Retina

10 Layers of the Retina

1. Retinal pigment epithelium

   • single layer of pigmented cells; absorbs stray light and nutrient/waste exchange

2. Photoreceptor layer

3. Outer limiting membrane

   • fusion of photoreceptor cell membranes; basis of electrical charge of retina

4. Outer nuclear layer

   • cell bodies of photoreceptors

5. Outer plexiform layer

   • synapses between photoreceptors and bipolar/horizontal cells

6. Inner nuclear layer

   • cells bodies of bipolar cells, amacrine cells, horizontal cells

7. Inner plexiform layer

   • synapses between bipolar/amacrine and ganglion cells

8. Ganglion cell layer

   • cell bodies of ganglion cells; send axons out

9. Nerve fiber layer

   • axons of ganglion cells that exit the lamina cribrosa and form the optic nerve

10. Inner limiting membrane

    • footplate of Muller cells (glial cells), highly reflective in younger patients

Photoreceptor Micro-Anatomy

Photoreceptors (rods and cones)

Common features across species

• outer segment

  • Discs found in both rods and cones

  • discs are circular in shape when viewed from above

  • on sagittal section the disc has a double membrane with a high concentration of Na+ on the inside of the discs

  • discs made up of lipoprotein and visual pigment (50%) - 4 different pigments depending on the cone type (3) or rod (1)

• inner segment (ellipsoid and myoid)

  • location of energy production

    • Ellipsoid: closer to outer segment; has mitochondria for energy production

    • Myoid: closer to cell body; has organelles like Golgi and ER for protein synthesis

  • connected to outer segment by cilium

  • contains a specialized cilia consisting of 9 double microtubules

    • transport from inner segment to outer segment

  • extend all the way to the discs and are involved in the regeneration of the outer segment

• nucleus

• synaptic termination

Differences: outer segments

• cone outer segment is smaller than its inner segment

• rod outer segment is the same size as its inner segment

• Response time to stimulus: rods respond much slower due to larger disc space; cones detect temporal change better

Photoreceptor Distribution varies with retinal location inspected

foveola (central 1.2 degrees) mostly cones and some glial cells

fovea (central 2.5 degrees) has both rods and cones

peak concentration of rods at 20 degrees from the fovea

Glial Cells of the Retina

Astrocytes – Type I, II, and III

• star-shaped cells

• protect blood vessels, maintain the blood-brain barrier

• physically hold the blood vessels in place in the retina

Oligodendrocytes

• myelination of the retinal ganglion cells occurs outside the retina after it passes through the lamina cribrosa

Muller cells - ependymal cells

• transverse the retina from ILM to ELM

• Guidance and physiological support

• Finds nutrients, removes waste

Retina is part of CNS

Information Flow -- Vertical Connections CONES

cones ---> ON and OFF bipolar cells ---> ON and OFF ganglion cells ---> LGN

Information Flow -- Vertical Connections RODS (4 paths)

1. rod→ rod ON bipolar → rod A II Amacrine → cone ON bipolar (gap junction) → ON ganglion cell

2. rod→ rod bipolar → rod A II amacrine → OFF ganglion cell

3. rod→ cone (gap junction) → ON and OFF bipolar cell → ON and OFF ganglion cell

4. rod → OFF cone bipolar → OFF ganglion cell

The sublamina of the IPL

Sublamina A - more distal or closer to bipolar cells

Sublamina B - more proximal or closer to ganglion cells

Two functionally different bipolar cells

• ‘ON’ bipolars - terminate in sublamina B

• ‘OFF’ bipolars - terminate in sublamina A

Biplexiform cell

a ganglion cell that receives direct input from the photoreceptors (unknown function and projection) - bypasses the bipolar cells

receives mostly rod input

recent studies (animals)

• Projection: optic tectum

• Function: visuomotor reflexes

Horizontal or Lateral Connections

Horizontal cells

• Each horizontal cell contacts all cones within its dendritic field; thus, it is not wavelength selective

• functionally produce the receptive field surround of the ganglion cells

Amacrine cells

• 30-40 different types based on morphology and cytochemistry

• cell bodies located in the proximal INL and synapse in the IPL

• connected with bipolar cells, ganglion cells and other amacrine cells

  • can provide direct input to ganglion cells

  • that is, midget ganglion cells receive as much input from amacrine cells as from bipolar cells

• AII

  • mediates rod vision

  • contains glycine as its neurotransmitter

  • Functionally helps to shape the time course of the ganglion cell response and may be involved in the production of the surround of the ganglion cell RF

Interplexiform cells

• carries information from amacrine cells back to bipolars, other amacrines, rods or cones

• involved in a feedback loop in the retina

• Mangel and Dowling (1987) suggested that these cells were involved with the changes that occur in the receptive fields of ganglion cells with dark adaptation

Receptive Fields

Central receptive field (inner circle) - on pathway

• on pathway: rod → on bipolar cell → on ganglion cell

Surround receptive field (outer ring) - off pathway

• off pathway: rod → horizontal cell → off bipolar cell → off ganglion cell

Area on the retina from which the neural discharge of a neuron can be influenced by light stimulation

Synapses at the outer plexiform layer

Cone pedicle

• many invaginations in which bipolar and horizontal cells synapse (12 - 25 / pedicle)

• commonly see a ribbon synapse, which has 2 horizontal cells and 1 bipolar cell synapsing

• will also be noninvaginating synapses - typically with a flat bipolar (off synapses)

Rod spherule

• typically more than 3 processes synapse (more than one central process)

• common information flow is from rods to rod bipolar

• no junction between the laterally placed horizontal cells (like that of cone pedicles)

• one invagination per spherule

Gap Junction

• Each adherent junction has 3 gap junctions

• links the membrane of photoreceptor cells

• Open channels between 2 cell membranes allow for communication, ion transport, electrical coupling

Synapses at the inner plexiform layer

Conventional dyad: most common in primates: 1 bipolar synapses with 1 ganglion and 1 amacrine

Amacrine-amacrine: second most common: 1 bipolar synapses with 2 amacrines

Reciprocal: information can flow both ways between a bipolar and an amacrine (feedback)

Serial: bipolar to amacrine to amacrine to ganglion cell: allows for informational integration

Types of Ganglion Cells

Cat ganglion cells

• types seen with light microscopy and Nissl stain

  • Alpha cells (cell body > 21 microns) LARGEST

  • Beta cells (cell body 12 - 20 microns)

  • Gamma cells (cell body < 11 microns) SMALLEST

• Diameter of dendritic spread

  • Alpha cells, 425 - 785 microns LARGEST

  • Beta cells, 90 - 370 microns SMALLEST

  • Gamma cells, 440 - 500 microns

• Axon diameter

  • Alpha cells, 2.5 - 3.5 microns LARGEST

  • Beta cells, 0.8 - 1.5 microns

  • Gamma cells, 0.25 - 0.32 microns SMALLEST

Primate Ganglion Cells

• P1: larger cells, larger axon diameter, larger dendritic spread

• P2: smaller cells, smaller axon diameter, smaller dendritic spread

Bipolar Cell Categorization

1. Invaginating Midget Bipolar

   • Single cone at ribbon

2. Flat Midget Bipolar

   • Single cone at conventional synapse

3. Diffuse Flat Bipolar

   • Several cones conventional synapses .

4. Diffuse Invaginating Bipolar

   • Several cones ribbon synapse

5. Rod Bipolar

   • Rods only

6. Giant Bistratified Bipolar

   • Contacts many cones

7. S-cone Bipolar

   • Contacts only S-cones

Photoreceptor Physiology: Transduction

photoreceptors convert light into a neuroelectrical response (act as a transducer)

unlike typical neural cells photoreceptors do not produce action potentials (only produce membrane potential changes)

unlike most CNS cells the photoreceptors do not depolarize when excited, they hyperpolarize

Dark current

• in the dark the Na+ channels are open and Na+ flows into the outer segment and is pumped out the inner segment - a continuous cycle

• light results in a closure of the Na+ channels so Na+ doesn't flow in but it is still pumped out - inside becomes more negative (hyperpolarizes)

• in the dark the photoreceptors release neurotransmitter continually - glutamate

Effects of light

• in the dark the inside of the photoreceptor is about 20 millivolts more negative than the outside

• when light hits the photoreceptor there is a series of chemical reactions that results in the inside of the cell becoming more negative (-60 millivolts)

Chemical reactions

• rhodopsin --> opsin + all-trans retinal

  • with light (photoreaction) - rhodopsin → bathorhodopsin

• concentration of free cGMP decreases; cGMP needed to bind at Na+ channels to keep them open

• decreasing the cGMP concentration closes some of the Na+ channels

• results in the inside of the cell becoming more negative

Effects of light on cells

Photoreceptor cell: hyperpolarization

Horizontal cell: hyperpolarization

Bipolar cell (off): hyperpolarization

Bipolar cell (on): depolarization

Amacrine cell (on): produces action potentials

Ganglion cell (on): produces action potentials