Retina: Light Detection and Analysis

• Detection and analysis of light by the retina involves the structure/function of the eye, retinal photoreceptors (rods/cones), light transduction into electrical signals, luminance sensitivity, and color vision.

Electromagnetic Radiation

• Speed of electromagnetic radiation in a vacuum equals 300,000 km/s.
• Frequency refers to the number of cycles of energy fluctuation each second.
• Wavelength refers to the distance covered by one cycle.
• Photoreceptor cells detect between 400 nm and 700 nm wavelength.

Structure of the Vertebrate Eye

• The pupil controls the amount of light entering the eye.

• The lens focuses light onto the retina.

• The retina is the back layer of the eye, which contains neurons that process/transmit visual information to the brain as part of the central nervous system (CNS).

• The macula describes an area of concentrated light-sensitive cells around the fovea.

• The fovea represents the most light-concentrated region of the macula.

• The fovea has almost exclusively cone cells.

• The optic nerve consists of axon tracts from retinal neurons; its head produces the blind spot.

• Cornea and lens focus light onto 125 million photoreceptors.

• Anything from the left visual field hits the right side of the retina, which projects the object upside down onto the retina.

Organization of the Vertebrate Retina

• Output from the retina goes to the rest of the CNS via retinal ganglion cell axons.

• Rods and cones represent primary light detectors (photoreceptors).

• Three interneuron types process visual information in two synaptic (plexiform) layers.

• Photoreceptors are in the back of the retina.

• Light must pass through several cell layers to reach them.

• Neurons are nearly transparent, minimizing optical blur.

• Pigment epithelium (PE) cells serve a cell biological role; photoreceptors export all-trans-retinal and PE cells take it up.

• Then, PE cells reform 11-cis-retinal and export it back to photoreceptors.

• PE cells also have dark pigments for absorbing photons, which prevents reflection that might activate receptors not directly in the light path.

• This helps explain the odd organization, in which the photoreceptors are behind other neuronal layers.

Distribution of Rods and Cones

• The distribution of rods and cones vary across the retinal surface
• The fovea contains only cones, whereas the macula has a high number of rods.

Responses of Vertebrate Photoreceptors to Light

• Retinal photoreceptors of mammals are small.
• Initial recordings were from larger photoreceptor cells in amphibian retinas.
• Resting potential isn't as negative as with other neurons.
• Light produces hyperpolarization.

Electrical Events and Light Response

• Light causes hyperpolarization.

• A non-selective cation channel, open in the dark, shuts when light is on, moving membrane potential closer to Ek.

• Rods can detect a single photon of light in their outer segments.

Rods; Rhodopsin

• The intracellular discs of rods are packed with rhodopsin, but are not continuous with the plasma membrane
• Rhodopsin constitutes a 7-transmembrane domain protein and binds to a retinal molecule.
• Retinal (Vitamin A) photoisomerization leads to a conformational change in rhodopsin, which then allows transducin (a heterotrimeric G-protein) to bind.
• Retinal is bound to opsin proteins.

Rod Anatomy

• Non-selective cation channels exist in the plasma membrane.
• Rhodopsin is in the intracellular discs.
• A soluble second messenger is needed to transmit the signal from the discs to the plasma membrane.
• The messenger is cyclic GMP (cGMP).

cGMP-gated channels

• cGMP-gated channels (CNG) are voltage gated channel member.
• Do not select for Na, K and Ca and are not significantly voltage sensitive.
• Have an extra C terminal region (binds cGMP for channel opening).
• In the dark, guanylate cyclase activity remains high, PDE activity remains low and cGMP presence remains high

Signaling

• In the presence of light, PDE activity increases, cGMP concentrations dramatically decrease and channels close

Transduction Scheme

• PDE becomes inactive when transducin is not associated with rhodopsin.
• Photoisomerization then occurs and transducin binds to rhodopsin.
• Next GDP-GTP exchange occurs
• PDE is activated as guanosine triphosphate is bound

Dark and Light Effects on Photoreceptors

• In the dark, cGMP concentration is high; many cGMP-gated channels are open, and the cell has a fairly depolarized resting potential sufficiently positive to open voltage-gated calcium channels.
• The neurotransmitter (glutamate) is released continuously in the dark.
• When photons are absorbed, PDE activates; cGMP is destroyed; cGMP-gated channels close; the cell membrane hyperpolarizes; voltage-gated calcium channels close; glutamate release declines or stops.
• Biochemical cascades amplify light response

Amplitude Response

• The human visual system responds to variations in amplitude (i.e. luminance) over a 1012 fold range.
• Absolute refractory period limit information transmission relates to action potential rate-based coding

Encoding Strategy

• Range fractionation consists of multiple receptor types that respond over different light intensity amplitude levels
• Vertebrate retinas have high sensitivity to low light, and low sensitivity to bright conditions

Dark Adaptation of Rods

• After moving from a bright environment to darkness, cones/rods take about 10/30 minutes to fully recover sensitivity because it takes time to regenerate 11-cis retinal.

Rod Adaptation

• In constant light, rod responses decline within 1 second.
• Arrestin (rhodopsin kinase) phosphorylates rhodopsin, rendering it inactive until dephosphorylated.
• Calcium entry regulated photoreceptor response
• High [Ca2+]in inhibits guanylate cyclase, decreasing cGMP.
• Low [Ca2+]in enhances cGMP production, so much PDE must be activated to create similar light response.
• The effect of calcium necessitates GCAP (guanlylate cyclase activating protein).

Cones

• Cone responses are faster and more transient, require more photons, and depend on wavelength.
• Humans possess three cone types (short, medium, and long).
• The cones absorb a broad spectrum, but peak absorbance differs.
• Photon absorption is similar for human medium and long.
• The peak of M comes with yellowish-green
• The peak of L comes with yellow-orange.

Human Opsins

• Unfilled circles are amino acids identical between the two proteins
• Pairwise amino acid sequence comparisons indicate ~40% opsin identity
• All opsins share a common evolutionary origin, of which about 25% of the amino acids are conserved
• Human Long vs Medium are about 96% identical

Opsin Evolution

• Vertebrates exhibit 5 major opsin families, and our distant fish ancestors had members of all five

• Ancestral mammals were nocturnal, and lost the genes for the M1 and M2 opsins (blue and green), which weren't used.

• Most mammals exhibit cone dichromats

• Many nocturnal mammals lose the S genes

• Roughly 35 million years ago in the primate lineage, cones re-evolved trichromatic vision

• The human "red and "green" opsins originated from a gene duplication

• Helps with fitness: the human is technically tetrachromat

• "Color blindness"

• In dichromats #s disappear/can’t see them

Color Blindness Genetics

• The L and M genes are located adjacent to each other on the X chromosome.
• Due to similarities, their genes are commonly crossed over, which facilitates variations in the X chromosome cone opsin allele
• Inheritance is linked to the sex since males are only in receipt of one X
• Anomalous cone trichromats and tetrachromats may result

Friday Discussion

• All students should participate and consider discussions but specific groups will be assigned tasks based on name