1. Light absorbing chromophore: Retinal 2. Retinal binding protein: Opsin

4 - Grover - Retina Function Flashcards by John Castillo

Functional Organization of Retina: Pathways

Vertical and Lateral Pathways

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Retina: Vertical Pathway

Neurotransmitter?

Effect?

Cell Types?

Neurotransmitter: Glutamate

Effect: Excitatory or Inhibitory

Cell Types: Ganglion Cells, Photoreceptors, Bipolar Cells

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Ganglion Cells

Part of Vertical Pathway

Output cells of retina (to nerve), long axonal projections, fire action potentials

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Photoreceptor Cells

Bipolar Cells

Don’t fire action potentials, graded potentials only, transmitter release graded

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Lateral Pathway

Neurotransmitter?

Cell Types/Functions?

Neurotransmitter: Inhibitory Transmitter (GABA?)

Cell Type/Function:

Horizontal Cell - Mediate lateral inhibition between photoreceptors

Amacrine Cells - Mediate connections between rod photoreceptors and ganglion cells

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Retina Organization: Rod and Cone Density

Consequence?

Fovea: High Cones (low light sensitivity)

Periphery: High Rods (high light sensitivity)

Peripheral retina is adapted for vision in dim light

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Retina Organization: Convergence

Peripheral vs Fovea

Peripheral Retina:

Greater Convergence, many photoreceptors converge onto one ganglion cell

Greater Sensitivity

Poor spatial resolution

Fovea:

Less Convergence, 1:1 ratio of photoreceptor to ganglion cell

Decreased sensitivity

Greater spatial resolution

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Change in Membrane Potential: Light vs Dark

Membrane potential in dark is depolarized

Light causes photoreceptor to hyperpolarize; graded and proportional to light intensity

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What molecules controls the gating or membrane potential of sensory transduction?

Cyclic Nucleotide (cGMP / cAMP) gated channels open, allowing cations to enter cell

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Photoreceptor in Dark

Intracellular [cGMP]?

Resting Current?

Cytostolic cGMP is high (high guanylyl cyclase activity)

Many CNG channels

Resting inward current (dark current) holds photoreceptor at depolarized membrane potential (channels held open)

- - -

_D_ark = _D_epolarized

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What is released continuously by photoreceptors in the dark?

Do these cells fire action potentials?

Neurotransmitter Glutamate

Photoreceptors do not fire action potentials

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Photoreceptor in Light

What is activated? Consequences?

Membrane potenial?

Neurotransmitter release?

Phosphodiesterase (PDE) is activated, cGMP converted to GMP, and CNG channels CLOSE

Membrane potential hyperpolarizes

Neurotransmitter release declines in proportion to hyperpolarization (proportional to light intensity)

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What initiates phototransduction in the light?

Where are the key molecules contained?

Phototransduction is intitaed when photon absorbed by photopigment

Photopigments are contained in outer segment disks

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Photopigment Parts

Light absorbing chromophore: Retinal
Retinal binding protein: Opsin

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Opsin

Resembles metabotropic neurotransmitter receptor

Contains binding pocket for retinal

*Opsin tunes retinal’s absorbance to specific region of light spectrum

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What effect does light have on retinal?

Induces conformational change, which induces conformational change in opsin

***Steps to Phototransduction***

Exam!

Conformational Change in OPSIN activtes transducin (G-protein)
Activated transduces stimulates Phosphodiesterase (PDE)
PDE rapidly degrades cGMP
CNG channels close

Phototransduction in Rods

Cause for high sensitivity?

High light sensitivity due to signal amplification

One photon = 1 mV change

Where is receptor most sensitive in a intensity vs frequency tuning curve?

At frequency where threshold intensity is lowest

Phototransduction in Cones

Spectral Sensitivity of Photopigment is major difference from rods

Three types of cone opsin proteins (each cone contains only one type opsin)

Opsin Wavelengths in Cones

Short = Blue

Intermediate = Green

Long = Red

How is wavelength determined by different opsin photopigments?

Wavelength is determined by comparison of output from different classes of cones (can’t be determined by single cone class)

Phototransduction in Cones

Comparison to rods?

Less sensitive (100:1 ratio)

Best under bright light

Dense in Fovea

Dyschromatopsia (Color Blindness)

Types?

Loss of one or more cone photopigment, impaired color discrimination

Dichromacy: Loss of one cone photopigment

Red/Green = Most common

Blue = Rare (Chrom. 7)

Monochromacy: Loss of two or three photopigments (rare)

Light Adaptation - Control? What does output from retina signal?

Cytoplasmic Ca²⁺ regulates light sensitivity Increase calcium, increase sensitivity - - - Output from retina signals a **change in light intensity, not absolute level of light intensity**

Scotopic Vision

**Low** light levels **Rods** only **High** sensitivity, **Low** acuity

Mesopic Vision

Some cones **Not** bright enough to **saturate rods** **Ex: Starlight, Moonlight**

Photopic Vision

**All** Cones **Rods completely saturated** **Low** sensitivity, **high** acuity Ex: Indoor lighting, sunlight

Receptive Field Properties Horizontal / Bipolar Cells

Horizontal Cells: **- Lateral inhibitory connections (synapses)** create center-surround receptive fields - Excited by photoreceptors - - - Bipolar Cells: - Can be either **excitatory OR inhibitory** **-** Depends on **glutamate receptor type** in bipolar cell

On-center Bipolar Cells (Off-Surround)

**Depolarize (excite)** when receptive field **center** is **illuminated** **Hyperpolarize (inhibit)** when receptive field **surround** is **illuminated**

Off-center Bipolar Cells (On-surround cells)

**Hyperpolarize (inhibit)** when receptive field **center** is **illuminated** **Depolarize (excite)** when receptive field **surround** is **illuminated**

Rod Pathway Bipolar Cells

**Only** _On-center bipolar cells (excite)_ Rod bipolar cells synapse onto **amacrine cells** **Amacrine cells** synapse onto on- and off-center **cone bipolar cells** **- -** **Inhibit** onto off-center **Excite** onto on-center **USE CONE PATHWAYS FOR OUTPUT**

Melanoma-Associated Retinopathy Paraneoplasia

Some melanoma patients **lose night vision** **Paraneoplasia -** Antibody againts mGlu6

Congenital Stationary Night Blindness

Loss of **night vision** Several gene mutations affected **on-center** bipolar cells (mGlu6 not functional) -- **REMEMBER: Rod system uses hitchhikes onto this system** **Day vision** less affected due to parallel (off-center bipolar cell) pathways

Receptive Field Ganglion Cells

Each ganglion cell is driven by only one type of bipolar cell ## Footnote **On-center ganglion cells receive input from on-center bipolar cells** **Off-center ganglion cells receive input from off-center bipolar cells**

On-center Ganglion Cells

Increase firing when light illuminates center of receptive field Inhibited when light illuminates surround receptive field

Off-center Ganglion Cells

Inhibited when light illuminates ceter of receptive field Increased firing when light illuminates surround of receptive field

Ganglion Cell Activation

Activated by large **relative differences** Areas of **sharp contrast (edges)** are effective stimuli for ganglion cells - - - Retinal ganglion cell firing does **not** encode absolute intensity, **only relative difference**