Eye & Ear SPR 2011EyeLayers of the EyeFibrous layer:Anterior corneaPosterior sclera
Middle vascular layer:ChoroidCiliary bodyIris
Neural layer:Retina outer pigment & inner neural layer
CorneaAnterior transparent layer of the eye ( outer fibrous layer)Refractive structure of the eye. Avascular - but can heal by diffusion of nutrients from the limbus.Has numerous free nerve endings Very sensitive!
Structure of the CorneaAnterior corneal epitheliumStratified squamous non keratinizedEpithelium capable of regeneration if damageddrying leads to ulceration
Bowmans anterior limiting membraneBarrier for infection
Substantia proprialamellae of collagen fibers (lymphocytes & fibroblasts are interspersed in between the fibers) in a ground substance.Regular arrangement, Regular spacing gives cornea the transparency
Descemets membraneIncreases in thickness with age
Endothelial layerActive pump located in the cells that maintains the dehydrated state of the cornea (substantia propria)
ScleraPosterior 5/6th of outer fibrous layerConnective tissue with collagen fibersRelatively avascularYellow in elderly because of accumulation of Lipofuscin pigments
Also called as the Limbus Corneal epithelium becomes the bulbar conjunctivaContains 2 very important structures:Trabecular meshworkCanal of Schlemm
Sclerocorneal JunctionLimbusCanal of Schlemm:Canal lined with endothelium situated at the inner aspect of the corneal stroma close to the iris
Trabecular Meshwork:Fine collagenous trabeculae enclosing endothelium lined spacesMerge & open into canal of Schlemm
Chambers of the EyeAnterior chamber between iris & cornea
Posterior chamber between lens & iris
Vitreous chamber lens & retina
Aqueous HumorClear watery Non- Refractile fluid secreted by the ciliary body into the posterior chamber.Similar to plasma with just 0.1% protein (as compared to 7% plasma protein in blood)Provides nutrition to the lens & cornea.Replaced every 2-3 hrs.Determines intra-ocular pressure ( 15-20mm of Hg)
Disturbances in production or flow Glaucoma!
Rise in intra-ocular pressure due to increased production or impaired drainage of aqueous humor.Needs to be treated as can affect vision.
Vitreous ChamberGelatinous mass with some fluid99% water with small amount of collagen & Hyaluronic acidFew cells called Hyalocytes synthesize the contents of the vitreous.Contains the hyaloid canal
Uveal TractMiddle vascular layerChoroidCiliary bodyIrisChoroidLoose vascularized connective tissue present between the retina & sclera in the posterior 5/6th of the eyeball.
Contains a choriocapillary layer for nutrition & oxygenation of the underlying retina.
Choroid & retina separated by Bruchs membrane - a hyaline glassy membrane, extends from the optic disc to the ora serrata.
Contains pigmented melanocytes which absorb light that passes through the retina.
Ciliary BodyExtends from the iris to the ora serrata.Attached to the suspensory ligaments of the lens.Contains ciliary muscle which is made up of meridoneal, circular & radial fibers.When it contracts, tension on the suspensory ligaments is reduced, thus permitting the lens to acquire a more convex shape for accommodation.Has parasympathetic innervation.
Stroma made up of elastic fibers, blood vessels & melanocytes.Lining epithelium is double layered:Deep pigmented layer continuous with the retina.Superficial non pigmented layer continuous with the neural layer of retina. Involved in transport of ions/water. Have features of fluid transporting cells.Ciliary ProcessesFinger like extensions.Have a connective tissue core rich in fenestrated capillaries.Produce aqueous humor.
IrisForms a diaphragm anterior to the lens.Circular in shape with a central opening called Pupil.Stroma has pigment cells. Color of iris (eye) depends on the pigment in the stroma.Blue eyes have less pigment, brown eyes have more pigment.
ConjunctivaBulbar conjunctiva covers exposed part of sclera
Palpebral conjunctiva covers inner surface of eyelids.
Stratified columnar epithelium with goblet cells which secrete mucus.
Lacrimal GlandCompound tubuloacinar gland.Acini with a single layer of columnar to pyramidal cells (like parotid)Secrete tears which is a watery secretion containing lysozyme & antibodies Ig A.
EyelidHairy skin thin & highly folded with a loose supporting tissue.
Skeletal muscle orbicularis oculi & levator palpebrae.
Submuscular connective tissue extremely lax, continuous with the loose areolar tissue layer of scalp.Tarsal plate thick fibroelastic plate with Meibomian glands (modified sebaceous glands) - CHALAZION
Lid margin has eye lashes associated with glands of Zeiss (sebaceous) & glands of Moll (modified sweat) - STYERetinaConsists of an outer pigment layer & inner neural layer.Inner neural layer composed Photoreceptors rods & conesConducting neurons bipolar cells & ganglion cellsAssociation neurons horizontal cells & amacrine cellsSupporting cells Muller cells & neuroglial cellsIntraretinal space is cleavage plane.
Rods outnumber cones (120million to 7 million)
Rods more sensitive to light than cones.
Cones for color vision & high visual acuity.RetinaComposed of 9+1 layers:Outer pigment layer
Photoreceptor layer rods & cones processes
Outer (external limiting membrane) apical boundary of Muller cells, site of tight junction with photoreceptors.
Outer nuclear layer nuclei of cell bodies of rods & cones
Outer plexiform layer axodendritic synapses between axons of photoreceptors & dendrites of bipolar & horizontal cells.Inner nuclear layer cell bodies of horizontal, amacrine & bipolar cellsInner plexiform layer synapses between axons of bipolar cells & dendrites of ganglion cellsGanglion cell layer cell bodies of multipolar ganglion cellsOptic nerve fiber layer axons of ganglion cells Inner limiting membrane basal lamina of Muller cells separates retina from vitreous.
Rods & conesHave inner & outer segments, a nuclear region & synaptic region.
Outer segment contains flattened membranous discs which contain Rods - Rhodopsin. Cones- Iodopsin
- Eventually shed their discs which are phagocytosed by the pigment epithelium.
Inner segment has organelles for rhodopsin & iodopsin production & numerous mitochondria.
Cones recognize RED, GREEN, BLUERods Rhodopsin or visual purple has two absorption maxims: 350 and 500 nm. The spectral extinction curve for rods corresponds to that of rhodopsin, suggesting that rhodopsin is the chemopigment in rods.
Rhodopsin consists of a glycoprotein (opsin) and a chromophore group (11-cis-retinal).
Retinal is the aldehyde of vitamin A1 (retinol).
Photoisomerization in Rods Inside the rod a special amplification takes place.
Light absorption by a single rhodopsin molecule activates thousands of G-protein molecules (transducin), which then activate large quantities of cGMP phosphodiesterase in the discs.
With absorption, specifically, it is the 11-cis form of retinene, which, isomerizes to the all-trans form.
This isomerization converts the rhodopsin to its active form, metarhodopsin II.
This reconfiguration of the retinene molecule thus produces the same effect as if a neurotransmitter had suddenly bound to a receptor.
The photoisomerization of 11-cis retinal to all-trans retinal in photoreceptors is the first step in vision occurs in the OUTER SEGMENTin the LIGHT rhodopsin changes conformation (TRANS), which activated enzyme TRANSDUCIN;
Transducin activates a PHOSPHODIESTRASE, converts cyclic-GMP to GMP; GMP closes the Na channels, and HYPERPOLARIZES rod cells (RECEPTOR POTENTIAL); hyperpolarized rod cell stops releasing GLUTAMATE, allowing bipolar cells repsond*
Vitamin A & RodsThe Vitamin A that our bodies produce from the beta carotene in many of the foods we eat (including, most famously, carrots) is needed to synthesize the retinene bound to the centre of the rhodopsin molecule.
Indeed, a severe Vitamin A deficiency impairs night vision, because of the smaller amounts of retinene being produced.
During the daytime, however, there is generally enough light to allow relatively normal vision despite low levels of visual pigments Cones In the cones, the photosensitive pigment is opsin, a transmembrane protein that is very similar to rhodopsin
The fovea only contains cones.
Cones function in the daytime with maximal visual acuity and colour vision.
The human eye possesses three types of cones, each with a specific pigment related to the three basic colours: red (erythrolab), green (chlorolab) and blue (cyanolab).
The cones in the fovea do not contain cyanolab.
Colour blindnessClassic red/green colour blindness is the result of a lack of red cones in the retina.
Forms of colour blindness are usually classified according to the type of cone affected.
Thus there are three kinds of colour blindness, corresponding to the three kinds of cones.
Blindness to green, due to deficiency of the green pigment, is called deuteranopia, and is the most common form
Macula:Yellow pigment zone (xanthophyll) surrounding the foveaRetinal vessels are absent hereGanglion cells heaped to the sides so that light can pass unimpeded to the fovea.
FoveaDepression in the inner layer of retina.Located 2.5mm temporal to the optic disc.High concentration o