visual’system:’(deliberately)’basic’ anatomy’and ... ·...
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Visual System: (deliberately) Basic Anatomy and Physiology of The Eye
Tom Fearnley Clinical Fellow Ocular Oncology, OculoplasDcs and Orbital Surgery
Lecture Outline
• Gross Structure • Layers of the Eye • Muscles
• Adnexal Tissues • Arterial Supply • Venous drainage • LymphaDcs
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 (>300 000/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
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)
The layers of the Eye
• Outer Layer (protecDon) • Sclera & Cornea
• Middle Layer (nutriDon) • Uvea
• Iris • Choroid • Ciliary Body
• Inner Layer (detecDon) • ReDna
Sclera
• The tough white outer coat of the eye • Covered by ConjuncDva and Tenon’s • Allows abachment of extraocular muscles
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
The layers of the Cornea
1: Epithelium 2: Bowman’s Layer 3: Stroma 4: Descemet’s Membrane 5: Endothelium
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
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
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
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)
Aqueous Drainage
The Choroid
• Thin, highly vascular structure between reDna and sclera
• Vascular supply to outer reDna
The Lens
• Flexible (change in shape alters refracDve power) • Biconvex • Held in place by suspensory zonules • Crystal like • 1/3rd 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
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!
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
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
Rods & Cones
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
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 – 6th CN • Superior Oblique – 4th CN • All the rest – 3rd CN
• Deliver blood supply to anterior segment of eye via anterior ciliary arteries
Adnexal Tissues
• Lids • ConjuncDva • Lacrimal apparatus
• Orbit
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
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
Lacrimal Apparatus
• Lacrimal gland produces tears
• Drain inferonasally • Puncta (upper and lower) • Cannalicular system
• Lacrimal Sac
• Nasolacrimal duct
• Inferior meatus (nose)
Tears
• 3 layers • Anterior lipid (meibomium glands)
• Middle aqueous (lacrimal glands)
• Posterior mucous (goblet cells)
• ProtecDve • AnDbacterial • Smooth surface
• NutriDon to cornea
Orbit
Arterial Supply • 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
Venous drainage
• Inner reDna • Branch reDnal veins
– Ophthalmic veins
» Cavernous sinus
• Outer reDna and Choroid
• Vortex veins – Ophthalmic veins
» Cavernous sinus
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
QuesDons?