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Ophthalmology > Physiology > Flashcards

Physiology Flashcards

1
Q

What must occur for us to see?

A

Light must enter the eye- Not too much or little
Light must fall on photoreceptors
Photoreceptors must generate current
Brain must interpret electrical signals

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2
Q

Where are photoreceptors found?

A

In the inner layer of the eye- Retina

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3
Q

Where in the laminar structure of the retina are the photoreceptors found?

A

Deep

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4
Q

What is found on top of the photoreceptors?

A

Bipolar cells then ganglion cells

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5
Q

What cells link photoreceptor cells with each other and and bipolar cells?

A

Horizontal cells

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6
Q

What cells link bipolar cells with each other and ganglion cells?

A

Amacrine cells

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7
Q

What must light pass through to reach the photoreceptors?

A

Pupil, lens, posterior chamber of the eye, ganglion cells, bipolar cells, amacrine cells and horizontal cells

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8
Q

What kind of photoreceptors do we possess?

A

Cones

Rods

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9
Q

What do photoreceptors do?

A

Transduce electromagnetism to neuronal signals

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10
Q

What are the 4 main regions of photoreceptors?

A

Synaptic terminals
Cell bodies
Inner segment
Outer segment

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11
Q

Where does signal transduction occur in photoreceptors?

A

Outer segment

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12
Q

In what type of light do rods help us see?

A

Dim/dark

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13
Q

In what type of light do cones help us to see?

A

Bright

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14
Q

Describe the basal state of photoreceptors

A

Marginally depolarised (-20mV)

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15
Q

What happens when photoreceptors are exposed to light?

A

Becomes hyperpolarized

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16
Q

What is the dark current?

A

The constantly depolarised state photoreceptors experience in the dark

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17
Q

What causes the dark current?

A

An open Na channel

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18
Q

Describe the dark current

A

Continuously open Na channels depolarise photosensitive cell to between Na and K Vm (-20mV)

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19
Q

What does this dark current result in?

A

Constant neurotransmitter (glutamate) release

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20
Q

How does light switch off the dark current?

A

Light activates GPCR rhodopsin.
Rhodopsin activates PDE through GMP
PDE hydrolyses cGMP thus reducing its conc
Decrease in cGMP levels close cGMP dependent Na channels leading to hyperpolarization of cell

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21
Q

Is phototransduction high or low gain?

A

High gain

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22
Q

What is rhodopsin?

A

A photosensitive GPCR

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23
Q

What is rhodopsin made of?

A

Retinal (derived from Vit A)

Opsin (GPCR)

24
Q

Where is rhodopsin found?

A

Disks in the outer segment

25
Q

What happens to Rhodesian when light hits it?

A

Converted to all-trans-retinal

26
Q

What two pathways are possible after the photosensitive cell in sight?

A

On pathway

Off pathway

27
Q

What happens in the On Pathway?

A

Glutamate from photosensitive cell activates metabotropic R (GPCR) on bipolar cells leading hyperpolarisation.
In turn this then leads to hyperpolarisation of ganglion cell and no AP
When light reduced glutamate levels AP can be generated

28
Q

What happens in the Off Pathway?

A

Glutamate from photosensitive cell activates ionotropic R on bipolar cells leading to depolarisation.
In turn this then leads to depolarisation of ganglion cell and generation of AP
When light reduced glutamate levels AP generation is inhibited

29
Q

What affects visual acuity?

A

Density of photosensitive cells

30
Q

What increases visual acuity?

A

Increasing density of photosensitive cells and convergence

31
Q

What is convergence?

A

The number of photosensitive cells that synapse onto a single ganglion cell

32
Q

Do cones or roads have higher convergence?

A

Rods

33
Q

What is high convergence?

A

Many photosensitive cells synapsing onto one ganglion cell

34
Q

What is low convergence?

A

One or two photosensitive cells synapsing onto one ganglion cells

35
Q

Why are rods high convergence?

A

Because they’re activated in low light therefore require more input to activate an AP. Only able to fire an AP by pooling signals from many photosensitive cells

36
Q

Why does acuity decrease as convergence increases?

A

Because the receptive fields from many photosensitive cells are being combined therefore cannot distinguish between them

37
Q

Where is the greatest density of cones

A

In the macula of the eye (0 degrees)

38
Q

Where are the greatest density of rods found?

A

Between 10 and 70 degrees (+/-)

39
Q

Where is degree 0 of the visual field?

A

Macula

40
Q

How many types of cone do we have?

A

3

41
Q

What colour light do short wave cones tend to see?

A

Blue

42
Q

What colour light do medium wave cones tend to see?

A

Green

43
Q

What colour light do long wave cones tend to see?

A

Red

44
Q

How does the lens change to focus on far away objects?

A

Tall and flat

45
Q

How does the lens change to focus on nearby objects?

A

Short and flat

46
Q

What is hyperopia?

A

Farsightedness- trouble seeing nearby objects as focus behind retina

47
Q

What is myopia?

A

Nearsightedness- trouble seeing far away objects as focus in posterior chamber

48
Q

What does hyperopia mean?

A

‘Oversight’

49
Q

What does myopia mean?

A

Shut eyes

50
Q

Do the eyes detect absolute light or difference in light?

A

Difference in light/contrast

51
Q

Describe the optic tracts

A

Nasal tracts cross

Temporal tracts do not

52
Q

Where do the optic tracts synapse in the brain?

A

Lateral geniculate nucleus

53
Q

How can temporary loss of vision in one eye during development lead to loss of binocular vision?

A

Axons from both eyes normally compete and provide binocular vision
When one of these is lost the other takes over and provides all input

54
Q

Where is visual information processed in the brain

A

Primary visual cortex layer 4

55
Q

What pathology does a lesion in the optic tract just behind the eye cause?

A

Monocular blindness

56
Q

What pathology does a lesion in the optic tract at the optic chiasma cause?

A

Bitemporal hemianopsia

57
Q

What pathology does a lesion in the optic tract after the optic chiasm cause?

A

Contralateral hemianopsia

Ophthalmology (10 decks)

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