Visual System 1: Anatomy

Question 1

peripapillary:

Answer 1

near or around the nerve head

Question 2

papilledema:

Answer 2

swollen nerve head

Question 3

Light passes through the transparent ________, _________, _________, and ________ before reaching the _________.

Answer 3

• cornea, anterior chamber, lens, and vitreous
• retina

Question 4

Transparent tissues on the light path are surrounded by ___________________ (e.g., choroid & ciliary body) and a tough ________ to which extraocular muscles are attached

Answer 4

• blood vessel-containing tissues
• sclera

Question 5

Structures of the Eye:

Answer 5

1. Cornea
2. Uvea
3. Anterior Chamber
4. Lens
5. Vitreous Body
6. Retina

Question 6

Layers of the Cornea:

Answer 6

1. Epithelium
2. Bowman’s membrane
3. Stroma
4. Descemet’s membrane
5. Endothelium

Question 7

Function of the Cornea:

Answer 7

1. Helps to shield the rest of the eye:
   
   • from germs, dust, and other harmful matter
   • Shares this protective task with the eyelids, the eye socket, tears, and the sclera, or white part of the eye
2. Acts as the eye’s outermost lens:
   
   • Functions like a window that controls and focuses the entry of light into the eye
   • Contributes between 65-75 percent of the eye’s total focusing power

Question 8

If the cornea contains no blood vessels, how does it get its nourishment?

Answer 8

receives its nourishment from the tears and aqueous humor that fills the chamber behind it

Question 9

Why must the cornea remain transparent?

Answer 9

• to refract light properly
• the presence of even the tiniest blood vessels can interfere with this process

Question 10

Epithelium:

Answer 10

• Outermost region
• 10 percent of the cornea’s thickness

Question 11

Epithelium functions primarily to:

Answer 11

1. Block the passage of foreign material into the eye and other layers of the cornea
   
   • such as dust, water, and bacteria
2. Provide a smooth surface that absorbs oxygen and cell nutrients from tears
   
   • Then distributes these nutrients to the rest of the cornea.

• Filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched

Question 12

Basement membrane:

Answer 12

serves as the foundation on which the epithelial cells anchor and organize themselves

Question 13

Bowman’s Membrane:

Answer 13

• directly below the basement membrane of the epithelium
• composed of collagen

Question 14

What happens if Bowman’s membrane is injured?

Answer 14

• can form a scar as it heals
• some vision loss may occur

Question 15

Stroma:

Answer 15

• Beneath Bowman’s layer
  
  • comprises about 90 percent of the cornea’s thickness
• Primarily of water (78 percent) and collagen (16 percent)
  
  • does not contain any blood vessels.
• Collagen gives the cornea its strength, elasticity, and form
• The collagen’s unique shape, arrangement, and spacing are essential in producing the cornea’s light-conducting transparency

Question 16

Descemet’s Membrane:

Answer 16

• Under the stroma
• Thin but strong sheet of tissue that serves as a protective barrier against infection and injuries
• Composed of collagen fibers (different from those of the stroma) and is made by the endothelial cells that lie below it

Question 17

What happens if Descemet’s membrane is injured?

Answer 17

Descemet’s membrane is regenerated readily after injury

Question 18

Endothelium:

Answer 18

• Extremely thin, innermost layer of the cornea.
• Endothelial cells are essential in keeping the cornea clear
  
  • Normally, fluid leaks slowly from inside the eye into the stroma
• Primary task is to pump this excess fluid out of the stroma

Question 19

What would happen if endothelial cells did not have pumping action?

Answer 19

• The stroma would swell with water, become hazy, and ultimately opaque
• In a healthy eye, a perfect balance is maintained:
  
  • fluid moving into = fluid being pumped out

Question 20

What happens if the endothelium is injured?

Answer 20

• Once endothelium cells are destroyed by disease or trauma, they are lost forever
• If too many endothelial cells are destroyed:
  
  • corneal edema and blindness ensue
  • corneal transplantation the only available therapy

Question 21

_________ – affect the ________ and are the most common of all vision problems in this country.

Answer 21

• Refractive errors
• cornea

Question 22

Myopia:

Answer 22

Nearsightedness:

• When the cornea is curved too much, or if the eye is too long
• faraway objects will appear blurry because they are focused in front of the retina
• Myopia affects over 25 percent of all adult Americans

Question 23

Hyperopia:

Answer 23

Farsightedness:

• Opposite of myopia
• Distant objects are clear, and close-up objects appear blurry
• With hyperopia, images focus on a point beyond the retina
• Hyperopia results from an eye that is too short

Question 24

Uvea (3 parts):

Answer 24

1. Choroid
2. Ciliary body
3. Iris

Question 25

Choroid:

Answer 25

* **Largest component:** **3 layers** (out to in) 1. **vessel layer:** medium sized arteries and veins, loose CT and melanocytes 2. **chorocapillary layer:** capillaries arranged in one plane, fenestrated type 3. **Bruch’s membrane:** 3 - 4 micron thick amorphous hyaline membrane that the retinal pigmented epithelia rests upon

Question 26

Ciliary body:

Answer 26

**expansion of the stroma of choroid near the lens** 1. **Contacts three regions:** vitreous body, sclera, posterior chamber/lens 2. Has projections called **ciliary processes**, * project toward **lens** 3. **Trabecular meshwork** within ciliary body near limbus * **Aqueous humor** drained from anterior chamber via the trabecular meshwork

Question 27

Iris:

Answer 27

**covers lens; ****regulates amount of light reaching the retina:** 1. **anterior aspect** made of vascular, loose CT with interspersed melanocytes * **number of melanocytes** in this layer determines **eye color** 2. **Posterior surface** lined with a double layer of pigmented epithelium **(absorbs light)** 3. **2 muscle masses** rest upon the pigmented epithelium and **regulate iris opening (pupil diameter)**

Question 28

How is iris color determined?

Answer 28

Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is in brown-eyed people and of the latter in blue- and green-eyed people

Question 29

Structure of the anterior segment of the iris:

Answer 29

* **Radially arranged myoepithelial cells** form the **dilator pupillae muscle** between the vascular and pigment layer **(sympathetic innervation)** * **Concentric smooth muscle** bundles at the pupil margin (inner aspect of iris) form the **sphincter pupillae muscle** **(parasympathetic innervation)**

Question 30

Anterior Chamber:

Answer 30

* Contains aqueous humor * Avascular * Involved in maintaining intraocular pressure

Question 31

Describe the pawthway for aqueous humor (starting from production):

Answer 31

1. aqueous humor **produced by the ciliary processes** in the posterior chamber, passes from the **posterior chamber ⇒ anterior chamber** (between iris and lens) 2. aqueous humor **drained** from anterior chamber via the trabecular meshwork 3. from here, the humor **passes into the canal of Schlemm** that drains into the venous system * there is no direct connection between the canal and trabecula, the humor **percolates through the tissue into the canal**

Question 32

Glaucoma (types):

Answer 32

1. primary open angle glaucoma (POAG) 2. angle closure glaucoma 3. normal tension glaucoma 4. secondary glaucoma

Question 33

Open-angle or chronic glaucoma:

Answer 33

* accounts for 80 to 85% of all cases * due to an **obstruction in the drainage system** of the eye * When the fluid reaches the angle, it passes too slowly through the meshwork drain * As the fluid builds up, the **pressure inside the eye rises to a level that may damage the optic nerve and may result in vision loss**

Question 34

Angle-closure glaucoma:

Answer 34

* much more rare, but very severe, acute form of the disease * can cause blindness in 24-48 hours if not treated. * It results from **poor access to the drainage system** in the eye * referred to as angle-closure because the **angle between the iris and the cornea narrows**, blocking the drainage of the aqueous humor. * This sudden increase in pressure inside the eye causes **intense pain and blurred vision**

Question 35

Normal tension glaucoma:

Answer 35

optic nerve damage has occurred despite a normal IOP

Question 36

Secondary glaucoma:

Answer 36

* **another disease** causes or contributes to increased eye pressure, resulting in **optic nerve damage and vision loss** * result of **ganglion cell death**

Question 37

Lens:

Answer 37

Very transparent structure: * avascular * little ECM

Question 38

Lens: Structural components ## Footnote

Answer 38

* **Capsule** - ECM surrounding lens * **Epithelium** - anterior surface of lens * **Lens fibers** - body of the lens (no organelles)

Question 39

Lens: Functions/Specializations

Answer 39

* Second to cornea in refractive power (~10 D) * lens is supported by system of fibers **(suspensory ligament or zonules)** attached to the ciliary body

Question 40

Accommodation

Answer 40

* **lens thinner** when focused on **distant objects, relaxed ciliary muscles** * **lens thicker** when focusing on **near objects, ciliary muscles contract**, relaxing tension on zonule fibers thus **thickening lens** * anterior directed movement of ciliary body

Question 41

Vitreous Body:

Answer 41

* Nearly **acellular** * major macromolecules: **type 2 collagen & hyaluronic acid** * Very transparent structure: **99% water & avascular** * **Nutritive** function

Question 42

What are the two regions of the retina?

Answer 42

1. Neural or sensory retina 2. Retinal pigment epithelium (RPE)

Question 43

Neural (sensory) retina: Regional specialization

Answer 43

* Fovea (Latin for small pit): * region used for high acuity vision * Anteriorly the number of neural elements in the retina declines * becomes a single layer of epithelium (unpigmented) that covers the ciliary body

Question 44

Neural (sensory) retina: Layers

Answer 44

* **Layer of photoreceptor** (rod and cone) outer and inner segments; **most posterior** (external) so light passes through all other retina layers before it reaches photoreceptors * **Outer nuclear layer (ONL):** nuclei of **rods and cones** * **Outer plexiform layer (OPL):** location of **synapses of rod and cone axons** with next layer of neurons (bipolar) * **Inner nuclear layer (INL):** nuclei of **bipolar neurons** * also nuclei of **horizontal** and **amacrine neurons** and **Müller glia** * **Inner plexiform layer (IPL):** **synapses of bipolar axons** with ganglion cells * **Ganglion cell layer (GCL):** nuclei of ganglion cells * **Nerve fiber layer (NFL):** axons of ganglion cells that **converge** to form optic nerve * **Inner limiting lamina:** **basement membrane** of Müller glial cells

Question 45

General features of photoreceptors:

Answer 45

* **Inner segments** – organelles for protein synthesis and energy production * **Outer segments** – flattened membrane discs with photosensitive visual pigments * Rods- long and slender * Cones - conical w/ membrane discs

Question 46

* Rods and cones have __________ distributions across the retina. * Peak foveal cone density varies by at least a \_\_\_\_\_\_\_\_\_. * The human foveal cone mosaic is an efficiently packed mosaic, with the locations of cone centers forming a \_\_\_\_\_\_\_\_\_\_\_\_.

Answer 46

* unequal * factor of 3 * triangular array

Question 47

Retinal Pigment Epithelium (RPE):

Answer 47

* **Simple, cuboidal, melanin‑containing** **epithelium** between neural retina and Bruch's membrane (BM) * ECM **separating** the RPE and the capillary bed of the choroid * Provides the **outer blood‑retinal barrier**

Question 48

Major functions of the RPE:

Answer 48

* **absorbs** scattered light * **transports** nutrients and ions between photoreceptors & choriocapillaris * **spatial buffering** of ions in the subretinal space * **reisomerization** of all-trans retinal * outer segment **renewal** * **secretion** of growth factors for maintenance and structural integrity of retina

Question 49

Retinal Blood Supply: The retina has a \_\_\_\_\_\_\_\_\_.

Answer 49

dual blood supply: * **Outer retina:** choriocapillaries * **Inner retina:** central retinal artery * central artery is a branch of the opthalmic artery

Question 50

Foveal Development:

Answer 50

Multiple anatomical events define foveal specialization: 1. Cone packing increases 2. Foveal pit develops after a thickening of the retina 3. Site of incipient fovea is avascular all along

Question 51

Describe the recycling pattern of the RPE.

Answer 51

* The RPE **engulfs, digests, and recycles** approximately **10% of the mass** of each photoreceptor outer segment per day. * Outer segment phagocytosis occurs on a **diurnal** schedule: 1. Peak of **rod** outer segment disc shedding occurring **at dawn** 2. Peak of **cone** outer segment disc shedding occurring **at dusk**

Question 52

What is AMD? How is it treated?

Answer 52

Age-related Macular Degeneration * **Leading cause of blindness in elderly** in developed countries * Drusen develop within **Bruch’s membrane** * **Vision loss is slow**, and deteriorates **central vision first** (contrast to glaucoma). * Metamorphopsia is common (Amsler grid testing). * **Risk factors:** Family Hx AMD, smoking, blue eyes, CFH mutation * **Treatment** – focused on halting progression to “wet” – anti-VEGF

Question 53

Albinism:

Answer 53

**Inherited disorder of melanin biosynthesis:** * associated with **absent or reduced melanin pigment** in the eye * often the skin and hair. 1:17,000

Question 54

Oculocutaneous albinism (OCA):

Answer 54

Autosomal recessive: * **OCA1A** = no pigment anywhere * **OCA1B** = some pigment * due to **“leaky”** mutation allowing residual enzyme **(tyrosinase)**

Question 55

Ocular albinism (OA):

Answer 55

X-linked: * typically normal skin and hair

Question 56

Visual symptoms are \_\_\_\_\_\_\_\_\_.

Answer 56

universal: 1. nystagmus 2. refractive errors 3. photophobia 4. reduced acuity 5. macular translucency 6. iris transillumination 7. altered retinostriate projections 8. foveal hypoplasia * arrested foveal development