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Neuro Final Exam > Lecture 12 - Visual System > Flashcards

Lecture 12 - Visual System Flashcards

1
Q

What are the 3 concentric layers of the eyeball and what is each layer composed of?

A

1) Outer fibrous layer - sclera (white) and cornea (transparent) –> participates in bending of light for retina
2) Middle, vascular layer (uvea) - iris, ciliary body, choroid –> changes size of lens (distance)
3) inner layer - Retina

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2
Q

What does the choroid contain? And what animal is it absent in?

A

tapetum lucidum in most domestic animals; tapetum lucidum is absent in the pig

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3
Q

What lines the iris, ciliary body and choroid?

A

the pigmented epithelium of the retina

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4
Q

Where does the functional optic part of the retina lie?

A

lines the fundus to the level of the ora serrata

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5
Q

What does the retina consist of?

A

10 histological layers

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6
Q

What forms the Blood occular barrier? and what’s its fn? What does breakdown of it lead to?

A

1) formed by retinal pigmented epithelium and endothelium of retinal capillaries (lie within the nerve fiber layer of retina)
2) limits access of substance in plasma to retina
- breakdown leads to uveitis

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7
Q

What is the fundus?

A

interior part of the eye –> retina, choroid and optic disk

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8
Q

What does the fundus reveal to us?

A

1) appearance of optic disk
2) appearance of vasculature
3) reflectivity of tapetum lucidum
4) retina attachment

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9
Q

Describe appearance of tapetum lucidum and its function?

A

it’s shiny colored part of choroid; makes animal’s eyes shine and gives animal the ability to see in dim light by offering extra reflection of light

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10
Q

1) What did retina develop from?
2) what is the optic nerve histologically?
3) What did the optic part of retina develop from?
4) What does the outer wall become?

A

1) optic cup of diencephalon
2) a CNS tract
3) from inner wall of optic cup
4) pigmented epithelium

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11
Q

Name the 10 histological layers of retina in order

A

1) pigmented epithelium
2) rods and cones
3) external lining membrane (ELM)
4) outer nuclear layer
5) outer plexiform layer
6) inner nuclear layer
7) inner plexiform layer
8) ganglion cell layer
9) optic nerve fibers
10) internal limiting membrane (ILM)

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12
Q

Where are photons located? And what must happen for photons to absorb light?

A

1) outer segments of rods and cones

2 ) light must penetrate 8 of the layers to reach this layer where photons are

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13
Q

What surrounds outer segments of rods and cones?

A

processes of pigmented epithelial cells

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14
Q

Trace the path of axons of ganglion cells

A

Start as nonmyelinated axons taht run to the optic disk and then exit eyeball as myelinated axons that comprise the optic nerve

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15
Q

What is the optic disc and what is absent there? What enters at this spot?

A

1) a blind spot
2) photoreceptor cells are absent
3) retinal vessels enter here and course along retinal surface

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16
Q

Which part of retina has the highest visual acuity?

A

area centralis (visual streak)

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17
Q

What does increased reflectivity of tapetum lucidum suggest?

A

retinal damage

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18
Q

T or F: appearance of optic nerve can change if there’s inflammation

A

True

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19
Q

Name the cell types in the retina?

A

photoreceptors, bipolar cells, ganglion cells, horizontal cells, amacrine cells, radial glial cells

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20
Q

What do photoreceptors do? When are they excited/inhibited?

A

transduce light E to neural electrical activity.

2) excited (depolarized) in dark : Na+/Ca+ channels open (cGMP) –> continual depolarization –> release of glutamate
3) inhibited (hyperpolarized) by light : degradation of cGMP –> channels close

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21
Q

What do the bipolar cells do? When are they ON or OFF? What mechanism is involved in each state?

A

receive input from photoreceptors and synapse on ganglion cells (and some amacrine cells). They are depolarized (ON) or hyperpolarized (OFF) in response to light

2) photoreceptors (dark) –> glutamate –> activation of excitatory channels –> depolarizes OFF-bipolar cells OR glutamate –> activation of inhibitory G-protein coupled receptors –> hyperpolarizes ON-bipolar cells
3) light: less glutamate released –> decreased inhibition of ON-bipolar cells and decreased excitation of OFF-bipolar cells

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22
Q

What do ganglion cell axons form? What is unique about their fn?

A

leave the retina and form optic nerve

generate APs

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23
Q

What is the rate of firing affected by in ganglion cells?

A

increased or decreased by visual stimuli

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24
Q

What are horizontal cells always? What do they do? What is their primary responsibility?

A

inhibitory

2) have interactions with photoreceptors and modulate activity of bipolar cells indirectly
3) lateral inhibition (inhibition as a result of activity in neighboring cells)

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25
Q

What do amacrine cells do? What do they respond to?

A

often inhibitory neurons that make synaptic contact with bipolar and ganglion cells
2) some to onset/offset of light; others to direction of light movement

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26
Q

How are amacrine cells involved in brain control of retinal activity?

A

optic nerve contains efferent axons that synapse on amacrine cells

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27
Q

What is another name for radial glial cells? What is their fn?

A

Mueller cells - modified astrocytes that provide structural and metabolic support –> take up excess ions and NTs mols to maintain homeostasis

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28
Q

What do radial glial cell processes form?

A

ILM and ELM

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29
Q

What are the two populations of photoreceptors?

A

rods and cones

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30
Q

What do the outer segments of rods and cones contain? What do the discs contain?

A

stacked membranous discs that are continually produced, sloughed and phagocytized by pigmented epithelium
2) photosensitive molecule rhodopsin (retinal + protein) that intercepts photons

31
Q

T or F: photoreceptors generate APs

A

False; they do NOT generate APs

32
Q

How does signal transduction work?

A

photoreceptor cells respond to visual stimuli –> graded depolarizations (excitation) and hyperpolarization (inhibition) –> proportional release of glutamate NT

33
Q

What do rods form? What does this result in?

A

convergent circuits with bipolar cells –> improves vision in dim light but at expense of image resolution

34
Q

What do cones form? What does this result in?

A

form relay circuits with bipolar cells –> good visual detail but requires bright light

35
Q

What are the main features of rods?

A

1) 95% of photoreceptors
2) widely distributed thru out retina
3) single population all containing rhodopsin
4) no color sensitivity
5) fnal in dim light (saturates in daylight)
6) highly convergent circuits
7) low spatial resolution
8) high sensitivity - can respond to a single photon (many rods –> one bipolar cell)

36
Q

What are the main features of cones?

A

1) 5% of photoreceptors
2) concentrated in area centralis (moer visual acuity)
3) multiple populations –> based on diff wavelength (color) sensitivities due to protein differences
4) bright light conditions
5) relay circuits (few cones per ganglion cell)
6) lower sensitivity - need hundreds of photons for response
7) 1 cone –> 1 bipolar cell –> 1 retinal gangion

37
Q

What are the two types of bipolar cells?

A

1) excited by light –> inhibitory glutamate receptors

2) excited by dark –> excitatory glutamate receptors

38
Q

Why is processing of visual stimuli in retina organized the way it is?

A

to optimize detection of contrast

39
Q

What type of visual information is most useful to an animal? Ex?

A

pattern of contrast in visual scene

Ex: contrast at edges of object defines its shape and motion involves both spatial and temporal contrast

40
Q

What cells are involved int eh vertical pathways of processing? What’s the fn?

A

photoreceptors –> bipolar cells –> ganglion cells

2) specialized to detect changes in contrast

41
Q

What cells are termed “ON” cells?

A

bipolar and ganglion cells specializing in detecting increases in brightness/illumination (bright objec ton dark background)

42
Q

What cells are termed “OFF” cells?

A

bipolar and ganglion cells specializing in detecting decreases in brightness/illumination (dark object on bright background)

43
Q

What are the cells in the lateral pathway? Whats the fn?

A

horizontal and amacrine cells; contribute to contrast enchancement

44
Q

What is detection of contrast based on?

A

organization of receptive fields

45
Q

What is the receptive field? Describe receptive field of retinal ganglion cell

A

region of retina that must be illuminated in order to obtain a response in a visual neuron
2) round and have a center and a surround with antagonistic properties –> responses in center are mediated by vertical pathway; responses in surround based on lateral inhibition mediated by lateral pathway

46
Q

T or F: receptive fields of ganglion cells are overlapping

A

True - so that every point of retinal surface is analyzed by several on-center and off-center ganglion cells

47
Q

Where are center-surround receptive fields first established?

A

at the level of bipolar cells and persist at CNS levels of visual processing (lateral geniculate nucleus and visual cortex)

48
Q

Describe responses of ON-center and OFF-center ganglion cells

A

ON-center cells –> inc rate of firing when center is illuminated and dec when surround is illuminated
OFF-center cells –> dec rate of firing when center is illuminated and inc when surround is illuminated

49
Q

What is the firing rate of ganglion cell a measure of?

A

the contrast btw illumination of center and surround

50
Q

What is paralled processing and what does it involve?

A

involves parallel pathways used to process info about different features of visual scene (form, motion, color)
-originate in retina from fnally distinct classes of ganglion cells

51
Q

What are the two fnal classes of primate ganglion cells?

A

M (magni, large) and P (parvi, small)
M cells - larger receptive fields and signal motion, position and depth
P cells - small receptive fields and info about fine detail and color

52
Q

What do parallel pathways remain separate from?

A

retina to cortex

53
Q

Where does retinotopic mapping occur?

A

lateral geniculate nucleus and primary visual cortex; lost at level of association cortex
-visual field is mapped onto retina

54
Q

What is hierarchial processing?

A

receptive fields become larger and more complex at each level

55
Q

How many populations of cones are there? therefore humans are ………..?
What does color vision in dogs compare to? therefore dogs are …………?

A

3; trichomatic

2) people who are red-green color blind; dichromatic –> see blue and yellow but not green or orange-red

56
Q

two populations of cones found in which species? nocturnal animals are what?

A

cat and pig; completely color blind (rat, hampster)

57
Q

What is non-image forming vision?

A

mediated by intrinsically photosensitive retinal ganglion cells (special kind of ganglia that are pigmented) –> contain melanopsin (sensitive to light at dusk and dawn)

  • detect ambient light
  • involved in circadian clock
58
Q

T or F: all types of retinal neurons are spontaneously active

A

true

59
Q

What are the two visual pathways?

A

conscious pathway: retina –> lateral geniculate –> cortex
reflex pathway: retina –> rostral colliculus (eye, ear and head turning to orient to a visual stimulus)
retina –> pretectal region (pupil size regulation to compensate for light intensity)

60
Q

What is optic nerve composed of?

A

axons from retinal ganglion cells

61
Q

What happens at the optic chiasm?

A

decussation of optic nerve axons; in some species % of axons from lateral side of each retina do NOT cross

62
Q

What composes optic tract?

A

axons from both eyes –> conveys contralateral visual field info (axons from lateral part of retina of ipsilateral eye and medial/central parts of retina of contralateral eye)

63
Q

What is binocular vision important for? What does it require?

A

depth perception

1) visual field overlap –> ind objects viewed simultaneously by both eyes
2) corresponding ganglion cells in each eye –> axons –> same optic tract
3) visual cortex in one cerebral hemisphere receives info about object from both eyes –> some columns monitor stimulation in corresponding loci of two eyes

64
Q

Describe conscious visual pathway

A

optic tract fibers synapse in lateral geniculate nucleus –> axons sent into optic radiation of internal capsule –> primary visual cortex

65
Q

What appearance does primary visual cortex give? How is it involved?

A

exhibits typical columnar organization of neocortex

  • cell column within visual cortex –> excited in response to light-dark boundaries oriented at certain angle, moving in a certain direction –> affect either or both eyes
  • some columns activated by particular colors
66
Q

Where is the association cortex and why is it imp?

A

surrounds primary visual cortex and req’d to associate meaning and significance to elements of primary image
-2 sep visual integrations

67
Q

What are the 2 separate visual integration?

A

1) phylogenetically older “where” system - analyzes motion and depth
2) phylogenetically newer “what” system - analyzes form and color

68
Q

What does damage to two visual systems produce?

A

1) older: failed ocular pursuit of moving target
- poor depth perception
- deficient visually guided movements
- deficits in visual attention
2) loss of color vision
- impaired pattern recognition (face/object recognition)

69
Q

Describe pathway of reflex visual pathways

A

axons leave optic tract –> brachium of rostral colliculus –> reach two visual reflex centers (rostral colliculus and pretectal region)

70
Q

What does rostral colliculus fn in?

A

orientation of eye, ear and head turning to sudden, prominent visual stimulus (subconscious eye movements)
-axons of RC –> appropriate motor nuclei via tectobulbar and tectospinal tracts

71
Q

What does the pretectal region fn in?

A

pupil size regulation to compensate for light intensity
-fibers decussate in caudal commissure –> axons –> parasympathetic nucleus of occulomotor nerve (pupillary constriction)

72
Q

Why is the PLR important

A

for diagnosing lesion in visual pathways

73
Q

Is there more or less overlap of L and R visual fields in horses vs dogs/humans?
will most axons cross (decussate) in horses?
what happens in dogs?

A

1) less
2) yes
3) fewer cross and remain on one side

74
Q

T or F: nasal retina sees visual field

A

true

Neuro Final Exam (6 decks)

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