University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2024
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494

Pages:

69
Author:

Hundebuchesn

Eyeball Coats of the Eyeball 1- Outer fibrous coat 2- Middle vascular coat 3- Inner nervous coat Each eyeball consists externally of a tough, fibrous globe that maintains its overall shape. Internally the eye contains transparent tissues that refract light to focus the image, a layer of photosensitive cells, and a system of neurons that collect, process, and transmit visual information to the brain. Outer fibrous coat The Sclera The white posterior five-sixths of the outer fibrous layer is the sclera -The sclera is composed of dense fibrous tissue and is white. -Posteriorly, it is pierced by the optic nerve -It is continuous posteriorly with the dura mater of optic nerve ✓It is continuous in front with the cornea at the corneoscleral junction, or limbus ✓The sclera is also pierced by the ciliary arteries and nerves and their associated veins Corneoscleral junction (limbus) Cornea Conjunctiva Sclera Functions: ✓Supports eye shape ✓Protects delicate internal structures ✓Extrinsic eye muscle attachment site Sclera is the dense white part Dura mater Optic nerve Corneoscleral junction (limbus) Pupil Cornea Sclera Iris Caruncle Outer fibrous coat The Cornea The transparent anterior one-sixth of the outer fibrous layer is the cornea ✓It is transparent ✓It is in contact posteriorly with the aqueous humor. Blood Supply ✓ The cornea is avascular and devoid of lymphatic drainage ✓ It is nourished by diffusion from aqueous humor, tears, and from the capillaries at its edge (limbus) Blood vessels may cloud the cornea, which may prevent it from refracting light properly and may adversely affect vision Nerve Supply Long ciliary nerves from the ophthalmic division of the trigeminal nerve (nasociliary branch) Function of the Cornea The cornea is the most important refractive medium of the eye. The cornea is one of the most sensitive tissues of the body (has one of the richest sensory nerve supplies of any tissue) aqueous humor MIDDLE VASCULAR PIGMENTED COAT Iris Ciliary body Choroid THE VASCULAR COAT CONSISTS OF: FROM BEHIND FORWARD 1-THE CHOROID Act as the black box 2- THE CILIARY BODY of the camera 3- THE IRIS MIDDLE VASCULAR PIGMENTED COAT 1- Choroid Choroid is a brown vascular membrane deep to the sclera Component: Areolar connective tissue; highly vascularized, heavily pigmented (contains numerous melanocytes) Function: ✓Supplies nourishment to outer part of retina ✓Pigment absorbs extraneous light MIDDLE VASCULAR PIGMENTED COAT 2- Ciliary body ✓ The ciliary body is ring-shaped and continuous posteriorly with the choroid, and anteriorly it lies behind the peripheral margin of the iris ✓ Contains the ciliary muscle (the main muscle of accommodation) ✓It is connected to the lens by the suspensory ligaments of the lens (zonular fibers) Ciliary body Component: Ciliary muscle Ciliary processes (covered with a secretory epithelium) Function: ✓Holds suspensory ligaments that attach to the lens and change lens shape for far and near vision ✓Epithelium secretes aqueous humor Iris Ciliary body ✓The ciliary processes secrete aqueous humor into the posterior chamber Lens Ciliary processes Ciliary muscle The ciliary muscle Nerve supply: The ciliary muscle is supplied by the parasympathetic fibers from the oculomotor nerve. After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves. Action: Contraction of the ciliary muscle relieves the tension in the suspensory ligament, and the elastic lens becomes more convex. This increases the refractive power of the lens. Iris Lens Accommodation permits focusing on near and far objects by changing the curvature of the lens To accommodate the eye for close objects, the ciliary muscle contracts so that the radiating fibers of the suspensory ligament are relaxed. This allows the elastic lens to assume a more globular shape Near vision When the ciliary muscle relaxes so that the radiating fibers of the suspensory ligament are tense. This allows the elastic lens to assume a flatter shape Far vision MIDDLE VASCULAR PIGMENTED COAT Ciliary body 3- Iris Is a thin, contractile and pigmented smooth muscle with a central aperture the pupil ➢It is suspended in the aqueous humor between the cornea and the lens. Lens ➢The periphery of the iris is attached to the ciliary body. ➢It divides the space between the lens and the cornea into an anterior and a posterior chamber Ciliary body Cornea Iris Anterior chamber Iris Posterior chamber NOTE: Iris and lens are bathed in clear aqueous humor that fills both the anterior chamber between the cornea and iris and the posterior chamber between the iris and lens Lens Iris Component: Two smooth muscles (sphincter pupillae and dilator pupillae) and connective tissue stroma (contains melanocytes), with a central pupil Melanocytes of the iris stroma provide the color of one’s eyes. Function: Controls pupil diameter and thus the amount of light entering the eye In individuals with very few melanin pigment in the stroma, light with a blue color is reflected back (producing the blue color of iris). As the density of melanin increases in the stroma, the iris color changes through various shades of green, gray, and brown. The scattering of light by the turbid medium in the stroma of the iris Stroma X X X X Pigmented epithelium Blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma Individuals with albinism have almost no pigment and the pink color of their irises is due to the reflection of incident light from the blood vessels of the stroma. The Tyndall effect is scattering of light by particles in a colloid or particles in a fine suspension. It can be seen when the light passes through the colloids or turbid substances causing the light to scatter in multiple directions. Pupil The muscle fibers of the iris are involuntary and consist of circular and radiating fibers. Iris The circular fibers form the sphincter pupillae Nerve supply: is supplied by parasympathetic fibers from the oculomotor nerve. After synapsing in the ciliary ganglion, the postganglionic fibers pass forward to the eyeball in the short ciliary nerves. The radial fibers form the dilator pupillae Nerve supply: is supplied by sympathetic fibers, which pass forward to the eyeball in the long and short ciliary nerves. Action: The sphincter pupillae constricts the pupil in the presence of bright light and during accommodation. Action: The dilator pupillae dilates the pupil in the presence of light of low intensity or in the presence of excessive sympathetic activity such as occurs in fright Production of aqueous humor and intraocular pressure 1. 2. 3. Ciliary Process: Produces Aqueous Humor Posterior Chamber: Aqueous Humor flows from this chamber through the pupil to the Anterior Chamber Canal of Schlemm: Reabsorbs Aqueous Humor Obstruction to the draining of the aqueous humor results in a rise in intraocular pressure, this may lead to optic neuropathy (glaucoma) Cornea Canal of Schlemm Iris Aqueous Humor is a clear fluid that fills the anterior and posterior chambers of the eyeball Lens Ciliary muscle Ciliary processes The lens is a unique avascular tissue and is highly elastic The Lens The lens is a transparent, biconvex structure It is situated behind the iris and in front of the vitreous body It focuses light on the retina Clinical note: With advancing age, the lens becomes denser and less elastic, and, as a result, the ability to accommodate is lessened (presbyopia). Presbyopia is corrected by wearing glasses with convex lenses (reading glasses). Presbyopia (Gr. presbyter, elder + L. opticus, relating to eyes) Iris Cornea Accommodation of the Eye To accommodate the eye for close objects, the ciliary muscle contracts so that the radiating fibers of the suspensory ligament are relaxed. This allows the elastic lens to assume a more globular shape. Vitreous body Lens protein= crystallins Clinical notes: A cataract is a cloudy area that forms in the lens of the eye Causes of cataract include excessive exposure to ultraviolet light or other radiation, trauma, and as secondary effects in diseases such as diabetes mellitus and hypertension. Iris Cornea In older individuals, denaturation of crystallins commonly begins to occur in lens fibers, making them less transparent. When areas of the lens become opaque or cloudy and vision is impaired, the condition is termed a cataract. Vitreous body Nervous Coat: The Retina The retina consists of an outer pigmented layer and an inner nervous layer Its outer surface is in contact with the choroid, and its inner surface is in contact with the vitreous body At the center of the posterior part of the retina is an oval, yellowish area, the macula lutea, which is the area of the retina for the most distinct vision. It has a central depression, the fovea centralis Vitreous body Fovea centralis Macula lutea Nervous Coat: The Retina Component: Pigmented layer: Pigmented epithelial cells Neural layer: Photoreceptors, bipolar neurons, ganglion cells, and supporting Müller cells Function: Pigmented layer: Absorbs extraneous light (supplementing the choroid in this regard) Provides vitamin A for photoreceptor cells Neural layer: Detects incoming light rays; light rays are converted to nerve signals and transmitted to the brain Anterior 1/4 Non receptive (non visual) part Ora serrata Optic (light sensitive) part Posterior 3/4 Nervous Coat: The Retina The optic nerve leaves the retina about 3 mm to the medial side of the macula lutea by the optic disc. The optic disc is slightly depressed at its center, where it is pierced by the central artery of the retina At the optic disc is a complete absence of rods and cones so that it is insensitive to light and is referred to as the blind spot Optic disc (blind spot) Optic nerve Macula lutea with fovea centralis Optic disc Ophthalmoscopic (Fundoscopic) examination of the right eye Optic disc (blind spot) Macula lutea with fovea centralis Temporal Nasal ….. ….. ….. Macular lutea (yellow, inferolateral) Fovea centralis The point of sharpest vision Highest concentration of cones Optic disc (superiomedial) Where optic nerve fibers pass through Blind spot (no rods and cones) Macula lutea (4) Optic disc (1) Central depression: fovea centralis Central depression: physiological cupping Fovea centralis is the point of sharpest vision Blind spot (no rods and cones) Diffuse margins Sharp margins Lateral and slightly Inferior Medial and slightly Superior Avascular Vascular (blood vessels entering/ leaving) These are end arteries They don’t communicate/ anastomse with each other Right Fundus 1 4 Central Retinal Artery Central Retinal Vein Right Fundus B Area B A Area A Anatomical end artery Example: Central artery of retina Area B Contents of the Eyeball The contents of the eyeball consist of: 1-THE AQUEOUS HUMOR 2-THE VITREOUS BODY 3- THE LENS Vitreous Body A large gelatinous mass of transparent connective tissue The vitreous body fills the eyeball behind the lens and is a transparent gel. The hyaloid canal is a narrow channel that runs through the vitreous body from the optic disc to the posterior surface of the lens; in the fetus, it is filled by the hyaloid artery, which disappears before birth. The function of the vitreous body is to contribute slightly to the magnifying power of the eye. It supports the posterior surface of the lens and assists in holding the neural part of the retina against the pigmented part of retina Detached retina !!! Ocular blood supply Supratrochlear artery External nasal artery Supraorbital artery Lacrimal artery Muscular branch Ophthalmic artery Short Posterior ciliary arteries Ocular blood supply ✓The arterial input to the eye is provided by several branches from the ophthalmic artery, which is derived from the internal carotid ✓These branches include: Central retinal artery Short and long posterior ciliary arteries Anterior ciliary arteries ✓Venous outflow from the eye is primarily via the vortex veins and the central retinal vein, which merge with the superior and inferior ophthalmic veins that drain into the cavernous sinus, the pterygoid venous plexus and the facial vein The short posterior ciliary arteries: (from 6-12 in number) pierce the posterior part of the sclera just around the optic nerve. The long posterior ciliary arteries: (2 in number) pierce the posterior part of the sclera at some little distance from the optic nerve. The anterior ciliary arteries are derived from the muscular branches of the ophthalmic artery. Vorticose vein Long posterior ciliary artery Central retinal artery Central retinal vein Long posterior ciliary artery Vorticose vein Short posterior ciliary arteries Superior ophthalmic vein Long posterior ciliary artery Central retinal artery Optic nerve Long posterior ciliary artery Central retinal vein Vorticose vein Short posterior ciliary arteries Structures piercing the Sclera Side view Inferior ophthalmic vein Vorticose vein Central retinal artery and vein (optic disc) Long ciliary nerve Vorticose vein Vorticose vein Short posterior ciliary arteries Short ciliary nerves Long posterior ciliary artery Vorticose vein Structures piercing the Sclera posterior view Some vortex veins drain into the superior ophthalmic vein which drains into the cavernous sinus Some vortex veins drain into the inferior ophthalmic vein which drains into the pterygoid plexus and cavernous sinus Superior ophthalmic vein The vorticose veins (vortex veins) (mostly 4 in number) drain the ocular choroid Varicose veins Inferior ophthalmic vein Sclera Choroid Retina Choroidal vessels (Derived from posterior ciliary arteries) Sclera Choroid Retina Note: The choroidal arteries arise from long and short posterior ciliary arteries The corresponding veins drain into the vortex veins Choroido-capillaries Chorio-capillaries The choriocapillaris is an extensive, anastomosing capillary system derived from the choroidal vessels The human cornea is about 0.5 mm thick at the center, increasing somewhat towards the periphery Histology of cornea Cornea is: Two layers of epithelium with organized connective tissue in between 0.8 mm 0.5 mm Functions 1- Transmission of light (as it is transparent) 2- Refraction of light for better focus (as it is curved) 3- Plays a role in structural integrity of the eyeball (as its tough) 4- Protection A section of the cornea shows five distinct layers: ✓An external stratified squamous epithelium non keratinized ✓ An anterior limiting membrane (Bowman’s membrane), which is the basement membrane of the external stratified epithelium ✓A thick stroma ✓A posterior limiting membrane (Descemet’s membrane), which is the basement membrane of the endothelium ✓An inner monolayer of epithelial cells (endothelium). Dense connective tissue Cornea is a dense type of connective tissue sandwiched between 2 layers of epithelium Corneal Endothelim 5 4 3 2 1 Corneal covering epithelium Posterior lamina (Descemet’s membrane) Substantia Propria Corneal stroma Anterior lamina (Bowman’s membrane) Corneal Endothelim Dense connective tissue Cornea is a dense type of connective tissue sandwiched between 2 layers of epithelium Monolayer of epithelial cells Corneal covering epithelium 5 4 3 2 Thick basement membrane between substantia propria and corneal endothelium Posterior lamina (Descemet’s membrane) 1 Stratified non keratinized squamous epithelium 5-6 cell layer Regenerates from stem cells at the edge (corneoscleral limbus) Substantia Propria Corneal stroma The bulk tissue Multiple lamellae of regularly arranged Fine collagen fibrils and in between are fibrocytes (keratocytes) Anterior lamina (Bowman’s membrane) Thick basement membrane between corneal covering epithelium and substantia propria Corneal stroma Collagen lamella NOTE: Collagen fibrils are parallel and are superimposed like book pages The uniform arrangement of collagen fibrils contributes to the transparency of this avascular tissue Around 250 Collagen lamellae are arranged vertically and horizontally forming a lattice The fibrils are spaced apart by a ground substance that is essentially a hydrated gel of proteoglycans Dense connective tissue Cornea is a dense type of connective tissue sandwiched between 2 layers of epithelium Corneal Endothelim Regulates composition of corneal stroma to maintain transparency Maintains a proper fluid balance 5 4 3 2 1 Corneal covering epithelium Posterior lamina (Descemet’s membrane) Supports the internal corneal endothelium Substantia Propria Corneal stroma Anterior lamina (Bowmans membrane) Supports corneal covering epithelium, helping to protect against infection of the underlying stroma Dense connective tissue This layer regulates fluid and solute transport between the aqueous humor and corneal stromal layer (Have water pumps) 5 4 3 2 1 This layer absorbs nutrients and oxygen from tears and conveys it to the rest of the cornea. It contains free nerve endings. It also prevents foreign matter from entering the eye Layers 1 and 5 prevent excess fluid build up in stromal layer, stromal swelling and subsequent loss of transparency More fluid Cornea becomes opaque Aging!!! The shape or curvature of the cornea can be changed surgically to improve certain visual abnormalities involving the ability to focus. In the common ophthalmologic procedure, laser-assisted in situ keratomileusis (LASIK) surgery, the corneal epithelium is displaced as a flap and the stroma reshaped by an excimer laser which vaporizes collagen and keratocytes in a highly controlled manner with no damage to adjacent cells or ECM. After reshaping the stroma, the epithelial flap is repositioned and a relatively rapid regenerative response reestablishes normal corneal physiology. Corneal grafts (transplants) between unrelated individuals can usually be accomplished successfully without immune rejection due in part to this tissue’s lack of both a vascular supply and lymphatic drainage Stratified non keratinized squamous epithelium Endothelim Substantia Propria Stromal layer Both sclera and cornea are composed of fibrous connective tissue Why the sclera is white while the cornea is transparent?????? The sclera is opaque due to the irregularity of the Type I collagen fibers with different thicknesses, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen fibrils Cornea Sclera Histology of retina Component: Pigmented layer: Pigmented epithelial cells Neural layer: Photoreceptors, bipolar neurons, ganglion cells, and supporting Müller cells 10 layers!!! Outer Pigmented layer 1 layer Retinal Pigment Epithelium The pigmented epithelial layer consists of cuboidal or low columnar cells and surrounds the neural layer of the retina Neural Retina : 9 distinct layers 1- The rod and cone layer: contains the outer segments of the rod and cone cells 2- The outer limiting layer 3- The outer nuclear layer: contains cell bodies of the rod and cone cells. 4- The outer plexiform layer: includes axons of the rod and cone cells and dendrites of the bipolar cells 5- The inner nuclear layer: contains the nuclei of the bipolar cells, amacrine cells, and horizontal cells, Muller cells 1 3 4 5 6 8 9 7 2 6- The inner plexiform layer: includes axons of the bipolar cells and dendrites of the ganglion cells 7-The ganglionic layer: contains cell bodies of ganglion cells 8- The nerve fiber layer: containing the ganglionic cell axons that converge at the optic disc and form the optic nerve. 9- The inner limiting layer Outer segment Photosensitive region Generation of the receptor potential Inner segment Nucleus Synaptic terminal The rod and cone cells (photoreceptors), named for the shape of their outer segments, are polarized neurons with their photosensitive portions aligned in the retina’s rod and cone layer (RCL) Retinal pigment epithelium Neural Retina : Main cells RBG Note: ganglion cells have long axons Bipolar cells Optic nerve (Axons of ganglion cells) (Central processes) Ganglion cells To optic disc Photoreceptors (rods and cones cells) Neural Retina : Cross-talking cells HA Make specific connections with other neurons Note: Between the three layers with cell nuclei are two “plexiform” regions containing only axons and dendrites connected by synapses Horizontal cell Amacrine cell Neural Retina : Glial cells Muller cell 2 Outer limiting layer Retinal pigment epithelium 1 Photoreceptor layer 4 Outer plexiform layer 3 Outer nuclear layer 6 Inner plexiform layer 5 Inner nuclear layer 9 Inner limiting layer 7 Ganglionic cell layer 8 Nerve fiber layer Müller cells ✓ Müller cells are retinal neuroglial cells ✓Their cell bodies are located in the inner nuclear layer of the retina ✓Müller cells extend processes that span across the entire neural retina ✓From these major Müller cell processes smaller lateral extensions ramify to form: The outer limiting membrane The inner limiting membrane (forms the inner surface of the retina) ✓ The major role of the Müller cells is to maintain the structural and functional stability of retinal cells. This includes regulation of the extracellular environment via uptake of neurotransmitters, removal of debris, regulation of K+ levels, storage of glycogen, electrical insulation of receptors and other neurons, mechanical support of the neural retina, and maintaining a bloodinner retina barrier. Major process Major process Inner nuclear layer These layers receive O2 and nutrients by diffusion from the choroidocapillaries of the choroid. Choroidocapillary lamina of the choroid Retinal pigmented epithelium 1 3 2 4 5 6 7 8 These layers are supplied by branches of Central Artery of Retina 9 The retinal pigmented epithelium and the photoreceptor layer of the retina, derived from the two layers of the optic cup, are not firmly joined to each other. Head trauma or other conditions can cause the two layers to separate with an intervening space. In such regions of detached retina, the photoreceptor cells no longer have access to metabolic support from the pigmented layer and choroid and will eventually die. Prompt repositioning of the retina and reattaching it with laser surgery is an effective treatment. Supplied by choroidocapillaries (by simple diffusion) Sclera Choroid Retina Outer retina Choroido-capillaries Chorio-capillaries Inner retina Supplied by branches of Central Artery of Retina Detached retina Rhodopsin (protein + vit. A (Retinol) Iodopsin Iodopsin Iodopsin ✓ Rods are responsible for black/white vision and function best in dim light (responsible for night vision) ✓ Deficiency of vitamin A (night blindness!!!) ✓ Cones are responsible for color vision and function best in relatively bright light ✓ Cone cells are densely packed in the fovea centralis Color blindness! ✓ Three types Rods Cones More concentrated in the peripheral part of retina More concentrated in the central part of retina (macula lutea) Sensitive to dim light (Night vision) Sensitive to bright light (Day vision) More abundant Less abundant Rods Temporal Nasal Right Fundus ….. ….. ….. Cones

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