Retina Phototransduction Flashcards

1
Q

What is Cataracts and what causes it? What are the symptoms?

A

Clouding of the lens that affects vision; it is due to disruption of the order of the organization of the lens cell fibers or aggregation of proteins. Symptoms are hazy vision, poor night vision, glare and faded colors

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2
Q

What is Glaucoma and how is it caused? What are the symptoms?

A

Group of diseases that damage the eye’s optic nerve and can result in vision loss. It is due to high intraocular pressure. The vision loss is very slow and happens at the periphery, patients often do not know that they are losing their vision until they are tested

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3
Q

Cells along the vertical path (photoreceptors, bipolar and ganglion cell) in the eye release what neurotransmitter? What do cells mediate later information transmission (horizontal and amacrince) release?

A

Glutamate - stimulatory

GABA or glycinergic (with some exception) - inhibitory

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4
Q

In darkness what do rods and cones do?

A

They are depolarized at -40mV and constantly secrete neurotransmitter.

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5
Q

When stimulated by light how do photoreceptors react? How is the signal passed on?

A

They do not fire action potentials but instead respond with graded hyperpolarizations. The hyper polarization then spreads passively to the synapse where it reduces the release of the neurotransmitter glutamate.

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6
Q

In the dark explain what the ions are doing in photoreceptors:

A

There is circulating current in the dark. Na and Ca cations flow inward through cGMP-gated channels at the outer segment. K ions flow outward through K-selective channels at the inner segment. The combined action of the currents results in -40mV resting potential.

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7
Q

In the light explain what the ions are doing in photoreceptors:

A

The inward cationic current is suppressed. cGMP levels are reduced and the channels are closed reducing Ca and Na flow. K ions continue to still flow. Hyperpolarization and neurotransmitter release is reduced.

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8
Q

What is important about Ca levels in the photoreceptors?

A

The reduction of inward current when stimulated by light results in a reduction in intracellular Ca levels/ Light-induced reductions in Ca levels are important for light adaptation.

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9
Q

What is phototransduction? What occurs at the cellular level?

A

It is the conversion of light into electrical signals. The visual pigment protein rhodopsin (cone opsin in cones) with a light-absorbing chromophore. Opsins are in the GPCR superfamily. Light absorption results in a conformational change in retinal which leads in turn to activation of rhodopsin which activates the G-protein. This leads to a reduction in cGMP levels

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10
Q

What is special about the phototranduction pathway?

A

The absorption of single photon closes more than 200 channels. This leads to a membrane hyper polarization of ~1mV careful observers can see light flashes so dim that only 1 in 100 rods absorbs a single photon.

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11
Q

What is the visual cycle? Where does it occur?

A

It is the conversion of all trans to 11-cis isomere. It is the restoration of retinal capable of signaling photon capture. It occurs largely in the retinal pigment epithelium in a process known as the visual cycle. It is essential for maintaining the light sensitivity in photoreceptors.

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12
Q

Where are cones mostly located?

A

The fovea. It is used for high spatial vision. the ganglion cells, Inner Plexiform Layer, and Inner Nuclear Layer are pushed away for higher acuity.

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13
Q

Describe the pathology in open angle glaucoma:

A

Slow development caused by the obstruction of drainage canals

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14
Q

Describe the pathology in closed angle glaucoma:

A

Sudden increase in intraocular pressure caused by closed or narrow angle between the iris and cornea.

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15
Q

Where are rods mostly located?

A

In the periphery

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16
Q

There is a high convergence of rods onto rod bipolar cells, what are the consequences of this?

A

High sensitivity and low resolution. Tradeoff high sensitivity for low spatial resolution.

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17
Q

There is one cone bipolar cell per 1 cone, what are the consequences of this?

A

Low sensitivity but high resolution. Tradeoff of high spatial resolution for low sensitivity

18
Q

What is scotopic vision? When does it occur?

A

Rod-only vision in dim light. High sensitivity, low acuity, no color.

19
Q

What is photopic vision? When does it occur?

A

Cone-only vision; low sensitivity, high acuity, color. Occurs in bright light only.

20
Q

What is Mesopic vision? When does it occur?

A

Cone and rod vision active together. Occurs at intermediate levels of light.

21
Q

What is Retinitis pigmentosa? What are the symptoms

A

It is a group of genetic eye conditions that leads to incurable blindness. The rods go first and then the cones. Symptoms are: first night blindness. Then tunnel vision. Finally the cones in the center die and total blindness occurs.

22
Q

What is age-related macular degeneration (1- Wet AMD, 2 Dry AMD)? What cells are destroyed? What are the symptoms?

A

This is a cone problem
1 - abnormal blood vessels behind the retina grow under the macula, leaking and rapidly damaging the retina.
2 - RPE and photoreceptors of the macula degenerate, accumulation of drusen (yellow deposits: 85% of cases)
Symptoms: Loss of central vision and acuity. Inability to see faces when looking at someone.

23
Q

What is Diabetic retinopathy? What cells are destroyed? What are the symptoms?

A

Retinal damage from complications of diabetes mellitus. Neither the rods nor cones are the issue here. It is due to neovascularization blocking small portions of vision. Blurry vision with macular edema. New vessels bleed into the retina and block vision.

24
Q

What is the visual system best at detecting?

A

light/ dark differences (spatial contrast)

25
Q

How do ON center surround cells respond to luminance?

A

They increase their discharge rate to luminance increments in the receptive field center. If light hits the receptive field they click rapidly. If exposed to darkness they stop clicking. If the antagonistic field is exposed to light as well then they click at a level slightly higher than baseline.

26
Q

How do OFF center surround cells respond to luminance?

A

They increase their discharge to luminance decrements in the receptive field center. If light hits the receptive field they stop clicking completely. If the receptive field is hit by darkness then they fire rapidly. In the event of light on the antagonistic surround they click at a rate higher than baseline.

27
Q

How do ON center/ OFF surround work? Summary of receptive fields of ganglion cells

A

Light in center - excitation (depol)
Light in surround - inhibition (hyper)
An optimal excitatory stimulus is a spot of light against a dark background. An optimal inhibitory stiulus is a dark spot agains a light background.

28
Q

How do OFF center/ ON surround work? Summary of receptive fields of ganglion cells

A

Light in center - inhibition
Light in surround - excitation
An optimal excitatory stimulus is a dark spot against a light background. An optimal inhibitory stiulus is a light spot agains a dark background.

29
Q

What do center-surround antagonistic receptive fields allow for?

A

They emphasize the regions where there are differences in illumination (edges).

30
Q

Where are functional differences between ON and OFF center ganglion cells found?

A

At the synapse between photoreceptors and bipolar cells in the outer nuclear layer, the first synapse in the visual system.

31
Q

What do ON bipolar cells do?

A

They invert the synapse. In the example the cone hyperpolarizes and the BC depolarizes. If the cone depolarized the BC would hyperpolarize. ON BCs depolarize in response to light and hyper polarize in dark.

32
Q

What do OFF bipolar cells do?

A

The off pathway has a non inverting synapse. The sign doesn’t change it transmits the signal it gets. OFF BCs hyper polarize in response to light and depolarize in dark.

33
Q

What does graded depolarization of bipolar cells lead to?

A

An increase in glutamate release at their synapses and depolarization of the ganglion cell they contact.

34
Q

When differences in luminance are subtle or non-existent what can help add dimension?

A

Color adds a perceptual dimension.

35
Q

What are the three cone types? What color can each see best?

A

Short - Blue
Medium - Green
Long - yellow-green (used to be call red cone)

36
Q

Is retinal cone mosaic uniform? What effect does this have on color perception?

A

It is far from uniform in all regards. Large differences in the ratio of M and L cone types do not seem to have a severe impact on color perception.

37
Q

What causes common color blindness?

A

The complete absence of one or more cone classes. No L cones: protanope, no M cones: deuteranope, no S: tritanope (rare)

38
Q

Magnocellular cells (M cells) are useful for what?

A

They are large receptive fields. They provide good light and contrast sensitivity and temporal resolution (sensitive to motion). Not color sensitive. They are Large cells - receive input from a relatively large number of photoreceptors.

39
Q

Parvocellular cells (P cells) are useful for what?

A

Small receptive fields (midget cells). They provide high acuity and color sensitivity. Poor light and contest sensitivity. Opponent color receptive fields. They are red green color detectors.

40
Q

Konicellular cells (K cells) are useful for what?

A

Carry short wavelength information and do blue/ yellow differentiation.

41
Q

What are retinal ganglion cells?

A

The third cell type in the eye besides rods and cones is the retinal ganglion cell. Controls pupillary diameter and involved in sleep and learning