | Special Sense Physiology 39  ElGhazawy | Special Sense Aqueous humor It is a transparent fluid that fills both the anterior & posterior chambers Composition It is a hypertonic solution rich in hyaluronic acid & its total protein is less than that of blood serum. It is like serum in having no fibrinogen (cannot clot) with the same A/G ratio. Formation of aqueous humor: It is secreted mainly from the Ciliary processes & to minimal degree from the posterior surface of the iris. Circulation of aqueous humor: It passes from posterior chamber through pupil ---> anterior chamber then at irdiocorneal angle--> spaces of Fontana -->canal of schlemm--- > aqueous vein. Clinical Notes: If the rate of production of aqueous humor is higher than the rate of drainage this results in increase intra-ocular pressure i.e. glaucoma. 40  ElGhazawy | Special Sense Intraocular pressure (IOP): Normal: - 12- 20 mmHg - It shows diurnal variation (highest in morning & lowest in evening) Functions: 1) IOP maintains the spheroid shape of the eye (which is important for proper optical function of the eye) 2) IOP is important for the normal focusing mechanisms of the eyes: It exerts two forces on the lens: a. A stretching force on the lens ligaments b. A pressure flattening force on the lens surface These 2 forces maintain the lens flat during rest 1] Pupil Dysfunctions: Constriction (Miosis) → opens the angle → decrease IOP Dilatation (Mydriasis)→ closes the angle → increase IOP 2] Increase venous pressure in head → decrease drainage →increase IOP Dysfunctions: - If IOP decreases: o The suspensory ligaments relax & lens becomes more spherical - If IOP increases: (Glucoma) o Suspensory ligaments are stretched (& accommodation will not occur) 41  ElGhazawy | Special Sense Near response Definition: It is the changes which occur when looking to near object 1- Accommodation: See Below 2- Miosis: Pupil constriction due to - Contraction of constrictor pupillae muscles by Oculomotor nerves - Aim of miosis: 1- To decrease amount of light entering the eyes 2- To increase depth of focus 3- To decrease spherical & chromatic aberrations Medial Convergence of both eyes: To allow the two Images to fall on two corresponding points on both eyes to prevent diplopia (double vision). It is caused by contraction of two medial recti muscles. 42  ElGhazawy | Special Sense Causes of Miosis (Pupil constriction) 1- Pupillary light reflex 2- Near response (accommodation reflex) 3- Lesion in cervical sympathetic ganglion (Horner's syndrome) 4- Pontine hemorrhage 5- During sleep (Parasympathetic is predominant) 6- During third stage of anesthesia. 7- Drugs: a) Parasympathomimetic drugs eg Eserine & Pilocarpine b) Morphine poisoning (pupils are markedly constricted) c) Histamine (direct action on constrictor pupillae) Causes of Mydriasis (Pupil dilatation) 1- Dark adaptation 2- Distant vision 3- Sympathetic stimulation eg emotions, fear, pain & asphyxia 4- Increase intra-ocular pressure (IOP) 5- Lesions of Occulomotor nerve or EWN 6- During 2nd & 4th stage of anesthesia. 7- Drugs & toxins: a. Sympathomimetic drugs: As adrenalin “Stimulate adrenergic receptors” b. Para sympatholytic drugs: As Atropine: (Blocks muscarinic receptors in constrictor pupillae muscle) c. Cocaine (sensitizes dilator pupillae to “epinephrine”) d. Alcohol intoxication e. Chloroform poisoning Changes of pupil size during anaesthesia: 1- 1st stage: Pupil may be normal in size & reacts to light 2- 2nd stage: Pupil dilated (due to increased sympathetic activity) 3- 3rd stage: Pupil constricted (surgical stage) 4- 4th stage: Pupil dilated again-loss of light & corneal reflex: o It is dangerous stage & death may occur 43  ElGhazawy | Special Sense Accommodation Definition: Increasing the dioptric power of the lens on looking to near object to focus the image on retina Mechanism: The lens has an elastic capsule which tends to make it spherical During distant vision: o ie looking at objects 6 meter or more away → the ciliary muscle [(radial & circular fibers) which forms a ring inside the ciliary body is relaxed forming wide ring of ciliary body] o The suspensory ligament becomes tense & the lens is flattened [& so the lens power at its minimal value which is 20 diopters] During near vision: o ie looking at objects nearer than 6 meter, the ciliary muscle contracts Contraction of: o Radial fibers → Release tension on lens Circular fibers→ Forms a narrow ring of ciliary body→ suspensory ligament becomes lax → lens becomes more spherical so the power reaches its maximum (ie 34 diopeter the age of 10 years) 44  ElGhazawy | Special Sense Pathway Receptors: Photoreceptors: Rods & cones Afferent: (Visual pathway) First order neuron: Bipolar cells Second order neuron: Ganglion cells o Ganglion cells axons form optic nerve which leaves the retina & then form the optic chiasma o In optic chiasma: The nasal fibers cross while the temporal fibers do not cross o The fibers then pass to optic tracts which carry visual sensation from contralateral halves (ie nasal field of one eye & temporal field of the other eye) of both visual fields 3rd order neuron: o Is lateral geniculate body (LGB) in thalamus where optic tract relays in. o Fibers from LGB pass to → optic radiation → to visual sensory area (17) o Then to visual association areas (18, 19) of occipital cortex Center: Superior colliculus (in midbrain) Efferent: Fibers arise from: Superior colliculus Terminate at Edinger Westphal Nuclei “EWN” (Occulomotor nuclei of both sides) Preganglionic fibers parasympathetic fibers in Occulomotor nerves Relay in ciliary ganglion, from which Postganglionic fibers (short ciliary nerves) arise & reach ciliary muscle leading to its contraction 45  ElGhazawy | Special Sense Power of accommodation It is the difference between lens power during: - Distant vision (relaxed ciliary muscle) - Near vision (contracted ciliary muscle) during maximal accommodation NB: Power of accommodation is: o 14 D at 10 years o 10 D at 20 years o 2 D at 50 years o Zero at 70 years Increase the lens power is due to: Increase in lens curvature (mainly the anterior surface of the lens). during accommodation This can be proved by: 1- Purkinje-Sanson image: A light candle is placed in front of eye while a person is looking at a distant object. 3 images of the candle will be seen in the subject's eye: 1) A clear small upright image (reflected from cornea). 2) Large upright faint image (reflected from anterior convex surface of lens) 3) Small inverted image (reflected from posterior concave surface of lens) o If the subject looks at a near object, the large upright faint image becomes smaller and moves towards the other upright images. The other two images do not change 2- Depth of anterior chamber becomes smaller proved by: 1. Slit lamp 2. Photography 46  ElGhazawy | Special Sense Refraction Normally: - Emmetropic: normal eye refraction - Near point (maximal accommodation): 10 cm - Far point (without accommodation): 6 meter or more 47  ElGhazawy | Special Sense Photoreceptor potentials & Signal transmission in retina 1- Photochemical changes (Bleaching) - The photochemical substances (pigments) are two types: Rhodopsin Iodopsin - Found in rods. - Found in cones. - Only one type. - Three types. - Formed from 11 cis retinal and a - Formed from 11 cis retinal and protein called scotopsin. proteins called photopsins - More sensitive to light. (3 types). - Sensitive to blue-green light (505 - Less sensitive to light. nm). - Sensitive to yellow light (550 nm). Bleaching: 1) Exposure of rhodopsin (or iodopsin) to light leads to → structural changes and its transformation to → bathorhodopsin → lumirhodopsin → metarhodopsin I →metarhodopsin II (active rhodopsin) → transformed to all trans-retinal which has straight configuration. 2) All trans-retinal → cannot combine with protein to form pigment 3) All trans-retinal isomerase → 11-cis retinal which combines with protein reforming the pigment in the dark 48  ElGhazawy | Special Sense 2- Electrical change: Receptor potential A] Dark Current: - During rest: The rod Inner segments contain active Na+ pump → creates negative rod intracellular potential The rod outer segments contain Na+ channels which are maintained open by cyclic guanosine monophosphate (cGMP). Na+ pumped by inner segments leaks through outer segment → so, negativity is not so high (-40 mv). Theoretical basis for generation of a “hyperpolarization receptor potential” caused by rhodopsin decomposition, which decreases the flow of positively charged sodium ions into the outer segment of the rod B] The excitation cascade (Linkage between Rhodopsin & Na +channels) - On light exposure: Photons (light) activates rhodopsin forming metarhodopsin II → Activates G protein in cell membrane called transducing → Activates phosphodiesterase → Hydrolyses cGMP decreasing it in cytoplasm → Closure of Na+ channels in outer segment → Hyperpolarization (-70 mv) → Hyperpolarization reduces release of synaptic transmitter (inhibitory) - On removal of light: Return to resting state Rhodopsin kinase inactivates active rhodopsin (metarhodopsin II) increase cGMP open Na+ channels return membrane potential to -40 mv NB: - cGMP increases because of activation of guanylate cyclase & inhibition of phosphodiesterase by Ca++ reduction on exposure to light - Summation of light occurs more easily in rods because of longer receptor potential in rods (high sensitivity) 49  ElGhazawy | Special Sense Automatic Regulation of retinal sensitivity (Retinal adaptation): It is the ability of the retina to adjust its sensitivity to different light intensities It can increase to 30,000 times during dark adaptation There are two types of adaptation: 1) Dark adaptation 2) Light adaptation 1) Dark adaptation: - It occurs when person shift from bright to dim light - It is increase sensitivity of rods & cones to light ie decrease visual threshold Changes occurring are: 1- Dilatation of pupil (mydriasis) 2- Increase sensitivity of rods & cones 3- Regeneration of photopigments in rods & cones 4- Increase signal intensity from retinal neurons Mechanism: - With low light intensities, photopigment bleaching (breakdown by light) is slowed, while regeneration precedes normally. - So, there is a net increase in photopigments inside rods & cones Components: 1- Rapid small rise of retinal sensitivity: o Caused by dark adaptation of cones o Takes 5-10 minutes for full loading with iodopsin 2- Slow greater rise of retinal sensitivity: o Caused by dark adaptation of rods o Takes 30 minutes for full loading with rhodopsin These chemical changes increase retinal sensitivity 20- 30000 times 50  ElGhazawy | Special Sense 2) Light adaptation: - It occurs when person shifts from dark to bright light - It is decrease sensitivity of rods & cones to very high light ie increase visual threshold Changes occurring are: 1- Constriction of pupils (miosis) 2- Decrease sensitivity of rods & cones. 3- Breakdown of photopigments in rods & cones. 4- Decrease signal intensity in retinal neurons. Mechanism: - On exposure to high light intensities, photopigments decrease inside rods & cones. - sensitivity of receptors decrease with increased intensity or duration of light exposure. - It takes 5 minutes Parts of the retina of Physiological Importance: Macula lutea: - Yellow spot opposite to posterior pole of eye. - It is 3 mm from temporal side of optic disc. Fovea centralis: Central portion of macula: - It is composed of cones only. - It is highly transparent as it contains no blood vessels. - The visual acuity is high because o Light falls on cones directly as all layers displaced to the side & not covered by blood vessels o Ratio of cones to bipolar & ganglion cells is 1 : 1 o Highly developed pigmented layer (suppress light reflection) Optic disc: Exit of optic nerve - It is 3 mm from nasal side of macula lutea - It contains no rods or cones, so, it is the cause of blind spot 51  ElGhazawy | Special Sense Photoreceptors: - They are rods (120 million) & cones (6 million) in each retina. - Each receptor consists of: o Outer segment which contains 1000 disc (rod like in rods or conical in cones) & photopigments o Inner segments contains mitochondria o Nucleus o Synaptic body. The retina consists of 2 areas: - Central: Fovea centralis - Peripheral: Extra fovea Duplicity theory of vision; Von Kries theory There are two types of vision: 1- Photopic vision: - It is the day vision. - It Is the function of cones. - Details, boundaries and colors can be detected. 2- Scotopic vision: - It is dim light vision - It Is function of rods - Details, boundaries and colors cannot be detected 52  ElGhazawy | Special Sense Colour vision It is the sense of discrimination between different wave lengths ie different color. It is the function of cones, thus it is better in central vision Primary color: - Red, green and blue are called primary colors. - Mixing various proportion of them can produce any spectral color (mixing wave length not pigments). Complementary color: - For any color there is a complementary color which when properly mixed with it, produces white color eg yellow is complementary for blue. Normal eye can distinguish the 7 colors of the spectrum & 100 intermediate color White color is produced when 7 spectral colors are in the same proportion while black is produced by absence of light Mechanism of color vision There are three types of cones. Each cone contains a different photochemical pigment (photopsin + retinal) & is maximally sensitive to only one of the three primary colors. The three cone pigments are: Blue sensitive pigment: Absorbs the blue violet portion of spectrum. Green sensitive pigment: Absorbs the green portion of spectrum. Red sensitive pigment: o Absorbs the yellow portion of spectrum. o It is highly sensitive to red portion of spectrum at a very low threshold 53  ElGhazawy | Special Sense Unequal stimulation of cones gives the color sensation. The relative frequency of impulses in each of the three cones when reaches the visual cortex, a sensation of color is perceived. For example: Orange light with a wave length 580 nm stimulates red cones by an intensity 99%, green cones by intensity 42%, blue cones by an intensity 0%. This unequal stimulation of cones when reaches visual cortex, it is interpreted as sensation of orange. On the other hand, blue light with a wave length 450 nm stimulates the red cones by an intensity 0% and green cones by 0% and the blue cones by 97%. This ration is interpreted by visual cortex as blue. When the three types of cones are equally stimulated, a white color is felt. Color perception is a retinal phenomenon while color translation is a cortical phenomenon Color coding depends on relative amount of activity in each cone 54  ElGhazawy | Special Sense Color Blindness (Achromatopsia) It is the inability to perceive a portion of the spectrum or to differentiate between colors due to absence of one or more of the three photosensitive substances of cones It is congenital disease affecting males & transmitted by females as a sex-linked recessive character. It affect 8% of males & 0.4 % of females It sometimes occurs after retro-bulbar neuritis (inflammation of optic nerve) Types: 1- Monochromatic (anopia) - Has only one cone substance & the other two cones are absent - All colors appear as grades of one color: white, black, grey. 2- Dichromatic (anopia) - Has 2 cone substances & the third is absent - They have blindness to one primary color either o red (protanopia) o green (deuteranopia) o rarely blue (tritanopia) 3- Trichromatic (anomalous) - Has 3 cone substances but one is less than normal (weak) - They have weakness to one primary color → o most commonly is blue (tritanomaly) o red (protanomaly) o green (deuteranomaly) 55  ElGhazawy | Special Sense Fat soluble vitamin Fat soluble vitamins are A, D, E and K VITAMIN A (antixero-ophthalmia) β carotenes gives 2 β ionone rings give 2 vitamin A. while α or gamma carotenes give 1 β ionone gives 1 vitamin A Importance of vitamin A: 1) development of healthy connective tissues. 2) maintenance of healthy epithelium 3) vit A prevents high molecular weight KERATIN from respiratory & urinary systems 4) antioxidant vitamin (CEA) 5) normal growth and differentiation of cells. 6) important in night vision RHODOPSIN CYCLE Photochemical changes in the retina translated to visual impulse 56  ElGhazawy | Special Sense Deficiency of vitamin A 1) Dryness of the eyes (zero-ophthalmia) 2) repeated chest infection 3) repeated urinary infections 4) night blindness (nyctaopia) 57  ElGhazawy | Special Sense Corneal Reflex It is superficial reflex Definition: Touching the cornea of one eye by a foreign body leads to blinking of both eyes. Pathway Receptors: Corneal receptors Afferent: Ophthalmic division of 5th nerve (trigeminal n) Center: Pons Efferent: Both facial nerves. Effector: Orbicularis oculi muscles of both sides leading to blinking It is used clinically to test: 5th nerve Depth of anaesthesia. 58  ElGhazawy | Special Sense Light Reflex Exposure of one eye to light results in reflex constriction of both pupils Composed of 2 components: 1- Direct light reflex (direct constriction of stimulated eye) 2- Indirect (or consensual) light reflex (indirect constriction of unstimulated eye) due to: 1- Partial crossing of optic nerves at optic chiasma 2- Each pre-tectal nucleus gives both Edinger Westphal nuclei (EWN) Pathway: Afferent: Impulses from photoreceptors of the stimulated eye are transmitted via optic nerve → Optic chiasma → optic tract but No relay in Lateral Geniculate Body (LGB) of thalamus Center: Pretectal nucleus (in midbrain) Efferent: Fibers arise from: Pretectal nucleus and Terminate at Edinger Westphal Nuclei (EWN) (Occulomotor nuclei of both sides) Preganglionic fibers in Occulomotor nerves Relay in ciliary ganglion, from which Postganglionic fibers (short ciliary nerves) arise Short ciliray nerves enter the eye to produce contraction of constrictor pupillae muscles of both sides Argyll-Robertson pupil Occurs in syphilis of nervous system (neurosyphilis) Due to lesion in pretectal nucleus (leading to absent light reflex) but accommodation reflex is still present 59  ElGhazawy | Special Sense Visual Pathway Photoreceptors: Rods & cones Nasal retina receives from temporal field & vice versa Upper retina receives from lower visual field & vice versa First order neuron: Bipolar cells Second order neuron: Ganglion cells (their axons form the optic nerve) Optic nerve: ⅓ of fibers are derived from macula. Then it forms optic chiasma Optic chiasma: Nasal fibers: Cross While temporal fibers: Do not cross Optic tract: It carries impulses from: 1. Temporal half of retina (Nasal field of vision) of its own side 2. Nasal half of retina (Temporal field of vision) of the opposite side 60  ElGhazawy | Special Sense The optic tract terminates in either of the followings: 1- Suprachiasmatic nucleus in hypothalamus For Controlling light evoked endocrine functions (circadian rhythm) 2- Pretectal nucleus in midbrain For Center of: Light reflex 3- Superior colliculus in midbrain For Center of: Accommodation reflex eye directional movement 4- Lateral geniculate body (LGB): It is subdivided into six layers fibers from these layers’ optic radiation pass to Visual sensory cortex Primary visual cortex: (Area 17) The primary visual cortex is organized in layers (6) & columns It surrounds the calcarine fissure of occipital lobe. The right visual cortex relays optic fibers from the right halves of both retina ie the nasal half field of the same side & temporal half field of opposite side The peripheral retina is represented anteriorly The macula is represented posteriorly, widely & bilaterally The upper retina is represented superiorly. The lower retina is represented inferiorly Its neurons is known as feature detectors (analyze features of stimulus) Visual association areas: Areas 18, 19 (surround area 17) They are responsible for: (1) Detection of nature of objects (2) Visual orientation & depth perception Sending information to other areas of cerebral cortex eg posterior parietal lobe, inferior temporal lobe & frontal field (8) 61  ElGhazawy | Special Sense Lesions of visual pathway: 1] Lesion of optic nerve: Blindness of corresponding eye 2] Lesion of optic chiasma: Center of chiasma → Bitemporal hemianopia Periphery of chiasma → Unilateral: Nasal hemianopia of same side Bilateral: Binasal hemianopia 3] Lesion of optic tract: Contralateral homonomous hemianopia with loss of light reflex 4] Lesion of lateral geniculate body (LGB): Contralateral homonymous hemianopia but light reflex is preserved 5] Lesion of optic radiation or visual sensory area (occipital lobe): As lesion of LGB (ie contralateral homnomous hemianopia) but central vision (by fovea) remains normal ie sparing of macula because: 1- Large cortical area of representation 2- Double blood supply 3- Bilateral representation 6] Lesion of tip of occipital lobe (macular area): Contralateral hemianopic scotoma (loss of central part of visual field) 7] Lesion of visual association areas (18, 19): Visual agnosia ie Loss of visual meaning (he sees but not understand) If the lesion is in the dominant hemisphere: → Visual aphasia ie inability to understand written words Axis 1] Optic axis: A straight line joining the two poles of the eye (anterior & posterior pole) 2] Visual axis: A line passing from fovea centralis to fixation point through nodal point 62  ElGhazawy | Special Sense 63  ElGhazawy | Special Sense Binocular single vision Definition: It is the ability to use both eyes without diplopia (double vision) Requirements: 1- The 2 visual fields must overlap to great extent 2- The two images must be identical (same size & colors). 3- The two images must fall on two corresponding points of the two retinae (nasal side of one retina & temporal side of other retina). The fovea centralis at both sides are corresponding points. Advantages: 1- Large visual field 2- Defect in one image corrected by other (whether physiological or pathological). 3- Abnormal refraction of one eye is masked by the normal eye. 4- Better depth perception & stereoscopic vision. Stereoscopic vision: - It is differentiation of the three dimensions of the objects (depth, height, width). - It can be perceived by monocular vision, but it is better by binocular vision. - eg if one looks at an object Infront of him, first he closes one eye and then the other, he will find the view seen by the right eye is slightly different from that seen by left. Depth perception: - It is determination of distance by visual system - It is mononuclear property, but it becomes more accurate by binocular vision - Depth perception depends on: 1- Change in size: Far objects are smaller 2- Color & details: Fade with distance 3- Occlusion: A front object covers an object behind it 4- Distribution of light & shades 64