Jarosław Kocięcki MD, PhD

ORGAN OF SPECIAL SENSE -

THE EYE BASICS OF ANATOMY, PHYSIOLOGY AND HISTOLOGY

Dimensions of the eye λ longitudinal dimension - 24 mm λ transverse dimension - 23,5 mm λ vertical dimension - 23 mm λ circumference - 37 mm λ capacity - 7,2 ccm λ weight - 7,45 g

Major layers of the eye λ corneoscleral layer (tunica fibrosa) - sclera - cornea λ uvea (tunica vasculosa) - choroid - ciliary body - iris λ retina (tunica interna)

Sclera λ definition - tough, fibrous coat continuous with the cornea λ function - forms the point of attachment for the ligaments of extraocular muscles - maintains the eye in a rigid shape λ structure

- consists of collagen fibrils maintained by fibroblasts that run parallel to the surface of the eye, branching and anastomosing to form a meshwork

Cornea - a. λ definition - avascular, transparent connective tissue covering anterior fifth of the eye λ function - the first refractive surface contributing to the object image and also a rigid protective layer

for the underlying tissues

transparency of the cornea - depends on the smoothness of the epithelium, the absence of blood vessels, the uniform

organization of collagen fibrils in the stroma, and the type of ground substance which state of hydration is maintained by epithelium and endothelium

The cornea is fed partially by the aqueous humor, tears and vessels of the limbus

Cornea - b. λ limbus - the zone of transition between the cornea and sclera where the conjunctival epithelium joins

with the corneal; beneath the epithelium is a trabecular meshwork and Schlemm’s canal which receives aqueous humor from the trabeculae and drains it from the anterior chamber into episcleral vessels

Cornea - c. λ structure - a stratified squamous nonkeratinizing epithelium - five cells thick - Bowman’s membrane - acellular layer of collagen fibrils - stroma - several layers of collagen fibrils of uniform size and spacing, parallel to the corneal

surface, embedded in ground substance (chondroitin sulfate and keratosulfate) - Descemet’s membrane - acellular, collagenous layer with a small elastic component

- endothelium - one layer of cuboidal cells covering posterior aspect of the cornea

Choroid λ definition - heavily vascularized and pigmented connective tissue which is the part of the uvea λ function - a nutritive layer, providing a pathway for blood vessels; it supports the retina and absorbs light

that has passed through the retina λ structure - pigment epithelium - a single layer of cuboidal epithelial cells containing melanin pigment

which functions to absorb light after it has passed through the photoreceptor cells so that it will not reflect back into the retina from the outer layers, interfering with the visual image

- choroid proper - a loose connective tissue containing a large number of blood vessels as well as pigment cells (melanophores); the innermost region of the choroidal stroma beneath the pigment epithelium contains an excessive capillary network, which provides nutrients for the outermost layers of the retina

Ciliary body - a.

λ definition - a ring-like part of the uvea, triangular in section, divided into pars plicata and pars plana λ function - accommodation - ciliary muscle innervated by parasympathetic fibers of the oculomotor

nerve relaxes tension on the suspensory ligament of the lens so that the lens rounds up to accommodate for near vision

- aqueous humor - a plasma filtrate that is actively modified in composition by the overlying epithelial cells; it is formed by highly vascular connective tissue underlying pars plicata; functionally it helps to maintain intraocular pressure as well as an avenue of exchange for metabolic nutrients and waste to and from the avascular cornea and lens

- supporting the lens through its suspensory ligaments (zonular fibres)

Ciliary body - b. λ structure - pars plicata (located anteriorly and folded into highly vascular ciliary processes) as well as pars

plana (located posteriorly and fairly smooth) are covered by two-layered cuboidal epithelium (its outer layer is unpigmented while the inner layer abutting the basement membrane is highly pigmented). Beneath those two parts is located the ciliary muscle

Iris - a. λ definition - the most anterior portion of the uvea, shaped like a disc with a central aperture - the pupil,

which rests on the anterior surface of the lens λ function - the pupil reacts to light as a variable diaphragm, which prevents overstimulation of the retina

in bright light, while ensuring adequate illumination for night vision Eye color is a function of the number of pigment cells located in the stroma - e.g. many pigment

cells yield brown eyes, whereas fewer pigment cells give rise to green or blue eyes

Iris - b.

λ structure - stroma - a loose connective tissue covered by an anterior epithelium continuous with the

corneal endothelium - sphincter muscle - circularly arranged, encircling the pupillary margin causes constriction of

the pupil under parasympathetic stimulation - dilator muscle - less distinct muscle fibers which run radially around the pupil and cause

dilation in response to sympathetic stimulation - pigmented epithelium - two-layered cuboidal posterior cells continuous with that of the

ciliary body

Arterial supply - from long posterior and anterior ciliary arteries - follow a spiral course; histological arrangement consists of a thin intima and media but a very thick adventitia which gives them slack when the pupil accommodates to light

The retina - a. λ definition - a complex membrane which sense light intensity and color λ function - transducing light stimuli into nerve impulses that travel via the optic nerve to the lateral

geniculate nucleus of the thalamus and finally to the visual cortex where the sensation of vision is created

The retina - b. λ structure 1. rod and cone outer segments 2. external limiting membrane 3. outer nuclear layer 4. outer plexiform layer 5. inner nuclear layer 6. inner plexiform layer 7. ganglion cell layer 8. nerve fiber layer 9. internal limiting membrane

The retina - c. 1. a - rods - are responsible for sensing low levels of illumination and are indispensable for night

vision. The outer segment of rods contains more than a thousand membranous discs formed by plasmalemma which contain molecules of rhodopsin (visual purple). Exposure to light changes conformation of the molecule leading to hyperpolarization of the rod cell membrane and triggers an action potential on a bipolar neuron with which the rod axon synapses

1. b - cones - respond only to bright light and are indispensable for color perception and normal

visual acuity. Cone pigment (iodopsin) absorb light efficiently at different wavelengths, giving rise to the perception of color. Whereas many rods can synapse onto one bipolar neuron (summation), cones may synapse individually producing great visual acuity

2. external limiting membrane - a series of junctional complexes between Müller’s cells (a type

of supporting glial cells) and the photoreceptor cells with which they interdigitate 3. outer nuclear layer - consists of rod and cone cell bodies; the cone nuclei form a single layer

adjacent to the external limiting membrane, whereas the rod nuclei occupy several layers except at the fovea

4. outer plexiform layer - consists of rod and cone axons, dendrites of bipolar neurons, and processes of horizontal cells

5. inner nuclear layer - contains nuclei of the bipolar neurons, horizontal and amacrine cells

(association neurons) and Müller cells

6. inner plexiform layer - consists of amacrine cells processes, bipolar axons and ganglion cell dendrites

7. ganglion cell layer - comprises multipolar ganglion cells and supporting glia; branches of the

central artery of the retina are presented in this layer and nourish layers 5 through 9, while capillaries of the choroid nourish the first four layers

8. nerve fiber layer - is composed of bundles of unmyelinated axons from the ganglion cells,

which course in parallel to the retina and run to the optic disc 9. internal limiting membrane - is formed from the bulbous, expanded ends of the Müller cells

and their basement membrane

The vitreous body λ definition - a transparent, jellylike tissue which fills the compartment of the eye bounded anteriorly by

the lens and posteriorly by the retina λ function - refracts light as well as cushioning and protecting the retina from shock and vibration, which

could cause it to detach from the choroid layer, leading to partial blindness λ structure - connective tissue comprising a small number of collagen fibrils embedded in a matrix of

hydrophilic mucopolysaccharide - hyaluronic acid

The lens λ definition - a transparent, biconvex body that has considerable flexibility, suspended at its peripheral part

(the equator) by zonular fibers originating from the ciliary body λ function

- accommodation depending on the flexibility of the lens, during which ciliary muscle contracts causing relaxation of tension on the zonular fibers and the lens rounds up to bring near objects into focus on the retina. When the ciliary muscle is relaxed, the zonular fibers are stretched and the lens is under tension and relatively flattened so that distant objects are in focus

λ structure - capsule - a tough, elastic basement membrane that envelops the entire lens - anterior epithelium - a low cuboidal cells covering anterior surface of the lens (facing the

pupil) which toward the equator of the lens become columnar and turn meridionally (from perpendicular to parallel to the lens surface)

- lens substance - consists of a bulk of the lens fibers, that is transformed, elongated

meridional epithelial cells

Chambers of the eye λ vitreous compartment - bounded by the posterior surface of the lens, zonule, the cilliary body and the retina; it

contains the jelly-like vitreous body λ posterior chamber - enclosed by the peripheral portion of the lens and its zonular attachments, cilliary processes

and posterior surface of the iris; cilliary body epithelium produces the aqueous fluid which passes through the pupil into anterior chamber

λ anterior chamber - occupies the space between the posterior surface of the cornea, trabecular meshwork in the

angle, anterior surface of the cilliary body and the iris and central portion of the lens; it contains the aqueous fluid from the posterior chamber which drains through the trabecular meshwork to Schlemm’s canal and than into the scleral and episcleral veins

Optic nerve λ definition - 2nd cranial nerve containing visual fibres passing to the lateral geniculate bodies and than to

the cortex, and pupillary fibres which form the afferent part of the pupillary reflex arc running to the tectum of the mid-brain

λ function - conduction of impulses generated by the light in the retina to the brain

λ structure - intraocular part - begins at the optic disc towards which the axons of the ganglion cells

converge to leave the eye through lamina cribrosa; the center of the optic disc is usually depressed (the physiological cup) from which the central retinal vessels and their main divisions are emerging

- orbital part - the optic nerve fibres have myelin sheaths proximal to the lamina cribrosa and the

nerve runs a slightly sinuous course to the optic foramen; the central retinal artery and vein enter and leave the nerve by passing through the meninges and the cerebrospinal fluid space about 1 cm behind the globe

- intracanalicular part - lies within the optic canal in the sphenoid bone together with the ophthalmic artery and its branch, the central retinal artery; the nerve sheath is here partly adherent to the dura lining the narrow canal

- intracranial part - extends from the optic canal to the chiasma; it is clothed in pia mater, the

other layers being reflected at the mouth of the optic canal; the anterior cerebral artery crosses above the nerve, the internal carotid is lateral to it, while the ophthalmic artery is infero-lateral

λ papilloedema (swelling of the nerve head due to raised intracranial pressure) - raised

intracranial pressure is transmitted by the CSF in the optic nerve sheath to the optic nerve; it is hypothesized that here it exerts pressure on the small vessels in the vascular circle at the nerve head and on the central retinal vein, thus causing a combination of vascular stasis and ischemia which results in oedema at the nerve head, as well as the accumulation of substances involved in axonal flow

λ glaucomatous cupping of the optic disc - an ischaemic change in the optic nerve head as the

intraocular pressure rises above capillary and arteriolar pressure causing a reduction in perfusion of vessels supplying the nerve; characteristic atrophy of both the nerve fibres and the supporting glial framework causes widening and excavation of the optic disc with overhanging edges - condition often seen in advanced glaucoma

Conjunctiva - a. λ definition - a mucous membrane with a non-keratinised, stratified epithelium and a subepithelial layer

containing adenoid and connective tissue, covers the anterior portion of the eye up to the cornea and the internal surface of the eyelids; it consists of three parts: palpebral conjunctiva (covers the tarsal plate to which it is closely adherent), fornix conjunctiva (recesses between the lids and the eyeball) and bulbar conjunctiva (loosely attached to the sclera, which forms medially plica semilunaris and the caruncle)

λ function

- protects the anterior portion of the eye and enables the smooth movement of eyelids on its surface

λ structure - the stratified columnar epithelium of the conjunctiva varies from two to seven calls thick and

is never keratinized in health; - goblet cells (secreting mucus and responsible for the mucous inner layer of the tear film) are

present and more numerous in the fornices; - the subepithelial layer consists of connective tissue in which lie the nerves, blood vessels and

the accessory lacrimal glands;

- the palpebral conjunctiva is supplied by the ophthalmic and maxillary branches of the fifth cranial nerve and the bulbar conjunctiva by the ciliary nerves both sensory and sympathetic

Eyelids λ definition - movable folds of tissue that protect the eye λ function - protects the eye against the traumas, foreign bodies, drying λ structure - eyelid muscles : - the levator palpebrae superioris - responsible for raising the upper lid; some of its fibres are

inserted into the top of the tarsal plate while others pass forward between the fibres of the orbicularis oculi to be inserted into the skin of the upper lid

- Müller’s muscle - also responsible for opening the palpebral fissure; in upper lid it arises from the levator palpebrae in the upper fornix and is inserted into the upper edge of the tarsal plate; in lower lid it arises from the inferior rectus muscle and is inserted into the lower border of the tarsal plate of the lower lid

- the glands of the eyelids - Meibomian glands (tarsal glands) - sebaceous glands situated within the tarsal plate which

produce a sebaceous substance that creates an oily layer on the surface of the tear film, helping to prevent rapid evaporation of the normal tear layer; they are not connected with the lash follicles

- glands of Zeis - smaller, modified sebaceous glands of the lash follicles - glands of Moll - the sweat glands which are unbranched, sinuous tubules; they empty their

secretion into the follicles of the eyelashes

Lacrimal apparatus λ lacrimal gland - lies in the supero-temporal quadrant of the orbit and consists of an orbital and palpebral

portion - the secretions of the gland pass into the upper fornix by several (6-12) small excretory ducts

which pass through or close to the palpebral portion - a tubuloalveolar gland composed of column-shaped cells of the serous type which show

lightly stained secretory granules, and the basal lamina separates them from the surrounding connective tissue

- secretory portions of lacrimal gland are surrounded by myoepithelial cells

λ lacrimal canaliculi - extend from lacrimal puncta (round apertures on the medial aspects of both the upper and

lower lid margins) which are about 1 mm in diameter and 8 mm long and join to form a

common canaliculus just before opening into the lacrimal sac, are lined by a thick stratified squamous epithelium

λ lacrimal sac - lies in the lacrimal fossa which is formed by the lacrimal bone and the frontal process of the

maxilla; it opens below into the nasolacrimal duct λ nasolacrimal duct - is the downward continuation of the lacrimal sac and runs backwards laterally and

downwards to open into the inferior meatus of the nose lateral to the inferior turbinate; it is lined with ciliated pseudostratified epithelium

λ tears - an aqueous solution with dissolved salts and small amount of protein - the enzyme lysozyme

responsible for dissolving the mucopolysaccharide coats of certain bacteria λ precorneal tear film - is made up of three layers - mucous layer - derived from the conjunctival goblet cells; act as a wetting agent allowing the

tears to form an even thin coating over the cornea; is nearest to the corneal epithelium - aqueous layer - contains the secretions of the lacrimal gland - lipid layer - derived from the Meibomian glands delays the evaporation of the aqueous layer the tear film is distributed evenly by the act of blinking and excess tears drain along the marginal

strip, enter the canaliculi and passes up to the nose cavity

Extraocular muscles λ rectus muscles - superior, inferior, lateral and medial - their origin is a tendineous ring enclosing the optic foramen and the medial end of the

sphenoidal fissure, giving attachment to the sphenoid bone; they are inserted into the sclera 5-8 mm from the limbus

λ superior oblique muscle - it also arises posteriorly but from bone just above and medial to the optic foramen; its tendon

passes through the trochlea attached to the frontal bone and then backwards and laterally, fanning out to be attached to the upper part of the postero-lateral aspect of the globe

λ inferior oblique muscle - arises anteriorly from the maxilla just within the orbital margin lateral to the orbital sac; it

follows a similar direction to the reflected tendon of the superior oblique, also to be inserted postero-laterally but lower down, extending medially to the posterior pole

Refractive tissues λ cornea - its anterior surface contributes approximately 2/3 of the refracting power of the eye

because more deflection of light rays occurs at the air-cornea interface, due to the large difference in index of refraction between these two media (corneal refractive index - 1,376; refractive power - +43.0 D)

λ lens - each of its surfaces is more convex than the cornea, but it is in the aqueous-vitreous medium, and the difference in refractive index at the aqueous-lens and lens-vitreous interfaces is much less (lens refractive index - 1,41; refractive power - +20,0 D)

λ aqueous humor and vitreous body - plays limited role in refraction because of very weak

refractive power λ total power of the eye - about 60,0 D (Gullstrand - 58,6 D; Le Grand - 58,6 D; Ivanov - 64,2

D)

Refractive errors of the eye λ emmetropia - when parallel light rays, from an object more than 6 meters away, are focused at the plane of

the retina when the eye is in a completely relaxed state, without any internal adjustment of its optics

λ hyperopia (farsightedness) - when the eye is “too short” and the focused image is formed behind the plane of the retina;

unless the optical system of the eye is actively altered to produce an increase in its power, hyperopic eyes have blurred images for distant objects also. It should be corrected with the condensing lens

λ myopia (nearsightedness) - when the eye is “too long” and the focused image is formed in front of the plane of the retina;

there is a point less than 6 meters in front of the eye that will be coincident with the retina when the optical system of the eye is relaxed. It should be corrected with the dispersing lens

λ astigmatism - when the curvature of the cornea varies in different meridians resulting in a surface

resembling a section through a cylinder; the steepest and the flattest meridians of the eye are usually at right angles to one another resulting in regular astigmatism; each meridian may be emmetropic, hyperopic or myopic therefore in astigmatic correction three numbers are required to indicate the power needed in each principal meridian plus the axis to provide the correct orientation of the lens in front of the eye (e.g.: +2.0 -1.0 x 180)