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SF2 - Test 1 > 4 - Grover - Retina Function > Flashcards

4 - Grover - Retina Function Flashcards

1
Q

Functional Organization of Retina: Pathways

A

Vertical and Lateral Pathways

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2
Q

Retina: Vertical Pathway

Neurotransmitter?

Effect?

Cell Types?

A

Neurotransmitter: Glutamate

Effect: Excitatory or Inhibitory

Cell Types: Ganglion Cells, Photoreceptors, Bipolar Cells

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3
Q

Ganglion Cells

A

Part of Vertical Pathway

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

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4
Q

Photoreceptor Cells

Bipolar Cells

A

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

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5
Q

Lateral Pathway

Neurotransmitter?

Cell Types/Functions?

A

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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6
Q

Retina Organization: Rod and Cone Density

Consequence?

A

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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7
Q

Retina Organization: Convergence

Peripheral vs Fovea

A

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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8
Q

Change in Membrane Potential: Light vs Dark

A

Membrane potential in dark is depolarized

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

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9
Q

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

A

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

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10
Q

Photoreceptor in Dark

Intracellular [cGMP]?

Resting Current?

A

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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11
Q

What is released continuously by photoreceptors in the dark?

Do these cells fire action potentials?

A

Neurotransmitter Glutamate

Photoreceptors do not fire action potentials

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12
Q

Photoreceptor in Light

What is activated? Consequences?

Membrane potenial?

Neurotransmitter release?

A

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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13
Q

What initiates phototransduction in the light?

Where are the key molecules contained?

A

Phototransduction is intitaed when photon absorbed by photopigment

Photopigments are contained in outer segment disks

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14
Q

Photopigment Parts

A
  1. Light absorbing chromophore: Retinal
  2. Retinal binding protein: Opsin
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15
Q

Opsin

A

Resembles metabotropic neurotransmitter receptor

Contains binding pocket for retinal

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

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16
Q

What effect does light have on retinal?

A

Induces conformational change, which induces conformational change in opsin

17
Q

***Steps to Phototransduction***

Exam!

A
  1. Conformational Change in OPSIN activtes transducin (G-protein)
  2. Activated transduces stimulates Phosphodiesterase (PDE)
  3. PDE rapidly degrades cGMP
  4. CNG channels close
18
Q

Phototransduction in Rods

Cause for high sensitivity?

A

High light sensitivity due to signal amplification

One photon = 1 mV change

19
Q

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

A

At frequency where threshold intensity is lowest

20
Q

Phototransduction in Cones

A

Spectral Sensitivity of Photopigment is major difference from rods

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

21
Q

Opsin Wavelengths in Cones

A

Short = Blue

Intermediate = Green

Long = Red

22
Q

How is wavelength determined by different opsin photopigments?

A

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

23
Q

Phototransduction in Cones

Comparison to rods?

A

Less sensitive (100:1 ratio)

Best under bright light

Dense in Fovea

24
Q

Dyschromatopsia (Color Blindness)

Types?

A

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)

25
Q

Light Adaptation - Control?

What does output from retina signal?

A

Cytoplasmic Ca2+ regulates light sensitivity

Increase calcium, increase sensitivity

Output from retina signals a change in light intensity, not absolute level of light intensity

26
Q

Scotopic Vision

A

Low light levels

Rods only

High sensitivity, Low acuity

27
Q

Mesopic Vision

A

Some cones

Not bright enough to saturate rods

Ex: Starlight, Moonlight

28
Q

Photopic Vision

A

All Cones

Rods completely saturated

Low sensitivity, high acuity

Ex: Indoor lighting, sunlight

29
Q

Receptive Field Properties

Horizontal / Bipolar Cells

A

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

30
Q

On-center Bipolar Cells (Off-Surround)

A

Depolarize (excite) when receptive field center is illuminated

Hyperpolarize (inhibit) when receptive field surround is illuminated

31
Q

Off-center Bipolar Cells (On-surround cells)

A

Hyperpolarize (inhibit) when receptive field center is illuminated

Depolarize (excite) when receptive field surround is illuminated

32
Q

Rod Pathway Bipolar Cells

A

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

33
Q

Melanoma-Associated Retinopathy

Paraneoplasia

A

Some melanoma patients lose night vision

Paraneoplasia - Antibody againts mGlu6

34
Q

Congenital Stationary Night Blindness

A

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

35
Q

Receptive Field Ganglion Cells

A

Each ganglion cell is driven by only one type of bipolar cell

On-center ganglion cells receive input from on-center bipolar cells

Off-center ganglion cells receive input from off-center bipolar cells

36
Q

On-center Ganglion Cells

A

Increase firing when light illuminates center of receptive field

Inhibited when light illuminates surround receptive field

37
Q

Off-center Ganglion Cells

A

Inhibited when light illuminates ceter of receptive field

Increased firing when light illuminates surround of receptive field

38
Q

Ganglion Cell Activation

A

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

39
Q
A

SF2 - Test 1 (20 decks)

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