Ocular Anatomy 10 Retina

Question 1

What is the thickness of the retina? Where is it thinnes?

Answer 1

0.56mm near the optic disc
0.1mm at the ora serrata

Thinnest at the fovea

Question 2

What are the contacts of the retina

Answer 2

Outer surface in contact with Bruch’s membrane of the choroid
Inner surface in contact with the vitreous body

Antioerly retina becomes continuous with the pigmented and non-pigmented columnar cell layers o fthe ciliary body

Question 3

What is the ora serrata?

Answer 3

Anterior edge, termination of the retina - insertion of the medial rectus muscle medially and lateral rectus laterally

Question 4

What are the layers of the retina?

Answer 4

From inside to out:
Inner limiting membrane (ILM) form by amalgamation of inner foot process of Muller cells

Nerve fibre layer

Ganglion cell layer

Inner plexiform layer

Inner nuclear layer

Outer plexiform layer

Outer nuclear layer

External limiting membrane (ELM) formed by amalgamation of the outer foot processes of Muller cells

Photoreceptor layer

Retinal pigment epithelium (RPE) layer

ILM NF G IP IN OP ON ELM P RPE

IN GO NIP IN HOP ON EPR

Question 5

What forms the ILM and ELM

Answer 5

ILM inner foot processes of Muller cells

ELM outer foot processes of Muller cells

Muller cells are the main neuroglial cell of the retina - derived from neuroectoderm
Arranged radially

Question 6

How does the retina heal?

Answer 6

Muller cells nad astrocytes form fibroglial scars when there is an insult to the retina
Gliosis

Question 7

What is the ganglion cell layer?

Answer 7

Bodies of ganglion cells
These are last retinal integrator of information before leaving via the nerve fibres in the NF layer

Question 8

What are types of ganglion cells

Answer 8

Parasol ganglion cells (M-cells)
Magnocellular layer - project to the lateral geniculate nucleus integrate to from a large receptive field - several photoreceptors

Midget ganglion cells (P-cells)
Parvocellular layer
Prject into lateral geniculate nucleus and make connections with one amacrine and one midget bipolar cells.
Information sent thought this system from a single cone - more detailed info.
More common near the fovea.

Nerve fibres ensheathed by glial cells and myelinated only after exiting the globe.

Question 9

What is the characteristic features of the plexiform layers

Answer 9

Absence or distinct reduction of cell bodies.

Nerve axons, cell processes make up most

Question 10

Inner plexiform layer consists of

Answer 10

Processes of bipolar, amacrinse and ganglion cells
Communicates information between inner nuclear layer and ganglion cell layer

Outer plexiform layer contains photoreceptor and bipolar axons - communication between outer and inner nuclear layers

Question 11

What does the inner nuclear layer contain?

Answer 11

Cell bodies of bipolar, amacine, horizontal and Muller cells

Question 12

What are bipolar cell funcitons?

Answer 12

Connect in radial fashion - from photoreceptor to synapse with ganglion cells

Rod bipolar cells connect several rod cells to one to four ganglion cells

Flat bipolar cells - connect many cone cells to many ganglion cells

Midget bipolar cells - connect a single cone cells with a single midget ganglion cell

Question 13

What are amacrine cells?

Answer 13

Connect amacrine to ganglion and ganglion to ganglion cells

Modulate photoreceptor signals by their neurotransmitter contant

Question 14

What are horizontal cells?

Answer 14

Multipolar cells with one long and several short processes which run both horizontally and parallel with retinal surface

Release GABA in response to stimulation by photorecetors. This inhibits the activity of bipolar cells, increasing contrast and spatial resolution

Question 15

What are Muller cells

Answer 15

Long narrow, pale staining cells that have long processes that run almost entire thickness of neural retina.
Form integral part of ILM and ELM
Make extensive contacts with blood vessels in the retina, forming part of the blood ocular barrier

Question 16

What does the outer nuclear layer contain?

Answer 16

Contains the nuclei of photoreceptors - separated by their photoreceptor bodies by the ELM

Question 17

What are the two types of photoreceptors for?

Answer 17

Rods for vision in dim light and give black and white images

Cones for bright light and resolve fine details and colour vision

Question 18

How many rods and cones are there in the retina?

Answer 18

120 milllion rods
No rods at fovea
Maximum density at juxtafoveal zone - 160,000 cells/mm2

6-7 million cones, concentrated at the fovea
160,000 cells/mm2

Question 19

What is the structure of rod cells?

Answer 19

Slender, long (100-120um)
Outer segment is photoreceptor and contains photosensitive pigment rhodopsin
Contains 600-1000 membrane bound discs stacked like coins

Connecting stalk joins outer segment to cell body.

Ellipsoid area next to connecting stalk contains basal body with mitochondria
Myoid segment toward vitreous contains ER, ribosomes, Golgi

Question 20

What is the structure of cone cells?

Answer 20

Long, selnder (65-75um)

Outer segment is conical and wider

Question 21

What is the size of the macula? where is it located?

What is the size of the fovea?

Answer 21

4.5mm (three disc diameters
3mm lateral to optic disc
Area in which ganglion cell layer in more than one cell thick

Fovea measures 1.5mm
Most dense area of cone cells
Absent ganglion cell layer and a receptor layer composed entirely of cones

Question 22

Significance of foveal avascular zone?

Answer 22

500um across

Area must be avoided by laser treatment

Question 23

What is the retinal pigment epithelium

Answer 23

Hexagonal cells that extend from margin of the optic nerve to the ora serata anteriorly

Basal end of each cell is infolded and rests on Bruch’s membrane of the choroid

Apical ends possess microvilli from 5 to 7 um long

Basal regions joined by zonula occludens surround the cell and join them tightly, isolating the retina from teh systemic ciruclation.

Apical microvilli erode tips of photorecptors and phagocytose the debris.
Cyclical over 24h
Form lipofuscin granules

Question 24

What is the blood supply to the retina?

Answer 24

CRA - first branch of ophthalmic artery

Question 25

Where does the retina receive nutrients?

Answer 25

Outer plexiform layer and peripheral from choroidal circulation by diffusion

Inner 2/3 from the CRA and its tributaries

Question 26

Where does the CRA run?

Answer 26

Runs forward adherent to dural sheath of optic nerve, enters inferior and medial to optic nerve pierces dural sheath 12mm behind globe.
TRavels through subarachnoid space and gives off small miningeal branches that supply pial sheath of optic nerve.

End artery

Runs In the nerve fibre layer with four branches supplying each quadrant of the retina.

Question 27

Why is macula function presenved in some CRAO

Answer 27

Small anastomoses occur between branches of the posterior ciliary arteries and the CRA

Occasionally a larger connection the cilioretinal artery (from posteroir ciliary artery) exists in approx 20% of people

Question 28

What maintains the blood reitnal barrier

Answer 28

Zonula occludentes between non-fenestrated retinal blood vessels and between the pigment epithelial cells of the reitina

Question 29

What is the path of the CRV? Where does it drain?

Answer 29

Tributaries that accompany arteries
Diameter is a third to quarter greater than arteries

Veins leave eye through lamina cribrosa with CRA

Drain into cavernous sinus or superior ophthalmic vein