7 - Visual System: 1 Flashcards
What are some other names for the optic disk? What is papilladema? What is Peripapillary?
Optic nerve head
Disk
Papilla
Papilladema: Swollen nerve head.
Peripapillary: near or around the nerve head
What layers make up the cornea from outside to inside? Describe each layer?
Epithelium: regenerative; stratified squamous cells
Bowman’s membrane: acellular layer of unorganized collagen fibers; barrier to infection
Stroma: type 1 collagen bundles that bind water and maintain corneal clarity
Descement’s membrane: basement membrane for corneal endothelial cells increase thickness with age
Endothelim: simple squamous; pumps water out of the stroma
What is the function of the cornea?
MAJOR refractive structure that protects the eye from environment.
What is myopia? What percentage of Americans have this? Describe the lens shape.
Nearsightedness
Can be due to a cornea that’s too strong or an eye that’s too long.
25% of Americans have this
What is hyperopia? What can cause this?
Farsightedness.
Can be due to a weak cornea or an eye that’s too short.
What are the three parts of the uvea?
- Choroid
- Ciliary body
- Iris
What is the choroid of the uvea? What are it’s layers from OUT to IN?
The largest component, which has three layers from out to in:
1. Vessel layer with medium sized arteries and veins, loose CT and melanocytes
- Choricocapillary layer: capillaries in one plane, fenestrated
- Bruch’s membranes: 3-4 micron thick amorphous hyaline membrane that the retinal pigmented epithelia rest on.
What is the ciliary body? What three regions does it come in contact with?
Expansion of the stroma of choroid near the lens. Has trabecular meshwork for draining aqueous humor from anterior chamber.
Contacts: vitreous body, sclera, and the posterior chamber/lens.
Has ciliary processes that project towards lens to increase the SA for fluid secretion.
What is the function of the iris?
Covers the lens and regulates light reaching the stroma.
What is the structure of the anterior and posterior aspects of the iris?
Anterior aspect made of vascular loose CT with melanocytes: number and type determines eye color.
Posterior surface lines with doubel layer of pigmented epithelium to absorb life.
What two muscle masses rest on the pigmented epithelium and regulate iris opening (pupil diameter)?
Radially arranged myoepithelial cells from the dialator pupillae between the vascular and pigment layers (contraction = dilation)
Concentric smoother muscle bundles at the pupil margin (inner aspect of iris) form the sphincter pupillae (contraction = constriction)
What is the structure and function of the anterior chamber?
Involved in maintaining intraocular pressure.
Avascular, contains aqueous humor; 99% water; produced by non-pigmented epithelium of the ciliary process.
What is the direction of aqueous humor flow?
From ciliary processes to posterior chamber to anterior chamber to trabecular meshwork to Schlemm’s canal to veins of the sclera.
What is the prevalence of Glaucoma? What is a risk factor for it?
Leading cause of blindness and vision impairment.
Risk factor: increased intraocular pressure caused by angle closure and inability of aqueous humor to drain.
What are clinical signs of glaucoma?
Elevated pressure (tonometry): normal is 12-22 mmHg
Increased cupping of optic nerve head. Cup to disc ratio is 0-1 and higher is WORSE.
Visual field defect (perimetry) will reveal a selective peripheral loss of sensitivity
What is the lamina cribrosa? What can alter this?
Network of collagen fibers through which the fibers of the optic nerve exit the eye.
May be altered by glaucoma.
What is the structure of the lens?
Transparents structure (avascular with little ECM and no organelles) that has a capsule of ECM surrounding the lens, epithelium at the anterior surface, and lens fibers for the body.
What is the function of the lens? How does it do this?
Second to cornea in refractive power (~10 D).
Zonule fibers attached to ciliary muscles help control the lens thickness.
What type of vision occurs when the lens is thinner? Thicker?
Thinner: when focused on distant objects - relaxed ciliary muscles
Thicker: when focusing on near objects, ciliary muscles contract and relax tension on the zonule fibers to thicken the lens.
Where/when can cataracts occur? What are they?
An opacification of the lens that can occur as the lens ages.
By 80 more than half of all americans have either a cataract or have had cataract surgery.
What are the ways to describe a cataract?
Nuclear: located in the center of the lens
Cortical: affect the layer of the lens surrounding the nucleus
Posterior capsular: found in the back outer layers of the lens and develop more rapidly (usually)
What occurs with cataract surgery?
Removal of the lens (phagoemulsification or extracapsular surgery) and implantation of an IOL.
What is the structure and function of the vitreous body?
Nearly acellular, avascular structure with type 2 collagen and hyaluronic acid.
Transparent structure of ~99% water for nutritive function .
What are the two main layers of the retina that develop from the neural ectoderm?
- Neural (sensory) retina
2. Retinal pigment epithelium
What are the regions of specialization in the neural retina?
Fovea: “small pit” region used for high acuity vision
Anterior the number of neural elements in the retina declines and becomes a single later of epithelium (unpigmented) that covers the ciliary body.
What are the inner and outer segments of photoreceptors (rods and cones)? What is the connecting cilium?
Inner segments: organelles for protein synthesis and energy production
Outer segments: flattened membrane discs with photosensitive visual pigments (opsin)
Connecting cilium: transport of proteins to OS: 10 bil opsin molecules per second.
What are the two types of photoreceptors? Describe their shape and function.
Rods: long, slender outer segments. Numerous at the fovea and very light sensitive.
Cones: conical outer segments with membrane discs. Less sensitive than rods. Responsible for high acuity and color vision.
What anatomical events define fovea specialization?
Cone packing increases
Foveal pit develops after a thickening of the retina
Site of incipient fovea is avascular all along.
What is the retinal pigment epithelium (RPE) structure?
Simple cuboidal melanin containing epithelium between the neural retina and Bruch’s membrane, the ECM separating the REP and the capillary bed of the coroid.
Provides outer blood-retinal barrier.
~40 photoreceptors/RPE cell
What are the major functions of the retinal pigment epithelium (RPE)?
Transports nutrients and ions between photoreceptors and choricapillaries
Outer segment renewal
Helps absorb light for the retina and has the enzyme that turns all trans retinal back to 11-cis retina
When does the retinal pigment epithelium (RPE) recycle outer segments? How much does it recycle?
It engulfs, digests, and recycles about 10% mass of each photoreceptor outer segment/day.
This occurs on a daily schedule: peak rod OS disc shedding at dawn and peak cone OS disc shedding at dusk.
What is age-related macular degeneration (AMD)? What type of visual defect is common with this?
Drusen develop within Bruch’s membrane causing slow vision loss that deteriorate central vision first.
Due to RPD sloughing off OS and making toxic byproducts.
Metamorphosia is common - amsler grid testing.
What are the risk factors and treatment of Age-related macular degeneration?
Risk: Family Hx, smoking, blue eyes, CFH mutation.
Treatment: focused halting progression from dry to “wet” - anti-VEGF
What are the two blood supplies of the inner retina?
Inner retina supplied by the superior and inferior temporal arteries (branches of the central retinal artery).
Describe the anatomical foveal specialization?
Avascular zone
Excavation of inner retinal neurons (foveal pit)
High cone density, absence of robs
Impaired foveal specialization is a hallmark of diseases like _____ and _____.
Aniridia and albinism.
What is albinism?
Inherited disorder of melanin biosynthesis associated with absent or reduced melaninpigment in eye, skin, and hair.
1:17,000
What are the two types of oculocutaneous albinism (OCA)? How is it inherited?
Autosomal recessive.
OCA1A: no pigment anywhere
OCA1B: some pigment due to leaky mutation allowing residual enzyme tyrosinase
How is ocular albinism (OA) inherited?
X-linked.
Typically normal skin and hair.
What are the universal visual symptoms of albinism?
Nystagmus (rapid side to side movement) , refractive errors, photophobia, reduced acuity, macular translucency, iris transillumination, altered retinostriate projections, and foveal hypoplasia (arrested foveal development).
What are clinical signs of albinism?
Iris transillumination
Macular translucency
Altered ipsi/contra impairs stereo vision
Foveal hypoplasia
What is the sclera and what does the sclera become anteriorly?
Outermost covering over the eye.
From the edges of the iris the sclera becomes the cornea.
The optic nerve is _____ in relation to the fovea which is more ______.
The optic nerve is more nasal, compared to the fovea which is more temporal.
What is the macula?
A region surrounding the fovea.
Why don’t higher-order aberrations normally impact day to day vision? When can eye aberration effects be noticed?
Because we can change our pupil size
In dark conditions you will notice the effects more, or after lasiks.
When looking at a VM slide, how would you tell which layer is more external on the retina?
The pigmented layers are on the outside of the retina.
When looking at a VM slide, how would you know if you’re looking at cells in the periphery (far from fovea) or central (near the fovea)?
Peripheral regions will have fewer ganglion cells.
Central regions will have more ganglion cells.
Where is there 0 robs and the highest density of cones. Where are there no photoreceptors?
At the fovea.
No photoreceptors are the optic disk.
What happens to fovea cells as a newborn gets older?
They have increased cone packing.
Describe the dual blood supply to the retina?
Inner retina fed by the renal artery system from the ophthalmic artery (30%).
Everything else (80%) fed by the ophthalmic artery through the ciliary artery system (anterior and posterior ciliary artery).
