The Retina

© Wesner, M. F.

We know there is retinal heterogeneity.

vis

ual axis

fovea centralis

foveal pit

parafoveal area

Outer plexiform layer

Inner plexiform layer

The more eccentric from fovea, the greater the “rod intrusion”..

conesrods

The retina: The outer nuclear layer structure

Components of rod and cone photoreceptors. The outer segment is a stack of disks containing light-sensitive pigment molecules. The inner segment includes the cell nucleus, and synaptic terminals housed in pockets called clefts.

Stacked disks

Membrane folds (invaginations)

hvhv

What determines successful pigment absorption is l (i.e. the

photon e state).

e = hn

3 different cone types

Four receptor TypesRods:• Rhodopsin (visual purple)

Cones:• Long-wavelength Sensitive (LWS, L-cones)

Erythrolabe {“red catcher”}• Middle-wavelength Sensitive (MWS, M-

cones)

Chlorolabe {“green catcher”}• Short-wavelength Sensitive (SWS, S-cones)

Cyanolabe {“blue catcher”}

The retina: Photoreceptor responses

The probability that a photon will be absorbed

depends on: Direction—photons traveling through the center of

the lens are more likely to be absorbed. Frequency—photons with a frequency near the

peak of a receptor’s spectral sensitivity are more

likely to be absorbed. Once a photon has been absorbed, the

photoreceptor has no way of distinguishing its

frequency.

Principle of Univariance (William Rushton, 1960s) - The response of a receptor conveys information about how much light (quanta) is absorbed, not by the wavelength (photon e) of the absorbed light.

In other words, wavelength (l) only determines the probability of quantal capture by the pigments!

This is why the terms R, G & B are misnomers.

Four receptor TypesRods:• Rhodopsin (visual purple)

Cones:• Long-wavelength Sensitive (LWS, L-cones)

Erythrolabe {“red catcher”}• Middle-wavelength Sensitive (MWS, M-

cones)

Chlorolabe {“green catcher”}• Short-wavelength Sensitive (SWS, S-cones)

Cyanolabe {“blue catcher”}

Transduction - the conversion of photon energy into electrochemical (neural) energy.

What happens when the pigments get photolyzed?

Chromophore (contains retinal)

hv

Photon aborption resulting in the conversion from 11-cis to all-trans retinal is known as photoisomerization.Note: Isomers are molecules that have the same number of atoms but different physical structures, thus different properties.

12

3

45

67 8

910 11

DARK

LIGHT

Na+

“dark current”

Na+

Light activated receptors are hyperpolarized!Turn OFF neuro- transmitter.

Cyclic guanosine monophosphate (cGMP) is a 2nd messenger which is ACTIVE in dark

(cis-retinal + opsin)

trans-

PhosphodiesterasecGMP GMP

dark current..

..thus NT is released in the dark..

+-

ophthalmoscopic examination - Ophthalmologist views the retina through an ophthalmoscope. He or she views the fundus.

OD (oculus dexter)- right eye fundus

OS (oculus sinister)- left eye fundus

Two common symptoms of disease found in the fundus:

• macular degeneration: heavy pigmentation around the fovea and parafovea. Results in degeneration of cones which affect central vision (scotomas).

• glaucoma: “cupping” or excavation of the optic disk (nerve head) due to increases in intraocular pressure (IOP).

OD (oculus dexter)- right eye fundus

blood vessels

maculafovea

optic disk (blind spot)

temporal hemiretina nasal hemiretina

centralisparafovea

Outer plexiform layerInner plexiform layer

Lateral interacting cells (lateral antagonism)

Inner nuclear layerOuter plexiform layer

Outer nuclear layer

Inner plexiform layer

Ganglion layer

* Roman numerals indicate which of von Graef’s IX layers are shown.

This is why the terms R, G & B are misnomers.

Trichromacy based on physiological response..

Based on psychophysically-derived equations from color matching of known congenital dichromats & heterochromatic flicker photometry (HFP). These curves were later corroborated by physiological monkey recordings.

Trichromacy is revealed based on behavioral response..

Spectral sensitivity curves are very similar to what can be derived physically..

& physiologically..

How do these photoreceptor events relate to bipolar activity? Properties of the first synaptic layer in the retina - the outer plexiform layer.

Remember: Light “turns off” photo-receptor neurotransmitter release.

Two types of bipolars:

1.Flat bipolars (sign conserving)2.Invaginating bipolars (sign inverting)

Note: In the outer plexiform layer, you have synaptic triads. Photoreceptors synapsing with two horizontal cells and either a flat or invaginating bipolar cell.

Outer plexiform layerInner plexiform layer

Lateral interacting cells (lateral antagonism)

LUMINANCE CONTRAST-INTENSITY

DIFFERENCES

Without these contrasts, the brain shuts down.

CHROMATIC CONTRAST -WAVELENGTH

DIFFERENCES

TEMPORAL CONTRAST -TIME (EVENT)

DIFFERENCES

flat bipolars invaginating bipolars

PR

HH

BP

sign conservingExcitatory NT (+)

PR

HH

BP

sign invertingHyperpolarizations (-)

flat bipolars

sign conserving

hv

PR

HH

BPNOTE: The flat bipolar hyperpolarizes because of the turning OFF of excitatory NT.

ALWAYS EXCITATORY

invaginating bipolars

sign inverting

PR

HH

BPNOTE: The invaginating bipolar depolarizes be-cause special membrane properties hyperpolarize with presence of glutamate.

hv

ALWAYS EXCITATORY

flat bipolarssign conserving

hv

PR

HH

BP

NOT (EXCITATORY) -Produces “OFF” center ganglion cell

G

invaginating bipolarssign inverting

PR

HH

BP

hv

G

(EXCITATORY) -Produces “ON” center ganglion cell

flat bipolars invaginating bipolars

PR

HH

BP

sign conservingDepolarizations (+)

PR

HH

BP

sign invertingHyperpolarizations (-)

Lateral inhibition (lateral antagonism)

How does lateral antagonism relate to human retina?

NOT (+) means not Horizontal NOT (-): Thus, depolarization (+)

- +

(+) means yes, Horizontal response (-): Thus, hyperpolarization (-)

- +

Spatial antagonism in the retina (i.e., the creation of ganglion cells that are either “On” center; “OFF” surround or “OFF” center; “ON” surround) allows the retina to begin processing for LUMINANCE CONTRAST.

Synaptic Dyad - inner plexiform layer

BP

A

G

Steady response from horizontal and amacrine cell integration-produces a steady-state, tonic ganglion cell response (X-cells).

Synaptic Dyad - inner plexiform layer

BP

A

G

*

*possible mechanism for transient response (self-inhibiting? delay response?)

Transient response from amacrine input and feedback-produces a phasic ganglion response (Y-cells).

NOTE: More neuronal convergence with eccentricity

Neuronal convergence (spatial pooling) lowers spatial acuity. However, convergence also increases overall light sensitivity.

..to optic nerve

eccentricity

Eccentric monosynaptic 1:1

..to optic nerve

eccentricity

More eccentric polynomosynaptic coupling..

..means larger receptive fields.

..to optic nerve

eccentricity

Eccentric monosynaptic 1:1

..to optic nerve