Visual System: (deliberately) Basic Anatomy and Physiology of The Eye. Tom Fearnley Clinical Fellow Ocular Oncology, OculoplasDcs and Orbital Surgery

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1 Visual System: (deliberately) Basic Anatomy and Physiology of The Eye Tom Fearnley Clinical Fellow Ocular Oncology, OculoplasDcs and Orbital Surgery

2 Lecture Outline Gross Structure Layers of the Eye Muscles Adnexal Tissues Arterial Supply Venous drainage LymphaDcs

3 Shameless plug Ophthalmology is one of the most rewarding careers in medicine offering an unrivalled mix of medicine and surgery Cataract surgery is the most frequently performed operadon in the UK (> /yr) At the very limit of surgical finesse Which of your senses would you least like to lose? You will see lots of eye disease Ophthalmologists regularly feature near the top of the happiest physicians table

4 The Gross Structure of the Eye in reladon to its funcdon The eye is a hallow sphere (2 chambers) - evolved to focus light to a fine point The amount of light allowed into the eye is controlled by pupil size Light is refracted by 2 surfaces Cornea Lens The light sensidve Dssue at the back of the eye is called the ReDna Contains specialised cells responsible for converdng light energy (photons) into a cell membrane potendal for neurotransmission The globe is protected within the orbit The adnexal Dssues (lids, lacrimal apparatus etc) protect and moisturise the ocular surface The extra- ocular muscles move the eyes in synchrony to maintain binocular single vision and allow depth percepdon (stereopsis)

5 The layers of the Eye Outer Layer (protecdon) Sclera & Cornea Middle Layer (nutridon) Uvea Iris Choroid Ciliary Body Inner Layer (detecdon) ReDna

6 Sclera The tough white outer coat of the eye Covered by ConjuncDva and Tenon s Allows abachment of extraocular muscles

7 Cornea Transparent window (you put your contact lenses on it) Transmits light Protects intraocular contents 2/3rds of focusing power of the eye (refracdve index) Thinner in the centre (500µm), Thicker at the edge (700µm) Avascular nutridonal requirements from tear film and aqueous InnervaDon via Trigeminal nerve 400 xs that of skin In condnuity with the sclera at the limbus Must remain reladvely dehydrated (or becomes cloudy) Epithelium replaced condnually by Limbal Stem Cells Endothelium must last a lifedme

8 The layers of the Cornea 1: Epithelium 2: Bowman s Layer 3: Stroma 4: Descemet s Membrane 5: Endothelium

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10 The Uveal Tract Grape (hangs from opdc nerve following dissecdon from sclera) Highly vascular Highly pigmented (melanin) 3 anatomically and funcdonally disdnct parts Iris Ciliary Body Choroid

11 Iris Thin contracdle diaphragm the coloured part of the eye Central aperture (pupil) Directly overlies the lens 2 muscles control pupil size Dilator pupillae (dilates) Radial Outer SympatheDc Spincter pupillae (constricts) CircumferenDal Pupil margin ParasympatheDc Pupil reflexes Light Near (constricdon plus accomodadon) SympatheDc

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14 Ciliary Body CircumferenDal structure surrounding the lens Produces Aqueous from glandular epithelium (acdve secredon and ultrafiltradon) Smooth muscle in stroma allows: AccomodaDon (AlteraDon of lens convexity) AlteraDon of aqueous oullow via abachment to trabecular meshwork

15 A word about Aqueous Circulates from the posterior to the anterior chamber through the pupil Similar to plasma (lower glucose and protein, higher ascorbate and lactate) Provides nutrients to cornea and lens Maintains IOP (balance between producdon and drainage) Drains (mostly) via the trabecular meshwork Specialised Dssue in the anterior chamber angle (between the iris and the cornea)

16 Aqueous Drainage

17 The Choroid Thin, highly vascular structure between redna and sclera Vascular supply to outer redna

18 The Lens Flexible (change in shape alters refracdve power) Biconvex Held in place by suspensory zonules Crystal like 1/3 rd focusing power of eye Lens fibres progressively laid down over life (thickens) Adult diameter 9-10mm Allows eye to change focus from distance to near (accomodadon) lost as we age Pathological opacificadon = CATARACT Accounts for 48% of world blindness 18 million people

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23 Vitreous thick, gel- like substance that fills the posterior chamber 99% water + Hyaluronic acid Collagen scoffold degrades with age 80% (4ml) of the volume of the globe Strongly adherent to anterior few mm of redna (detachments) Embryological support for vessels delivering nutrients to lens Best avoided during cataract surgery!

24 ReDna Lines innermost surface of the eye and contains photoreceptors Cells responsible for converdng light into nerve impulses Fovea centralis a depression, or pit, in the center of the redna that contains only cones most accurate vision & color sensidvity

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28 Photoreceptors Rods: about 100 million, operate in grayscale, are mostly around the periphery most sensidve to 500nm which appears bluish- green allow us to see in low light and provide for peripheral vision Cones: about 5 million 3 variedes (trichromadc) Each sensidve to pardcular wavelength of light (short, medium & long) Greatest concentradon at fovea

29 Rods & Cones

30 Basic phototransducdon Photoreceptors convert photon energy into membrane potendal that uldmately results in a reducdon of neurotransmiber release Photoreceptors contain light sensidve visual pigments AbsorpDon of light leads to an isomeric alteradon of the visual pigments which in turn hyperpolarises the photoreceptor cell due to movement of calcium ions

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32 Extraocular Muscles 6 per eye 4 recd Arise from Annulus of Zinn at apex 2 obliques Superior arises from back of roof of orbit Inferior arises from anterolateral orbital floor DucDons Monocular eg adducdon Versions Binocular same direcdon (conjugate) eg right gaze (dextroversion) Vergences Binocular opposite direcdon (disconjugate) eg convergence InnervaDon Lateral rectus 6 th CN Superior Oblique 4 th CN All the rest 3 rd CN Deliver blood supply to anterior segment of eye via anterior ciliary arteries

33 Adnexal Tissues Lids ConjuncDva Lacrimal apparatus Orbit

34 Lids Glide over the front surface of the eye Protect Distribute tears Anterior Lamella Skin and muscle (orbicularis - closure) Posterior Lamella Tarsal plate (rigid) ConjuncDva

35 ConjuncDva Clear covering over the sclera Several regions Tarsal (back of lid) Dghtly adherent Fornix (recess behind lids) lots of glands Bulbar (over globe loosely adherent allows eye to move freely Goblet cells secrete mucous LymphaDcs Immune response

36 Lacrimal Apparatus Lacrimal gland produces tears Drain inferonasally Puncta (upper and lower) Cannalicular system Lacrimal Sac Nasolacrimal duct Inferior meatus (nose)

37 Tears 3 layers Anterior lipid (meibomium glands) Middle aqueous (lacrimal glands) Posterior mucous (goblet cells) ProtecDve AnDbacterial Smooth surface NutriDon to cornea

38 Orbit

39 Internal carodd artery Ophthalmic artery Central rednal artery OpDc nerve Arcades nourish inner redna Posterior Ciliary arteries (short and long) Choroidal circuladon nourishes outer redna Lacrimal artery Lacrimal gland Anterior and posterior Ethmoidals External carodd Facial artery angular artery Medial lid and orbit Arterial Supply

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41 Venous drainage Inner redna Branch rednal veins Ophthalmic veins» Cavernous sinus Outer redna and Choroid Vortex veins Ophthalmic veins» Cavernous sinus

42 LymphaDcs Not well understood None within the eye itself Accepted pabern Lower medial and central lid to submandibular Upper lid, medial canthus and lateral lower lid to pre- auricular

43 QuesDons?