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Ophthalmology > Physiology II > Flashcards

Physiology II Flashcards

1
Q

What is needed for an object to be seen?

A

The pattern of the object must fall on the vision receptors (rods and cones in retina)
Brain must receive and interpret signals

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2
Q

Why must the amount of light entering the eye be regulated?

A

Too much light will bleach out the signals

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3
Q

What must the energy from waves of photons be transduced into for an object to be seen?

A

Electrical signals

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4
Q

What is the direct (vertical) pathway for signal transmission?

A

Photoreceptors to bipolar cells and finally to ganglion cells (light travels in the opposite direction from this)

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5
Q

What is the role of horizontal cells in the retina?

A

Receive input from photoreceptors and project to other photoreceptor and bipolar cells

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6
Q

What is the function of the amacrine cells in the retina?

A

Receive input from bipolar cells and project to ganglion cells

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7
Q

What do photoreceptors do?

A

Convert electromagnetic radiation to neural signals (transduction)

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8
Q

What are the four main regions of photoreceptors?

A

Outer segment, inner segment, cell body, synaptic terminal

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9
Q

What are the photoreceptors of the eye?

A

Rods and cones

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10
Q

What is the resting membrane potential of vertebrate photoreceptors?

A

Depolarised = resting Vm is more positive compared to other neurons (-20mV)

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11
Q

What happens to Vm on light exposure?

A

It hyperpolarises

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12
Q

What causes positive Vm?

A

The dark current = cGMP-gated Na channel that is open in the dark and closed in the light

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13
Q

What signal enables the brain to perceive objects in the visual field?

A

Change in Na due to closing/opening of dark current channel

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14
Q

How is the dark current modulated in the dark?

A

PNa = PK (Na channels in outer segment) = Vm between ENa and EK

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15
Q

How is the dark current modulated in the light?

A

PNa is reduced (outer segment channels close), PK > PNa, Vm > EK so hyperpolarise, change is local and graded

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16
Q

What are the visual pigment molecules?

A

Rhodopsin = retinal (vitamin A derivative) + opsin (G-protein coupled receptor)

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17
Q

Where is rhodopsin present?

A

In membrane folds (called discs in outer segment)

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18
Q

What effect does light have on 11-cis-retinal?

A

Converts it to its active form = all-trans-retinal

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19
Q

What does all-trans-retinal activate?

A

Transducin = causes molecular cascade which decreases cGMP, closing cGMP-gated Na channels

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20
Q

What does lowered Na entry result in?

A

Hyperpolarisation

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21
Q

How is transduction said to be a high gain process?

A

1 molecule of opsin gives 1000 molecules of transducin

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22
Q

What opens the dark current channel?

A

Binding of cGMP in response to light

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23
Q

What ion is the dark current channel permeable to?

A

Na = keeps photoreceptor Vm more positive that most neurons

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24
Q

What does the dark current channel ensure?

A

That there is a steady release of neurotransmitter

25
Q

What is the relationship between glutamate and the levels of light?

A

More glutamate in the dark and less in the light

26
Q

What is visual acuity?

A

Ability to distinguish two nearby points = determined largely by photoreceptor spacing and refractive power

27
Q

What kind of light are rods and cones used to see in?

A

Rods used for dim light

Cones used for normal daylight

28
Q

What is the convergence of the rod system?

A

High convergence = large spacing (low density), large ganglion cells

29
Q

What is the convergence of the cone system?

A

Low convergence = high density, small ganglion cells

30
Q

What is the benefit of the higher convergence of the rod system?

A

Increases sensitivity (however, decreases acuity)

31
Q

What does light comprise of?

A

Discrete wavelengths

32
Q

Why can humans not see UV or infra-red light?

A

Our photoreceptors are only activated in a small portion of the spectrum of electromagnetic waves

33
Q

Which opsins respond to particular wavelengths of light?

A

Short wave cone = blue light
Medium wave cone = green light
Long wave cone = red light

34
Q

What are some features of rods?

A

Achromatic, found in peripheral retina, high convergence, high light sensitivity, low visual acuity

35
Q

What are some features of cones?

A

Chromatic, found in central retina (fovea), low convergence, low light sensitivity, high visual acuity

36
Q

What does the visual system detect?

A

Local differences in light intensity (not absolute amounts of light)

37
Q

What facilitates the detection of local differences in light intensity?

A

Physiological and structural adaptions on the retina

38
Q

What is the monocular visual field?

A

+/- 45 degrees = each eye sees part of visual space

39
Q

Why do the visual fields of each eye overlap extensively?

A

To provide a binocular visual field (+/-45 degrees)

40
Q

Where is the retina divided in half?

A

Relative to the fovea = creates nasal and temporal hemiretina

41
Q

What do the nerve fibres from the nasal half of the retina cross?

A

The optic chiasm

42
Q

What do the resulting two optic tracts allow for?

A

Right and left visual fields to reach the right and left hemispheres separately = 60% (nasal retina) cross and 40% (temporal retina) do not

43
Q

What is the striate cortex?

A

Part of the visual cortex that processes visual information

44
Q

Where is the visual field mapped?

A

In the retina, LGN, superior colliculus and cortex

45
Q

Why is the central visual field over-represented?

A

Magnification factor not constant

46
Q

Why is a discrete point of light able to activate many cells in the target structure?

A

Due to overlapping receptive fields

47
Q

What is perception based on?

A

The brain’s interpretation of distributed patterns of activation

48
Q

Where do the signals form the right and superior visual fields go?

A 
Right = left cortex
Superior = lower cortex
49
Q

What happens to eye inputs in the primary visual cortex?

A

Eye specific inputs are segregated in layer 4

50
Q

What occurs in the primary visual cortex?

A

Both eyes project to each visual cortex but in the primary visual area they remain largely segregated into ocular dominance columns

51
Q

Where do cells outside of layer 4 of the primary visual cortex receive input from?

A

Receive input from both eyes

52
Q

What shapes visual perception?

A

Shaped by early experience

53
Q

What do children with congenital cataracts struggle to see even after corrective surgery?

A

Have difficulty perceiving shape and form

54
Q

What is amblyopia?

A

Cortical blindness caused by a variety of disorders where there is no issue in the eye itself but one eye has better vision than the other

55
Q

What is one cause of amblyopia?

A

Uncorrected wandering eye (strabismus)

56
Q

What is Hebb’s postulate?

A

When axon of cell A is near enough to excite cell B and repeatedly takes part in firing it, some growth process/metabolic change occurs in one/both cells so that A’s efficiency as a firing cell of B increases

57
Q

What strengthens the connections between presynaptic and postsynaptic cells?

A

Correlated activity between them = cells that fire together, wire together

58
Q

What does Hebb’s postulate apply to in memory?

A

Also used in learning and memory, where it is called long term potentiation (LTP)

59
Q

What effect does monocular deprivation have on LGN axons?

A

Can rapidly change the terminal arborisations = lack of activity leads to less branching

Ophthalmology (20 decks)

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