Eye

Human head

Objectives How the structural components of the eye contribute to the overall function of observing the visual world

Objectives Identify strategies (mechanisms) employed in the eye to provide nutrients and structural support without distorting light

Outline Overview Cellular structures through which light passes Cornea Lens Retina Structures which influence the image Iris Ciliary bodies Trabecular meshwork

Eye

Turn the eye so that it is facing you & examine these structures on the front surface of the eye: eyelids - two moveable covers that protect the eye from dust, bright light, and impact sclera - this is the tough, white outer coat of the eye that extends completely around the back & sides of the eye cornea - a clear covering over the front of the eye that allows light to come into the eye (preservative often makes this appear cloudy) iris - round black tissue through the cornea that controls the amount of light that enters the inner part of the eye (may be colored in humans) pupil - the round opening in the center of the eye that allows light to enter and whose size is controlled by the iris

V. Glossary of terms on the eye Anterior chamber -space between cornea and iris or lens - contains aqueous humor Aqueous humor -clear fluid, pressure regulated Bowman's membrane -acellular, glycosaminoglycans, rich layer of cornea Canal of Schlemm -carries fluid from trabecular meshwork to bloodstream Choroid -vascular layer Choroid plexus -vascular plexus, supplies nutrients to retina Ciliary body -attach zonules from lens, contraction of ciliary muscle alters shape of lens Ciliary epithelium -secretes aqueous humor Ciliary process -pigmented and non-pigmented epithelium Conjunctiva -stratified squamous or columnar, covers white of eye and inside eyelid Cornea -structural layer, course focus of image on retina

Corneal endothelium -water transport cells Corneal epithelium -stratified squamous, protection Corneal stroma -fibroblasts and collagen, avascular Descemet's membrane -basement membrane of endothelium of cornea Fovea -bull's eye of macula Glaucoma -serious eye disease, obstruction of outflow of aqueous humor which raises intraocular pressure Iris -regulate amount of light entering eye Iris muscles dilator -open pupil, myoepithelium constrictor -close pupil, smooth muscle Lens -focus image on photoreceptor cells, avascular Lens epithelial cells -cells on anterior surface, source of lens fibers Limbus -sclerocorneal junction

Macula -cones, most sensitive, devoid of blood vessels, ganglion cell and internuclear layer Optic -nerve cap or head- blind spot, no photoreceptor cells Optic nerve -connect brain with eye Photoreceptor -photosensitive cells rods -black and white, rhodopsin pigment cones -red, green, blue pigments Pigmented epithelium of iris -block out light Posterior chamber -space between lens and iris - contains aqueous humor Retina -photosensitive part of eye pigmented epithelium -absorbs light outer nuclear layer -nuclei of photoreceptor cells internuclear layer -nuclei of bipolar neurons ganglion cell layer -transmit to brain through optic nerve Retinal pigmented epithelium -phagocytotic, prevents backscatter, melanin, Vitamin A storage

Root of iris -attachment of iris to sclera Sclera -structural layer, white of eye, connective tissue Scleral spur -enlargement of sclera, site of anterior attachment of ciliary muscle Trabecular meshwork -resistance to aqueous humor outflow baffle of endothelial cells draped over connective tissue Transparent retina -all retina except retinal pigmented epithelium Uvea -vascular layer of eye = iris, ciliary body, ciliary processes, and choroid Vitreous body -fine filaments, jellylike, pressure on retina Zonules -ligaments of lens that attach lens to choroid and ciliary

Eye

Three Layers of the Wall of the Eye 1. Supporting, 2. Vascular, and 3. Retinal layers Uvea

Retina

Eye 192 human and 34412 monkey Aqueous humor 192 34412 Connective tissue Aqueous humor Nerve Posterior chamber 192 Retina Cornea Sclera Pupil Optic nerve Anterior chamber What is the function of the canal of Schlemm The canal of Schlemm functions to drain the aqueous humor from the anterior chamber. Where is aqueous humor produced? Aqueous humor is produced by ciliary processes. It then flows into the posterior chamber of the anterior cavity, through the pupil into the anterior chamber, and finally drains through the canal of Schlemm. What is the ora serrata? The ora serrata is the serrated junction between the retina and the ciliary body. What are the functions of the pigment epithelial layer of the retina? Absorbs scattered light that passes through the neural layer Part of the protective blood-retina barrier isolating retina photoreceptors from the highly vascular choroid and regulating ion transport between these compartments. Role in retinal regeneration Phagocytosis of shed components from the adjacent photoreceptors and degradation of this material Removal of free radicals by various protective antioxidant activities and support of the neural retina by secretion of ATP, various polypeptide growth factors, and immunomodulatory factors The sphincter pupillae muscle is innervated by what portion of the nervous system? Parasympathetic innervation What are fovea centralis and the macula lutea? Fovea centralis: a shallow depression with only cone cells at its center; ganglion cells and other conducting neurons are located only at its periphery. It lies directly opposite of the pupil and is where visual acuity or sharpness is maximal. Macula lutea: This surrounds the fovea centralis. Here all layers of the retina are present and the two plexiform layers are rich in various carotenoids, which give the area a yellowish color. The carotenoids have antioxidant properties and filter potentially damaging short wavelength light, thus helping to protect the cone cells of the fovea. What is the effect of light on the cGMP-gated sodium channel in the photoreceptor cell? Light > absorbed by retinal of rhodopsin > isomerize from 11-cis-retinal to all-trans-retinal > activates opsin > activate transducin > stimulates phosphodiesterase to hydrolyze cGMP > cGMP-gated sodium channels close > hyperpolarization of cell > decrease release of neurotransmitters at synapse > depolarizes sets of bipolar neurons > action potentials sent to optic nerve > brain produces image. Lens Iris 34412 Ciliary body 34412 Vitreous body Chorioid layer

Cellular structures through which light passes: Cornea Lens Retina

Cornea cornea cornea Function: Protection Structural support Filter out undesirable light rays Focus image on retina Nutrition: Limbus O2 from air for corneal epithelium cornea

Cornea

Cross section of human head

No blood vessels in cornea

corneal layers Iris of Eye (toluidine blue) 292 Ciliary muscle: smooth muscle Sclera Lens Sclera-cornea junction (limbus) Bowman's membrane Aqueous humor Corneal stroma Aqueous humor Corneal endothelium Iris Descemet's Membrane Iris Anterior chamber Posterior chamber

Stroma

Stroma

Limbus

Limbus

Lens Function: Focus image on photosensitive portion of photoreceptor cells Nutrition: Aqueous humor Lens

Eye (toluidine blue) elongation of lens cuboidal cells at bow region Lens cuboidal epithelium on its anterior surface gives rise to elongated lens fibers in the bow region, and the shared basement membrane of these cells is the capsule Capsule Lens cuboidal epithelium 292 Capsule flattened lens fibers No cuboidal epithelium on the posterior surface of the lens and the capsule is thinner there Artifactual space 34412

Lens

Cornea

Retina

Retina Function: photoreception of image processing by neurons prevent backscatter of light Nutrition: choroid, retinal blood vessels retina

34412 Cones Rods Ganglion cell layer Bipolar cells Vitreous body Retinal pigmented epithelium Artifactual space Photosensitive portion of rod and cone cells Retinal pigmented epithelium Artifactual space Venule in the choroid plexus Sclera

Retina (toluidine blue) 292 Ganglion cell layer Bipolar cells Artifactual space Photoreceptor cells Rods Optic nerve Cones Capillary

Cones have pigments for the primary colors (red, green, and blue)

Where do we find the primary colors (red, green, and blue)?

Light vs. dark adaptation

Function of retinal pigment Epithelium Vitamin A storage Phagocytosis of rod tips Absorption of light Nutrients to retina Blood retinal barrier

blind spot macula

Blind spot

Eye monkey 34412 Capillary network in retina Pigmented layer of epithelial cells Elaborate network of vessels, the choroid plexus. Central artery of the retina

Macula Blind spot

Blind spot

Macula

Typical retina Macula

Structures which influence the image Iris Ciliary bodies Trabecular meshwork

Iris Iris Function: regulate amount of light that reaches retina blackened posterior surface to stop light rays Dilator (myoepithelial) and Constrictor (smooth) muscles Nutrition: local blood vessels

Iris Dilator (myoepithelial) and Constrictor (smooth) muscle

Iris Myoepithelial Pigmented layer

Iris Blue eye Black eye

Pupil Iris

292 Eye (toluidine blue) Iris Anterior chamber Dilator muscle Lens Anterior chamber Dilator muscle Lens capsule Lens Iris Posterior chamber Iris pupil Cuboidal epithelium on anterior surface of lens Constrictor muscle Iris Pupil

Lens and iris 192 and 34412 Iris Iris Lens anterior surface 192 Pigmented epithelium of iris Iris Lens capsule Posterior chamber Iris Lens cuboidal epithelium 34412 Iris constrictor muscle is smooth muscle

Ciliary Bodies Function: Contraction of muscle changes lens through zonules Ciliary processes secrete aqueous humor Blackened region stops light rays Nutrition: local blood vessels

Ciliary Bodies Contraction of muscle reduces tension on zonules allowing the lens to be more spherical to focus on close objects.

292 Ciliary muscle: smooth muscle Ciliary processes Blood vessel lens Nerves Ciliary muscle 192

Ciliary bodies Lens Lens capsule Zonules

34412 Eye monkey Sites of attachment of zonules to the lens’ capsule Anterior surface Eye monkey Iris Anterior surface Posterior surface Ligaments (zonules) lens 192 One site of attachment of a zonule to the capsule on the lens on its posterior surface lens

34412 Sclara Ciliary muscle Zonules 192 Lens fibers Capsule and cuboidal epithelium on the anterior surface of lens Thinner capsule on the posterior surface of the lens

Secretory epithelium and pigmented epithelium for the ciliary bodies

Pigmented layer contributes to blood aqueous barrier

192 192 Eye – secretory cells Pigmented cells of the ciliary processes become the dilator muscle of the iris Pigmented cells Secretory cells Secretory cells become the pigmented cells of the iris Iris Ciliary processes

Outflow of aqueous humor IRIS

Trabecular Meshwork Function: resistance to outflow of aqueous humor Nutrition: local blood vessels, probably aqueous humor Trabecular Meshwork

Site of aqueous humor production Outflow of aqueous humor Site of aqueous humor production

Iris 34412 Canal of Schlemm Trabecular meshwork Blood vessels Posterior chamber with aqueous humor Monkey eye 34412

Endothelium

Trabecular Meshwork Resistance to outflow of aqueous humor 229 Canal of Schlemm Posterior chamber

In summary The overall function of the EYE:

to observe the visual world