Lecture 11: Retina

What are the first three cells of the visual pathway?

1. Photoreceptor

2. Bipolar cell

3. Ganglion cell

What is the retina?

The innermost layer of the eye that is located between the choroid and vitreous.

What is the function of the retina?

It converts light energy into neural signal (e.g. phototransduction).

What is the RPE?

The outermost layer of the retina.

What type of junctions are found in the RPE?

• Zonula occludens / adherens

• Desmosomes

• Gap junctions

How many layers and what type of cell shape make the RPE?

• Single layer

• Columnar toward macula → posterior pole

• Cuboidal toward ora serrata

What makes the RPE pigmented?

• Melanocytes

• Lipofuscin

What are melanosomes?

Organelles that produce, store, and transport melanin.

Where are melanocytes most densest?

Macula and at the equator (also their density varies)

What is lipofuscin?

• Yellow/brown lipid containing pigment granules

• Product of phagocytosis

• Increase with age

Where does the basal and apical side of the RPE faces/adheres?

• Basal side: strongly adheres choroid

• Apical side: adjacent to photoreceptors

What type of junction does the apical side of the RPE contain? What is it important for?

Zonula occludens for the blood-retinal barrier.

Apical side of the RPE have ______ that envelop photoreceptors.

Microvilli

What is the subretinal space?

Potential space filled with interphotoreceptor matrix (IPM), where there are no intercellular junctions between RPE and photoreceptors.

What is a retinal detachment?

The separation of neurosensory retina from the RPE.

What are the general functions of the RPE?

• Blood-retinal barrier

• Phagocytosis of photoreceptor discs

• Absorption of light

• Interphotoreceptor matrix (IPM) formation

• Vitamin A metabolism / storage

• Growth factor production

What are the photopigments that are found in the photoreceptors?

• Rods: rhodopsin

• Cones: erythrolabe (red), chlorolabe (green), cyanolabe (blue)

Where are rods most dense?

Peripheral retina

Where are cones most dense?

Central retina

What are rods used for?

• Dim (scotopic) lighting

• Motion and forms

What are cones used for?

• Bright (photopic) lighting

• Fine detail and color

Are there more rods or cones in the photoreceptor layer?

More rods. (Rods = 110-125 million, Cones = 6-7 million)

Where does the outer segment of photoreceptors face?

• Tip → RPE cells

• Base → inner segment

What surrounds the outer segment of the photoreceptors?

Interphotoreceptor matrix (IPM)

What are the outer segments composed of?

Stacks of membranous discs, which house visual pigments.

What is the cilium of photoreceptors?

• Connect outer and inner segment

• Extends from innermost disc

What is the function of the inner segment of the photoreceptors?

Produces photopigments

What are the two zones of the inner segment of photoreceptors?

• Ellipsoid (outer) zone → mitochondria

• Myoid (inner) zone → Golgi apparatus, endoplasmic reticulum

What is the outer fiber of photoreceptors?

Part that connects inner segment to cell body

What part of the photoreceptor that contains the nucleus?

Cell body

What does the inner fiber of photoreceptors contain?

• Axon connecting cell body to other cells

• Ends in synaptic terminals with synaptic vesicles

What is the telodendria?

Connection between cones to other rods or cones (horizontal connections)

What is a triad?

A specialized junction formed by bipolar dendrite and two horizontal cell processes on the cone pedicle.

Where is the rod-free zone?

Central retina

Where are cones most dense?

Centrally in retina

What is the physiological blind spot at the optic disc?

Where there are no photoreceptors

What is also called the 2nd order neuron?

Bipolar cells

What are bipolar cells?

Cell bodies that contain large nucleus with little cytoplasm.

Where do bipolar cells receive input and output?

Input: Photoreceptors and horizontal cells

Output: Ganglion cells and amacrine cells

What is the only bipolar cells associated with rods?

Rod bipolar cell

What is the function of the flat midget bipolar cell?

Connects to flat portion of cone pedicle

What is the function of the invaginating midget bipolar cell?

Connects to invaginations in cone pedicle

What is the function of the diffuse cone bipolar cell?

It contacts neighboring cones

What is the function of the blue cone bipolar cell?

It contacts widely spaced cones rather than neighboring cones

What is the giant cone bipolar cell?

A very large dendritic distribution that have many connections to cones

What are also called the 3rd order neuron?

Ganglion cells

What is the function of ganglion cells?

Transport information out of retina for higher order processing

What are the types of cells that terminate within the lateral geniculate nucleus (LGN)?

• P cells

• M cells

• K cells

What are P cells (Parvocellular layers) and what are the two types (more/less common)?

• Medium-sized cells for detail and color

• P1 more common and bipolar

• P2 less common and multipolar

What are M cells (Magnocellular layers)?

Large cells to process motion

What are K cells (Koniocellular layers)?

Small cells to process short wavelengths like color

What type of ganglion cells are most common?

P cells

What is the orientation of ganglion cell axons?

• They run parallel to inner surface of retina

• Converge at optic disc → form optic nerve

Where do the majority of ganglion cell axons terminate? Where do the rest terminate?

• Majority at LGN.

• Rest in other areas for processing (eye/pupil reflexes and circadian rhythm).

What are the functions of horizontal cells?

• Tranfer info horizontally

• Modulates information carried from photoreceptors to bipolar cells

What do horizontal cells consist?

Consist of many short dendrites with branching terminals.

What do horizontal cells synapse with?

• Photoreceptors (inhibitory feedback)

• Bipolar cells (inhibitory feedforward)

• Other horizontal cells (gap junctions)

What are the two types of horizontal cells?

HI: lateral element in triad

HII: more selective for blue cones

What are the functions of HI cells?

• Lateral inhibition: inhibit activity of neighboring photoreceptors

• Spatial contrast: detect differences in light intensity

What is the function of HII cells?

Blue-yellow color opponency: how visual system interpret colors by comparing signals

What are two types of cells that transfer information horizontally?

Amacrine cells and horizontal cells

What are the functions of amacrine cells?

• Transfer info hortizontally

• Modulates info carried from bipolar cells to ganglion cells (most inhibitory)

What do amacrine cells synapse with?

• Bipolar cells

• Ganglion cells

• Interplexiform neurons

• Other amacrine cells (via gap junctions)

How are amacrine cells classified?

Extent of coverage

What are amacrine cells subdivided?

• Narrow field

• Small field

• Medium field

• Large field

What is the most common type of amacrine cells?

AII

What is the function of AII cells?

Connect signals from rods bipolar cell → cone bipolar cells (to continue through visual pathway)

What type of processes extend into both plexiform layers of retina?

Interplexiform neurons

What is the function of the interplexiform neurons?

Provides feedback from inner to outer retinal layers

Where do interplexiform neurons receive input and send output to?

Input: amacrine cells and rod/cone bipolar cells

Output: amacrine cells