Anatomy Revision PDF

Anatomy revision OPT 350  Cornea  Diameter: 11.5 mm  Thickness: 0.5–0.6 mm centrally and 0.6–0.8 mm peripherally.  Transparent front part of the eye.  Covers the iris, pupil, and anterior chamber.  Together with the lens, the corne...

Anatomy revision OPT 350  Cornea  Diameter: 11.5 mm  Thickness: 0.5–0.6 mm centrally and 0.6–0.8 mm peripherally.  Transparent front part of the eye.  Covers the iris, pupil, and anterior chamber.  Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power (40 D). Cornea  Very sensitive.  No blood vessels.  it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humor at the inside Corneal layers 1- Corneal epithelium: a thin epithelial multicellular tissue layer of fast-growing and easily-regenerated cells, kept moist with tears. 2- Bowman's layer: a tough layer that protects the corneal stroma. 3- Corneal stroma: a thick, transparent middle layer, consisting of regularly- arranged collagen fibers along with sparsely distributed interconnected keratocytes, which are the cells for general repair and maintenance. 4- Descemet's membrane: a thin acellular layer that serves as the modified basement membrane of the corneal endothelium, from which the cells are derived. 5-. Corneal endothelium: a simple squamous or low cuboidal monolayer of mitochondria-rich cells responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments.. The corneal endothelium is bathed by aqueous humor. Unlike the corneal epithelium, the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells. If the endothelium can no longer maintain a proper fluid balance, stromal swelling due to excess fluids and subsequent loss of transparency will occur. Why is the cornea transparent? Cornea There are 2 theories of how transparency in the cornea comes about: 1. The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils. 2. The spacing of the neighboring collagen fibrils in the stroma must be < 200 nm for there to be transparency Conjunctiva  Clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids.  It helps lubricate the eye by producing mucus and tears.  It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye. Divisions of the conjunctiva Part Part Sclera  white part of the eye.  It is opaque due to the irregularity of the collagen fibers.  Vascular.  In children, it is thinner and shows some of the underlying pigment, appearing slightly blue.  In the elderly, however, fatty deposits on the sclera can make it appear slightly yellow. Iris  The colored part of the eye, the other structures visible are the pupil in the center and the white sclera surrounding the iris. The overlying cornea is pictured, but not visible, as it is transparent.  controlling the diameter and size of the pupil and the amount of light reaching the retina  The iris consists of two layers: the front pigmented fibrovascular tissue known as a stroma and, beneath the stroma, pigmented epithelial cells. Iris  It has 2 muscles:  The sphincter which is responsible for pupillary constriction  The dilator which is responsible for pupillary dilation  It is usually strongly pigmented, with colors ranging from brown to green, blue, grey, and hazel. Occasionally its light color is due to lack of pigmentation (melanin).  Heterochromia (also known as a heterochromia iridis or heterochromia iridium) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant. Pupil  It is an opening NOT A STRUCTURE, at the center of the iris.  The dilator pupillae, innervated by sympathetic nerves.  The pupil is constricted by the parasympathetic nerves.  Atropine, cocaine, and amphetamines drug (cycloplegia) mimics the action of the sympathetic nerves.  Alcohol and opioids mimics the action of the parasympathetic nerves  Gets wider in the dark but narrower in light Aqueous Humor  The aqueous humor is a thick watery substance filling the space between the lens and the cornea.  Secreted into the posterior chamber by the ciliary body  The anterior segment is the front third of the eye that includes the structures in front of the vitreous humor: the cornea, iris, ciliary body, and lens. Within the anterior segment are two fluid-filled spaces divided by the iris plane: 1) the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris. 2) the posterior chamber between the iris and the front face of the vitreous. Function of AH  Its main function is to provide diopteric power to the cornea.  Maintains the intraocular pressure and inflates the globe of the eye.  Provides nutrition (e.g. amino acids and glucose) for the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous.  Carries away waste products from metabolism of the above avascular ocular tissues.  May serve to transport ascorbate in the anterior segment to act as an anti- oxidant agent.  Presence of immunoglobulins indicate a role in immune response to defend against pathogens.  Maintains proper brain energy consumption. Crystalline lens and Zonules  It is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina.  Its power is approximately 20 D.  The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina (accommodation).  The lens is suspended in place by the zonular fibers, which attach to the lens near its equatorial line and connect the lens to the ciliary body Crystalline lens and Zonules  The lens is comprised of three main parts: the lens capsule, the lens epithelium, and the lens fibers. The lens capsule forms the outermost layer of the lens and the lens fibers form the bulk of the interior of the lens. Ciliary body  The ciliary body is the circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes.  It is triangular in horizontal section, and is coated by a double layer, the ciliary epithelium.  It extends from the ora serrata to the root of the iris.  They are attached to the lens by connective tissue called the zonule of Zinn, and are responsible for shaping the lens to focus light on the retina.  When the ciliary muscle contracts, the lens becomes more convex, generally improving the focus for closer objects.  When it relaxes, it flattens the lens, generally improving the focus for farther objects. Function of the CB  Accommodation  Aqueous humor production  Production and maintenance of the lens zonules.  Irido corneal angle  The width of the iridocorneal angle is one factor affecting the drainage of aqueous humour from the eye's anterior chamber:  A wide angle allows sufficient drainage of humor through the trabecular meshwork.  A narrow angle may impede the drainage system and leave the patient susceptible to acute angle-closure glaucoma.  Vitreous  The vitreous is the transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.  It is produced by certain retinal cells.  It has no blood vessels.  98-99% of its volume is water.  Helps to keep the retina in place.  it adheres to the retina in three places only: around the anterior border of the retina; in the macula; and at the optic nerve disc Retina  The retina is a complex, layered structure with several layers of neurons interconnected by synapses.  The only neurons that are directly sensitive to light are the photoreceptor cells.  These are mainly of two types: the rods and cones.  Rods function mainly in dim light and provide black-and-white vision.  Cones support daytime vision and the perception of colour Layers of the Retina 1. Inner limiting membrane - Müller cell footplates 2. Nerve fiber layer 3. Ganglion cell layer - Layer that contains nuclei of ganglion cells and gives rise to optic nerve fibers. 4. Inner plexiform layer 5. Inner nuclear layer contains bipolar cells 6. Outer plexiform layer - In the macular region, this is known as the Fiber layer of Henle 7. Outer nuclear layer 8. External limiting membrane - Layer that separates the inner segment portions of the photoreceptors from their cell nuclei. 9. Photoreceptor layer - Rods / Cones 10. Retinal pigment epithelium Retina  Temporal to this disc is the macula.  At its center is the fovea, a pit that is most sensitive to light and is responsible for our sharp central vision.  The edge of the retina is defined by the ora serrata.  The optic nerve carries the ganglion cell axons to the brain and the blood vessels that open into the retina.  The central retina is cone-dominated and the peripheral retina is rod-dominated Visual System The visual system consists of:  The eye, especially the retina  The optic nerve  The optic chiasma  The optic tract  The lateral geniculate body  The optic radiation  Visual cortex  Visual association cortex Visual System  The final result of all this processing is five different populations of ganglion cells that send visual (image-forming and non-image-forming) information to the brain: 1. M cells, with large center-surround receptive fields that are sensitive to depth, indifferent to color, and rapidly adapt to a stimulus; 2. P cells, with smaller center-surround receptive fields that are sensitive to color and shape; 3. K cells, with very large center-only receptive fields that are sensitive to color and indifferent to shape or depth; 4. Another population that is intrinsically photosensitive; and 5. A final population that is used for eye movements. Extraocular Muscles  The extraocular muscles are the six muscles that control the movements of the (human) eye.  The actions of the extraocular muscles depend on the position of the eye at the time of muscle contraction. Extraocular Muscles  A good mnemonic to remember which muscles are innervated by what nerve is to paraphrase it as a molecular equation: LR6SO4R3. or (LR6SO4)3 i.e. "LR 6 SO 4 Whole 3."  Lateral Rectus - Cranial Nerve VI  Superior Oblique - Cranial Nerve IV  The Rest of the muscles - Cranial Nerve III. Development of visual ocular components 1-The axial length: 12 mm at 26 weeks of gestational age. 15 mm at 34 weeks. 16 mm at 36 weeks. 16,5-17,3 mm in the full term neonate. 23 mm at adulthood. Development of visual ocular components 2- The corneal diameter: 8,2 mm at 34 weeks. 9 mm at 37 weeks. 9,8 mm at full term birth (range of 9-11). 12 mm at 2 years (adult size). A large diameter cornea may indicate glaucoma. A small diameter cornea may indicate microcornea or microphthalmos. Development of visual ocular components 3- The corneal power: 60 D at 28 weeks of gestational age. 54 D at 30-35 weeks. 48,4 D- 51 D at birth. 44 D at 1 year of life. In the late teens, there is another period of flattening. Development of visual ocular components 4- The anterior chamber depth: At birth, it averages 2,6 mm. It deepens by about 1,4 mm from birth to adolescence. In the first 18 months, most of this change happens.  Development of visual ocular components 5- The lens:  43,3 D in preterm neonates.  34,4 D in full term neonates (almost spherical).  During the first years of life, there is an increase in the transverse diameter and flattening of the lens surfaces.  Thus the lens becomes less spherical and its dioptric power decreases.  This together with the flattening cornea, compensates the increasing axial length.  Changes in the lens continue up to the middle teenage years.  The emmetropization occurs as coordinated growth between lens power and axial length.  If emmetropization did not occur , there would be large fluctuations in the refractive errors. Thank you