OPT 8123 Ocular Anatomy: Lecture 11 - Retina

Comprehensive practice flashcards covering retinal histology, cellular pathways, photoreceptor morphology, and layers from Lecture 11 of Ocular Anatomy.

Between which two ocular structures is the retina situated?

The choroid and the vitreous.

What is the process called by which the retina converts light energy into a neural signal?

Phototransduction.

What are the first three cells of the visual pathway in sequential order of signal transmission?

1. Photoreceptor, 2. Bipolar cell, 3. Ganglion cell.

What is the outermost layer of the retina?

Retinal pigment epithelium (RPE).

What is the innermost layer of the retina?

Internal limiting membrane (ILM).

List all ten retinal layers in order from innermost to outermost.

Internal limiting membrane (ILM), Nerve fiber layer (NFL), Ganglion cell layer (GCL), Inner plexiform layer (IPL), Inner nuclear layer (INL), Outer plexiform layer (OPL), Outer nuclear layer (ONL), External limiting membrane (ELM), Photoreceptor layer (PR), and Retinal pigment epithelium (RPE).

Which layers constitute the "neurosensory retina" or "neural retina"?

The layers extending from the photoreceptor layer (PR) to the internal limiting membrane (ILM) (all layers except the RPE).

In which directions does visual information travel through the retinal layers?

Both vertically and horizontally.

Which three cell types mediate vertical transmission of visual signals through the retina?

Photoreceptor cells, bipolar cells, and ganglion cells.

Which two cell types mediate horizontal transmission of signals across retinal layers?

Horizontal cells and amacrine cells.

Which three retinal cell types have a retinal layer that shares their exact name?

Retinal pigment epithelium (RPE), photoreceptor cells, and ganglion cells.

Approximately how many cells are contained in the retinal pigment epithelium (RPE)?

$\sim 5\text{ million}$ cells.

How does RPE cell morphology change from the macula to the ora serrata?

RPE cells are columnar toward the macula/posterior pole and cuboidal toward the ora serrata.

Which intercellular junction types connect adjacent RPE cells?

Zonula occludens, zonula adherens, desmosomes, and gap junctions.

Which two pigment types or pigment-bearing organelles are responsible for RPE pigmentation?

Melanosomes (from melanocytes) and lipofuscin.

What are melanosomes, and where is their density greatest within the retina?

Organelles that produce, store, and transport melanin; their density is greatest in the macula and at the equator.

What is lipofuscin, and what causes its accumulation in RPE cells?

Yellow/brown lipid-containing pigment granules that are byproducts of phagocytosis; they increase with age.

To what structure does the basal surface of the RPE adhere, and what membrane does its basement membrane contribute to?

It strongly adheres to the choroid, and its basement membrane forms part of Bruch's membrane.

What specialized apical structures of the RPE envelop photoreceptor outer segments?

Microvilli.

What apical intercellular junctions between RPE cells contribute to the blood-retinal barrier?

Zonula occludens.

What potential space separates the apical RPE from the neural retina?

The subretinal space.

What substance occupies the subretinal space between the RPE and photoreceptors?

Interphotoreceptor matrix (IPM).

What types of intercellular junctions exist between RPE cells and photoreceptor cells?

There are no intercellular junctions between RPE cells and photoreceptors.

What pathological condition is depicted here, defined as the separation of the neurosensory retina from the RPE?

Retinal detachment.

Why is fluid able to accumulate between the RPE and neurosensory retina during a retinal detachment?

Because the RPE is strongly attached to the choroid while having no intercellular junctions connecting it to the neurosensory retina.

What are the six general functions of the retinal pigment epithelium (RPE)?

1. Blood-retinal barrier formation, 2. Phagocytosis of photoreceptor discs, 3. Absorption of light, 4. Interphotoreceptor matrix (IPM) formation, 5. Vitamin A metabolism/storage, and 6. Growth factor production.

What order of neuron are photoreceptors classified as in the visual pathway?

1st order neurons.

Approximately how many rods are present in the human retina?

$\sim 110\text{--}125\text{ million}$ rods.

Under what lighting conditions do rods operate, and what visual stimuli do they detect?

Dim (scotopic) lighting; they detect motion and forms.

In which retinal region is rod density highest?

In the peripheral retina.

Approximately how many cones are present in the human retina?

$\sim 6\text{--}7\text{ million}$ cones.

Under what lighting conditions do cones operate, and what visual attributes do they detect?

Bright (photopic) lighting; they detect fine detail and color.

In which retinal region is cone density highest?

In the central retina (fovea).

What light-absorbing photopigment is present in rods?

Rhodopsin.

What are the three cone photopigments, and which colors of light do they absorb?

Erythrolabe (red), chlorolabe (green), and cyanolabe (blue).

What structures within the photoreceptor outer segment house the visual pigments?

Stacks of membranous discs.

What biochemical components make up the interphotoreceptor matrix (IPM) surrounding the outer segments?

Proteins and glycosaminoglycans.

What structure connects the outer segment of a photoreceptor to its inner segment?

The cilium (modified cilium).

What are the two distinct zones of the photoreceptor inner segment?

The ellipsoid (outer) zone and the myoid (inner) zone.

Which cellular organelle is concentrated within the ellipsoid zone of the photoreceptor inner segment?

Mitochondria.

Which cellular organelles are concentrated within the myoid zone of the photoreceptor inner segment?

Golgi apparatus and endoplasmic reticulum.

What is the primary metabolic role of the photoreceptor inner segment?

It produces photopigments.

What anatomical structure links the photoreceptor inner segment to the cell body?

The outer fiber.

What structure extends from the photoreceptor cell body to synaptic terminals containing synaptic vesicles?

The inner fiber (axon).

How does the relationship between the plasmalemma and disc membranes differ between rods and cones?

In rods, the plasmalemma is separate from the disc membranes; in cones, the plasmalemma is continuous with the disc membranes.

What are the respective names and general shapes of rod and cone synaptic terminals?

Rods terminate in rounder spherules; cones terminate in flatter pedicles.

How do rods compare to cones regarding the length of their outer segments and fibers?

Rods have longer outer segments and outer fibers compared to cones.

How do rods compare to cones regarding inner segment width and cell body size?

Rods have narrower inner segments and smaller cell bodies/nuclei; cones have wider inner segments and larger cell bodies/nuclei.

What are telodendria on cone pedicles?

Lateral processes that connect cones to neighboring rods or cones for horizontal communication.

What three elements constitute a synaptic triad within an invagination of a cone pedicle?

One central bipolar cell dendrite and two flanking horizontal cell processes.

What is the central rod-free zone of the retina?

The fovea, which contains exclusively cones and no rods.

Why does the optic disc represent a physiological blind spot, as shown on this distribution graph?

Because it contains no photoreceptors (zero rods and zero cones).

What order of neuron are bipolar cells in the visual pathway, and approximately how many are there?

2nd order neurons; approximately $36\text{ million}$.

What morphological characteristic describes the cell body of a bipolar cell?

It contains a large nucleus with little cytoplasm.

From which two cell types do bipolar cells receive synaptic input?

Photoreceptors and horizontal cells.

To which two cell types do bipolar cells transmit synaptic output?

Ganglion cells and amacrine cells.

Which bipolar cell type is the only one associated with rods?

Rod bipolar cells.

How many rods contact a rod bipolar cell centrally compared to peripherally?

$15\text{--}20$ rods centrally versus $80$ rods peripherally.

Where on the cone pedicle do flat midget bipolar cells synapse?

Onto the flat portion of the cone pedicle.

Where on the cone pedicle do invaginating midget bipolar cells synapse?

Into invaginations within the cone pedicle.

How many neighboring cones do diffuse cone bipolar cells contact centrally versus peripherally?

$5$ cones centrally versus $10\text{--}15$ cones peripherally.

How do blue cone bipolar cells contact cones differently from diffuse cone bipolar cells?

They contact widely spaced cones rather than neighboring cones, synapsing with up to three cone pedicles.

Which cone bipolar cell is characterized by an extensive dendritic distribution with numerous connections to cones?

Giant cone bipolar cells.

What order of neuron are retinal ganglion cells in the visual pathway, and approximately how many are there?

3rd order neurons; approximately $1\text{--}2\text{ million}$.

What is the primary function of retinal ganglion cells?

To transport visual information out of the retina for higher-order processing in the brain.

Into which brain structure's layers are ganglion cells most commonly categorized based on axonal termination?

The lateral geniculate nucleus (LGN).

What percentage of ganglion cells are P cells, and in which LGN layers do they terminate?

$80\%$, terminating in the parvocellular layers.

What visual information is mediated by P ganglion cells?

Fine detail and color vision.

What are the two subtypes of P ganglion cells and their cell shapes?

P1 (midget ganglion cell; more common, bipolar) and P2 (less common, multipolar).

What are M ganglion cells also called, what percentage do they represent, and where do they terminate?

Parasol ganglion cells; they represent $10\%$ of ganglion cells and terminate in the magnocellular layers of the LGN.

What visual property do M (parasol) ganglion cells primarily detect?

Motion.

What are K ganglion cells, what percentage do they make up, and what visual stimulus do they process?

Koniocellular ganglion cells; they comprise $10\%$ of ganglion cells and process short-wavelength (color) information.

What percentage of ganglion cell axons terminate in the lateral geniculate nucleus (LGN)?

$90\%$.

What physiological functions are mediated by the $10\%$ of ganglion cell axons that do not project to the LGN?

Eye and pupil reflexes, and circadian rhythm regulation.

What path do ganglion cell axons follow across the retina before exiting the eye?

They run parallel to the inner surface of the retina and converge at the optic disc to form the optic nerve.

What is the primary role of horizontal cells in the retina?

To transfer information horizontally and modulate signals passed from photoreceptors to bipolar cells.

Which three cell connections do horizontal cells make via their branching terminals?

1. Photoreceptors (inhibitory feedback), 2. Bipolar cells (inhibitory feedforward), and 3. Other horizontal cells (gap junctions).

What type of signal do horizontal cells send back to photoreceptors?

Inhibitory feedback.

What type of signal do horizontal cells send forward to bipolar cells?

Inhibitory feedforward.

What is the role of HI horizontal cells in visual processing?

They act as the lateral element in synaptic triads, mediating lateral inhibition between neighboring photoreceptors to enhance spatial contrast.

How is spatial contrast defined in retinal processing?

Differences in light intensity between adjacent areas.

Which cone photoreceptor type do HII horizontal cells selectively contact, and what process do they mediate?

Blue cones; they mediate blue-yellow color opponency.

What is the functional role of amacrine cells in retinal circuitry?

To transfer information horizontally and modulate the transmission of signals from bipolar cells to ganglion cells.

Are most amacrine cells excitatory or inhibitory in function?

Most amacrine cells are inhibitory.

With which four cell types do amacrine cells form synapses?

Bipolar cells, ganglion cells, interplexiform neurons, and other amacrine cells (via extensive gap junctions).

How are the $30\text{--}40$ types of amacrine cells classified according to their dendritic expanse?

Narrow-field, small-field, medium-field, and large-field.

What is the most common subtype of amacrine cell?

The AII amacrine cell.

Why is the AII amacrine cell indispensable for rod-mediated vision?

Because rod bipolar cells do not synapse directly onto ganglion cells; AII amacrine cells relay signals from rod bipolar cells to cone bipolar cells.

What is the complete cell sequence for signal transmission initiated by a rod photoreceptor?

Rod $\rightarrow$ Rod bipolar cell $\rightarrow$ AII amacrine cell $\rightarrow$ Cone bipolar cell $\rightarrow$ Ganglion cell.

What is the function of interplexiform neurons in the retina?

They extend processes into both retinal plexiform layers to provide feedback from inner to outer retinal layers.

From which cells do interplexiform neurons receive synaptic input?

Amacrine cells, rod bipolar cells, and cone bipolar cells.

To which cells do interplexiform neurons provide synaptic output?

Amacrine cells.

Name the three types of neuroglial cells found in the retina.

Müller cells, microglial cells, and astrocytes.

What general functions do neuroglial cells provide in the retina?

They provide structure and support, and participate in neural tissue reactions to injury and infection.

Approximately how many Müller cells are present in the human retina?

$\sim 10\text{ million}$.

What is the structural extent of a Müller cell across retinal layers?

It spans nearly the entire retinal thickness, with its apical aspect forming the external limiting membrane and its basal aspect forming the internal limiting membrane.

How do Müller cells support retinal homeostasis and cellular organization?

They surround cell bodies, dendrites, and axons, fill extracellular spaces between retinal neurons, and maintain retinal homeostasis.

What is the function of retinal microglial cells?

They act as wandering phagocytic immune cells that clear degenerated neurons and photoreceptors, proliferating in response to inflammation, injury, or disease.

In which retinal layers are the star-shaped astrocytes shown in red primarily located?

In the inner retina, specifically the nerve fiber layer (NFL) and ganglion cell layer (GCL).

Which two structures do retinal astrocytes envelope to provide support?

Ganglion cell axons and retinal capillaries.