Optho part uno Flashcards
Ocular anatomy: Eyelids
- The eyelid is composed of skin externally and mucosa internally (conjunctiva). Sebaceous glands lie in the subcutaneous layer (deep to skin, not mucosa), which can become obstructed to form a sty or hordeolum. Deeper is the orbicularis oculi muscle, which is innervated by CN-VII. The tarsal plate is deep to the orbicularis oculi muscle and external to the conjunctiva. It is an attachment site for the levator palpebrae superioris muscle (innervated by both CN-III and sympathetic fibers). The tarsal plate also contains tarsal glands (sometimes called Meibomian glands) that secrete a lipid-rich substance which help keep tears over the cornea. The deepest portion of the eyelid is the conjunctiva, which is discussed below
Ocular anatomy: Sclera
The sclera is a fibroelastic connective tissue layer mainly composed of collagen that surrounds the eye, and is the insertion site of extraocular muscles. Posteriorly, it is penetrated by the optic nerve through the lamina cribrosa. It is thinner and therefore more prone to rupture at the insertion of the optic nerve and the extraocular muscles. The sclera is composed of the episclera, which is an external layer formed from loose connective tissue, and the sclera proper, which contains a dense network of collagenous fiber bundles. This dense network is largely void of vasculature and fibroblasts, accounting to the sclera’s poor ability to heal itself. In a few cases, the sclera can appear blue during high intraocular pressure and after scleritis, both of which thin the sclera and cause the underlying brown uvea to appear blue due to an optical effect. Additionally, osteogenesis imperfecta and a heavily pigmented congenital nevus can cause a bluish scleral tint.
Ocular anatomy: Limbus
the limbus is the area of transition from the clear cornea to the opaque sclera. Histologically it is the transition of regular corneal stromal lamellae to irregular scleral stromal lamellae. The limbus is an important landmark surgically, diagnostically and pathologically.
Ocular anatomy: Iris
plural: irides) a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. “Eye color” is the color of the iris. In response to the amount of light entering the eye, muscles attached to the iris expand or contract the aperture at the center of the iris, known as the pupil. The larger the pupil, the more light can enter. No epithelium covers the anterior iris surface. Instead the anterior surface is composed of fibroblasts and melanocytes. It contains both sphincter and dilator muscles that control the pupil size. The dilator is controlled by the sympathetic nervous system and the sphincter is controlled by the parasympathetic (more on this later!)
Ocular anatomy: Pupil
this is the central opening in the iris. It regulates, and responds to, the amount of light entering the eye. In addition to photo-responsiveness it also responds to autonomic and pharmacologic stimuli. In the normal state the pupil in each eye should be of roughly equal diameter (within 1mm). when there is a difference in the size of each pupil it is referred to as anisocoria and can be an indicator of significant systemic disease (more on this later!)
Ocular anatomy: . Conjunctiva
Conjunctiva can be divided into three regions. The lining of the inside of the eyelid (palpebral conjunctiva) is tightly attached to the tarsal plate. When inflamed, this conjunctiva forms tiny papillary folds. The conjunctiva then reflect onto the sclera of the eyeball forming the superior and inferior conjunctival fornices. This region is composed of pseudostratified columnar epithelium and is rich in goblet cells, which secrete mucus to aid in keeping tears on the cornea. This region is also the site of lacrimal ductules (lacrimal gland innervated by CN-VII), which pierce the superior and inferior fornices to constantly secrete aqueous tears. The last component is the bulbar conjunctiva, which covers the surface of the eye. This area is composed of non-keratinizing stratified squamous epithelium and contains few goblet cells compared to the fornix.
Ocular anatomy: Cornea
The transparent cornea and its tear film are responsible for the majority of refraction. Externally is an epithelium with an underlying basement membrane, Bowman’s membrane. This acellular layer protects against the spread of malignant cells from the epithelium by separating the basement membrane from the underlying stroma. The stroma is composed of aligned collagen fibers to allow for transparency. It is void of vessels, lymphatics, and is normally dehydrated. Scarring and edema of this layer can lead to opacities on the cornea. Deep to the stroma is the Descemet membrane, which is the basement membrane of the corneal endothelial cells. The Descemet membrane thickens with age, and it is the site of the Kayser-Fleischer ring (copper deposition) of Wilson disease. It is important to note that the corneal endothelium is distinct from vascular endothelial cells, due to a different embryological origin. These endothelial cells maintain the stroma and therefore a decrease in their function can lead to stromal edema. Myopia develops when the eye is too long for the refractive power of the cornea and hyperopia develops when the eye is too short. LASIK (Laser-assisted in situ keratomileusis) can sculpt the cornea to correct the vision.
Ocular anatomy: Extraocular muscles
– a group of six delicate muscles that control the alignment and movement of the eye(s). There are 4 rectus muscles (the superior, medial, lateral and inferior rectus) and two oblique muscles (the superior and inferior oblique) for each eye. Balanced function of each of these muscles is required for proper ocular alignment as well as proper movement of the eyes (including rotation). Dysfunction of these muscles my point to defects of specific cranial nerves. They originate from the orbit and insert onto the scleral portion of the globe
Ocular anatomy: Anterior Chamber
The anterior segment is bound by the cornea anteriorly and the lens posteriorly. The iris divides the anterior segment into an anterior and posterior chamber. Aqueous humor produced by the ciliary body in the posterior chamber bathes the lens before flowing through the iris and draining into the trabecular meshwork that forms the lateral border of the anterior chamber. The ciliary body is also composed of smooth muscle that pulls on suspensory ligaments attached to the lens.
Ocular anatomy: Lens
This is a small disk shaped structure that lies immediately posterior to the iris. It is about 10mm in diameter and made up of lens epithelium that is clear in its normal state and surrounded by a lens capsule. Because the lens is completely encapsulated by this basement membrane it infoliates. Thus the old epithelium and proteins within the lens capsule accumulate over time and the lens increases in size with age as a result. This leads the a loss of elasticity of the lens and therefore a loss of accommodative ability and near vision (presbyopia). This also contributes the opacification of the lens with age (cataract).
Ocular anatomy: Ciliary Body
A circumferential structure inside the eye surrounding the lens. It has two main components: an external epithelium and an internal stroma. Structurally, it consists of the ciliary muscle and the ciliary processes. The function of the ciliary muscle is accommodation while the ciliary processes are responsible for aqueous production.
Ocular anatomy: Posterior Chamber
The posterior chamber should not be confused with vitreous chamber. The Posterior Chamber consists of small space directly posterior to the Iris but anterior to the lens. Aqueous that enters, but cannot exit, the posterior chamber is serious cause of acute angle closure glaucoma.
Ocular anatomy: Vitreous Humor
- The vitreous is an avascular transparent gel-like structure located between the retina and the posterior surface of the lens. It is composed of collagen fibrils and hyaluronic acid. Normal aging effects include liquefaction and collapse, which can cause the sensation of floaters. Additionally, the posterior portion of the vitreous, called the hyaloids, may separate from the inner limiting membrane of the retina with age. This process is a normal occurrence and called a vitreous detachment.
Ocular anatomy: Retina
The retina is composed of three sensory cell layers embryologically derived from the diencephalon: The ganglion cell layer, bipolar cell layer, and photoreceptor (rods and cones) layer. The final and outermost layer of the retina is the non-neuronal Retinal Pigment Epithelial Cell layer (RPE), which functions to absorb light that penetrates all layers of the retinal and reduce scattering of light to increase visual acuity. The first order neurons in the perception of light are the photoreceptors. Rods populate the periphery of the retina and are used in night vision. Cones are heavily concentrated in the center of the retina and detect color and daytime vision. Second order neurons are bipolar cells, which can perform lateral inhibition to increase contrast for vision. Third order neurons are the ganglion cells, which relay input to the visual center.
Ocular anatomy: Macula
The macula is near the center of the retina and is a 1.5 mm in diameter collection of densely packed cones and no blood vessels. Each macular cone synapses with a single bipolar cell and a single ganglion cell creating the greatest visual acuity, especially in the fovea, of the retina.
