Eye 3

bipolar cells

(soma in INL) interconnect Photoreceptors Cell Bodies with Retinal Ganglion Cells. Serve as Interneurons. Each bipolar cell receives input from 20-50 photoreceptor cells.

amacrine cells

Cell body in INL. Work laterally (like horizontal cells), but amacrine cells affect output from bipolar cells (but not from photoreceptors). This cell does not have an obvious axon but has a highly branched dendrite, which extends over a great distance.

horizontal cells

Cell body is in INL. Receive glutaminergic input from multiple photoreceptor cell bodies, and provide inhibitory feedback. Facilitate good vision in both bright and dim conditions.

optic nerve (cranial nerve II)

Ganglions Cell Axons run towards the optic nerve head. Upon arrival, they turn 90° inward towards sclera. Axons pass through choroid and fenestrated openings in sclera, the Lamina Cribrosa.

rod photorecptors

Many rods will synapse with dendrites of bipolar neurons. Rods are few in number in the Macula (region of retina responsible for central visual field). Mainly Cones found here. Rods are absent in the Fovea Centralis (central spot of the Macula). Only Cones found here. However, Rods predominate in all other regions of the retina.`

retina non-neuronal (supporting) cells

Müller Cells (Müller Glia) *Astrocytes (found in innermost layer of retina surrounding retinal blood vessels) *Retinal Pigment Epithelium (RPE): Outer Layer (Trophic Support to Photoreceptor Layer)

fovea centralis

Near Fovea Centralis, inner retinal layers become thinner and disappear. At most central region essentially only the ONL, and Inner and Outer Segments are present. Maximum level of light can reach the OS of cone photoreceptors at fovea centralis. Blood vessels form a network of capillaries that are found at the periphery of the fovea centralis.

optic nerve head

RGC axons make 90° turn and exit retina at Optic Nerve Head (ONH). Central Retinal Artery and Central Retinal Vein also enter/exit retina at ONH

retina

Sagittal Section Through Human Eye. Retina forms thin covering lining inside of eye. However, despite being so thin, in cross-section it displays Ten Distinct Layers.

cone, rods

__ photoreceptors are highly concentrated at Fovea Centralis. No ___ present here. By contrast, Macula Lutea has mainly Cones, but a few Rods are present.

retinal ganglion cells

are the output cells of the retina. Axons form wiring that connects retina to brain. Receive input from Bipolar Cells and Amacrine Cells. RGC soma in Ganglion Cell Layer (GCL). Axons extend into Nerve Fiber Layer (NFL) and ultimately (upon exiting eye) form Optic Nerve. All have long axon extending to brain.

retina

light sensitive membrane lining inside of eye

retina neurons

*Retinal Ganglion Cells (RGCs) •*Amacrine Cells •*Bipolar Cells •*Rod Cells (receptive to low light, render monochrome vision only) •*Cone Cells (receptive to bright light, render color vision (red, green, blue types)) •*Horizontal Cells

10 layers of the retina

1. Inner Limiting Membrane (Müller glial cell (Müller cell) processes) 2. Nerve Fiber Layer (NFL) (Unmyelinated axons of Retinal Ganglion Cells (RGCs)) 3. Ganglion Cell Layer (GCL) (Cell bodies of RGCs) 4. Inner Plexiform Layer (IPL) (Synapses between Bipolar Cells and RGCs) 5. Inner Nuclear Layer (INL) (Cell bodies of Bipolar, Müller, Horizontal and Amacrine Cells) 6. Outer Plexiform Layer (OPL) (Synapses between Photoreceptors and Bipolar Cells) 7. Outer Nuclear Layer (ONL) (Cell bodies of Rods and Cones) 8. Outer Limiting Membrane (Junctional complexes between Photoreceptors and Müller Glial Cells) 9. Photoreceptor Layer (Rod & Cone Photoreceptors. Divided into Inner Segments (IS) & Outer Segments (OS)) 10. Retinal Pigment Epithelium (RPE) (Heavily pigmented. Provides trophic support to Photoreceptor Layer)

cone photoreceptors

A cone outer segment consists of a membranous stack. But stacks of cone membrane decrease in diameter, which results in characteristic cone shape.. Unlike in Rods, the stack of Outer Segments in the Cone are not enclosed with a second outer wrapping membrane, but instead is open to the extracellular (subretinal) space adjacent to RPE. Cone contains Chromophore and Cone Opsin Protein (1 of 3 different types (thus accounting for ability to perceive color). Deficiency in expression subtypes of cone opsins accounts for various types of Color Blindness. Similar process of Cone outer segment shedding/renewal shedding occurs as in Rods (phagocytosed by RPE), but process takes about 30 days to complete.

macula lutea and fovea centralis

At posterior pole of retina is a yellow-appearing region, the Macula Lutea (Macula). Macula Lutea means Yellow Spot in Latin. Yellow color produced by Xanthophyll Pigments in Outer Plexiform Layer in region 2mm in diameter centered on fovea. Corresponds to center of visual field. When you look directly at an object, the image of that object is sensed by the Macula Lutea. At center of Macula Lutea is Fovea Centralis (~100-150 µm in diameter)

muller cells

Glial cells spanning entire neural retina. NOT neural cells. Maintain stability of retinal extracellular environment. Terminal ends of Müller Glia form Inner and Outer Limiting Membrane.

rod photoreceptors

Rods are named based on appearance of their Outer Segment (membrane-bound cylinder containing 100s of tightly stacked inner Membranous Discs). Outer Segment is site of Phototransduction. Molecules of Photopigment called Rhodopsin within Outer Segment membranes undergo a conformational change in response to photons. Rhodopsin is highly sensitive to photons, however, rod stimulation results in low visual acuity (with no color perception). Hence rods facilitate Monochrome Vision

rod photoreceptors

Through process of Rod Shedding, OSs are constantly being Renewed. New discs are formed at base of Outer Segment and move outward so that the shed discs are replaced. This process results in the rod Outer Segment being of Constant Length. Rod Outer Segment renewal occurs about every 10 days.

retina

Total thickness of retina is maximally 250mm, but is only 100-150mm thick in Fovea Centralis (central visual field). Retina is highly transparent. Light reaches photoreceptor cells with little interference. Retina has 100-120 million Rods and 6-7 million Cones

retinal ganglion cells

Unmyelinated axons of ganglion cells exit retina at optic nerve head (a structurally weak region in posterior pole of eye). Axons become myelinated in optic nerve by Oligodendrocytes.

lamina cribrosa

a trabecular structure of collagen beams through which pass axons of retinal ganglion cells and central retinal artery and vein as they exit through the sclera at the posterior pole of the eye.

retina

consists of 10 layers arranged in a laminated pattern

light

enters the eye through the cornea. It then passes through the anterior chamber, the lens, and the vitreous, before striking the Inner Limiting Membrane of the Retina. From there it passes through ALL retinal layers (except the retinal pigment Epithelium) to reach the Photoreceptors (Rods & Cones)

muller cells

extend from Inner Limiting Membrane to Outer Limiting Membrane and function primarily in structural and trophic support of retinal neuronal cells.

blind spot

•Optic Nerve Head (ONH) is often referred to as Blind Spot because photoreceptors not present in this region.

retinal pigment epithelium

•RPE cells form structural interaction with Outer Segments (OS) of Photoreceptors and secrete many growth factors that promote photoreceptor differentiation/survival. •RPE cells also Phagocytose shed photoreceptor OS and recycle Retinoids. This phagocytic function is critical in maintenance of life of cones and rods.

retinal pigment epithelium

•RPE is positioned between Photoreceptor Layer and Bruch's Membrane, immediately overlying inner part of Choroid. RPE incorporates MANY Melanin Granules. •RPE plays significant role in maintenance of Photoreceptor Layer. •RPE forms important structural relationship with Bruch's membrane allowing transport to/from Choroidal blood vessels.

photoreceptor cells

•Rod and Cone Photoreceptor Cells responsible for photoreception/phototransduction. • •This process involves Detection of Photons and resulting information being Transduced into an Electrochemical Signal. • •Light is Transduced in Outer Segment of Photoreceptor (Rod or Cone) that is enveloped by apical processes of RPE cells. •The Connecting Cilium connects Outer Segment to Inner Segment. Inner Segment contains mitochondria, synthesizes Outer Segment membrane proteins, and produces energy that maintains cell. • •The Cell Body and Nucleus of all photoreceptors are found within Outer Nuclear Layer (ONL). • •The axon of rods and cones synapses with a dendrite or dendrites of a Bipolar Neuron (Blue) in the OPL .