IOA2 Exam 2 - Retinal Physiology Pt 1

To enable vision, photoreceptors in the retina make

synaptic contacts with their partners called "ON bipolar neurons" and transmit info to the brain

The retina is an extension of the ________

central nervous system (CNS)

Where does the retina originate from?

Developing brain (diencephalon)

Where do the retina's axons originate from?

Optic nerve, which is a CNS structure that projects info to the brain

Lateral Retinal Pathway

refers to the horizontal connections within the retina, facilitated by cells like horizontal and amacrine cells, which allow signals to spread laterally

Cells involved in the lateral pathway

Amacrine

Horizontal

Vertical Retinal Pathway

describes the flow of visual information from photoreceptors (rods and cones) to bipolar cells, and then to retinal ganglion cells (RGCs), ultimately transmitting signals to the brain.

Cells involved in the Vertical Pathway

Photoreceptors

Bipolar cells

Retinal ganglion cells

Light sensitive cells in the retina

Photoreceptors

2 types of photoreceptors

Rods and cones

Responsible for low-light-level sensing, enabling scotopic vision (night vision)

Rods

Responsible for high-light-level sensing, enabling photopic vision (day vision)

Cones

Compare the number of photopigments in the photoreceptors

Rod cells have more photopigments (there are 100x more rods than cones)

Compare the amplification shown in rods vs. cones

-Rods show more

-Cones show less

Which type of photoreceptor shows directional selectiveness?

Cones

Which type of photoreceptor have a high sensitivity?

Rods (due to low light)

Which type of photoreceptor has a low sensitivity?

Cones

Rods have a (high/less) convergent retinal pathway?

High

Cones have a (high/less) convergent retinal pathway?

Less

Which type of photoreceptor has a slow response?

Rods

Which type of photoreceptor shows a fast response?

Cones

Photoreceptor that shows a slow acuity

Rods

Photoreceptor that shows a high acuity, especially in the macula

Cones

How many pigment types do rods have?

One

How many pigment types do cones have?

Three (SML)

The visual pigment in the rod cells is

Rhodopsin

The visual pigment in the cone cells is

Lodopsin

What type of tissue does the retinal pigment epithelium (RPE) have?

Simple cuboidal epithelium

What type of pigment is found in the RPE and what is its function?

Melanin- absorbs excess light to reduce scatter

How does the retinal pigment epithelium (RPE) contribute to the blood-retinal barrier?

The RPE forms the outer blood-retinal barrier through tight junctions composed of occludin and claudin

The tight junctions in the RPE, which include occludin and claudin, are part of the _________.

zonula occludens

Which structure in the eye has the highest melanin density?

Macula (increases visual acuity)

The RPE provides nutrition for the _______.

neuroretina

The RPE is also responsible for:

phagocytosis of waste or degraded materials

What is the fundamental role of the RPE?

Maintain the health of the neural retina and choriocapillaris

What structures connect RPE cells, forming the blood-retinal barrier?

Zonula occludens

What is the function of the blood-retinal barrier formed by the RPE?

Efficient isolation of the inner retina from systemic influences at the choroidal side

The blood retinal barrier is of importance for (2)?

-Immune privilege of the eye

-Selective transport between the blood and subretinal space

How does the RPE control nutrient and waste exchange?

The RPE selectively regulates the movement of nutrients and metabolites from the choriocapillaris into the retina while efficiently removing waste products

Normal pH of the retina =

7.28

Water is eliminated from the subretinal space via

active transport by the RPE Aquaporins

In the RPE cells, ion movement occurs by (4):

1) Na+/K+ ATPase pumps

2) Na+/K+/Cl- and Na+/2HCO3- cotransporters

3) Cl-/2HCO3- exchangers

4) Gated and undated ion channels

Moves a significant amount of lactate (a product of anaerobic metabolism) across the RPE

Proton-lactate water cotransporter

Lactate is a product of ________.

anaerobic metabolism

In the RPE cells, water passage occurs through

aquaporins

In the RPE cells, glucose passage occurs through

glucose transporters (GLUT1)

Where are glucose transporters located?

in both apical and basal membrane

Function of the glucose transporters

Maintain steady supply of glucose to the active photoreceptors

2 families of glucose transporters

1) Faciliatated diffusion glucose transporter family (GLUT family) - primarily GLUT1

2) Na+ dependent glucose transporter one (SGLT family)

What does the RPE do with photoreceptor outer segment discs?

The RPE phagocytoses fragments from the continuous shedding of photoreceptor outer segment discs

How many discs does the RPE phagocyotose daily?

2000 discs daily

Undigested material in the RPE cells accumulates as

lipofuscin

Lipofuscin

yellow-brown pigment granules composed of lipid-containing residues of lysosomal digestion

A2E stands for

N-retinylidene-N-retinylethanolamine

In which substance has A2E been identified?

lipofuscin

How does A2E substance affect RPE cells?

It impairs degradation in RPE cells, causing accumulation and inducing cell death

What condition is associated with the accumulation of A2E in the RPE cells?

ARMD

RPE metabolizes and stores ________.

vitamin A

Why is it important for the RPE to metabolize and store vitamin A?

Converting retinol --> retinyl esters for storage, and then back to 11-cis-retinal for use in photoreceptors

Vitamin A is also called

all-trans-retinol

Why is vitamin A important in the rod disc renewal system?

Vitamin A, in the form of 11-cis-retinal, is essential in the rod disc renewal system as it helps regenerate rhodopsin for proper rod function, particularly in night vision.

What is the precursor of rhodopsin pigment and how does it help vision?

11-cis-retinal; helps in night vision

LRAT =

Lecithin Retinol Acyltransferase

What does LRAT do in the retina?

helps store vitamin A in the form of retinyl esters in the RPE.

RPE65 =

Retinoid isomerohydrolase

What does RPE65 do in the retina?

helps convert vitamin A (all-trans-retinol) into 11-cis-retinal

RPE cells contribute to the formation of the

IPM (interphotoreceptor matrix) between the RPE and photoreceptors

What is IPM formed by in the retina?

Hyaluronan (HA), a type of glycosaminoglycan

Hyaluronan is binded to (2)

-Apical RPE by RHAMM

-Müller cells by CD44

A derivative of vitamin A that is administered as a multi-vitamin tablet, to preserve the retina layers and slow ARMD progression

Areds-2

RPE produces ________ that drive certain cellular processes.

growth factors

Which growth factors are produced by the RPE?

-Angiogenic Factor (VEGF)

-Anti-angiogenic Factor (PEDF)

Angiogenic Factor (VEGF) maintains the function of the ________.

choriocapillaris

Oveproduction of VEGF results in

neovascularization (leading to vision loss due to leaking blood vessels and fluid buildup in the retina)

Name of the anti-angiogenic factor

Pigmented epithelium-derived factor (PEDF)

PEDF acts as a:

Homeostatic factor- by inhibiting abnormal blood vessel growth (neovascularization)

PEDF is aka

serpin F1

PEDF is a multifunctional protein with the following functions (3):

1) anti-angiogenic

2) anti-tumorigenic

3) neutrotrophic

How much VEGF and PEDF should be present in the retina?

PEDF secretion should be higher than VEGF

What is an imbalance between the growth factors, particularly a decrease in PEDF, implicated to?

Development of conditions such as ARMD and choroidal neovascularization (CNV)

Structures within the RPE cells that absorb light

Pigment granules

Role of the pigment granules

Reduce excess light scatter

What is the relationship between the RPE and photoreceptors?

Is reciprocal—both layers depend on each other, and dysfunction in one affects the other

How do retinal degenerative diseases and dystrophies affect the RPE?

Cause changes in the RPE that are clinically visible