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Neuro Week 4 > ICL 7.2: CN II, Visual System, Pathways And Field Deficits > Flashcards

ICL 7.2: CN II, Visual System, Pathways And Field Deficits Flashcards

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1
Q

what is the first structure involved in the visual pathway and what is its function?

A

the eye

it collects and focuses light –> light travels through the ocular media and onto the retina

the retina then transmits the information to the optic nerve

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2
Q

what are the components of the ocular media of the eye?

A
  1. tear-film
  2. cornea
  3. anterior chamber
  4. lens
  5. posterior-chamber vitreous (jelly like substance in the eye)
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3
Q

what structures help to focus light?

A
  1. tear-air interface and cornea contribute more to the focusing of light onto the fovea than the lens
  2. ciliary muscles adjust lens shape in order to focus light optimally from varying distances upon the retina= accommodation
  3. pupils regulate the amount of light reaching the retina
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4
Q

how are visual images projected onto the retina?

A

visual images are projected upside-down and backwards on to the retina

so everything will be criss-crossed and flipped

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5
Q

what is accommodation?

A

it’s when the ciliary muscles contract to allow the lens to achieve its natural state – the lens in its natural state is a little rounded

so contraction of the circular ciliary muscle decreases its diameter to reduce tension on zonular fibers that attach to the circumference of the lens – the slack in the zonular fibers allows the lens to thicken and assume its natural shape, which provides increased focusing/refractive power needed for shorter working distances

this dynamic change in the focusing power of the crystalline lens is known as accommodation

conversely, relaxation of the ciliary muscle increases its diameter and stretches the zonular fibers, causing the lens to become thinner, thus, reducing its focusing power to allow for distant focus

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6
Q

what is the near synkinesthetic response?

A

the triad required for complete accommodation of the eyes

  1. accommodation = thickening lens to increase focusing power
  2. miosis = reduce pupil diameter to increase depth of focus
  3. convergence = disconjugate movement of the eyes towards each other so that near object falls on fovea of both eyes

when you’re looking at something distant on the other hand, your eyes diverge

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7
Q

what is emmetropia?

A

the state of vision in which a faraway object at infinity is in sharp focus with the eye lens in a neutral or relaxed state

so the eye’s focusing elements and its axial length are perfectly balanced

parallel light rays emanating from objects viewed at a distance of 20 feet or more are focused on the retina at the fovea!

this is what we’re trying to achieve with glasses! so only 30% of the population has emmetropia and the other 70% have ametropia

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8
Q

what is hyperopia?

A

objects focus behind the retina because the eye is too short

so the refractive power of the lens of the eye is deficient relative to its short length so light focuses behind retina

this is far sightedness!

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9
Q

how do you fix hyperopia?

A

hyperopia = objects focus behind the retina because the eye is too short = far sightedness

so you use a convex lens to increase the refractive power and focus the light more anteriorly onto the retina!

if you look through their glasses and their head seems to widen, then they’re hyperops

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10
Q

what is myopia?

A

objects focus in front of the retina because the eye is too long

the refractive power of the lens is too great relative to the length of the eye so light gets focused in front of the retina

aka near sightedness!

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11
Q

how do you fix myopia?

A

myopia = objects focus in front of the retina because the eye is too long = near sightedness

so you use a concave lens to reduce the refractive power of the lens and get the light to focus posteriorly on the retina

if you look through their glasses and their head seems narrow, they’re myops

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12
Q

what is astigmatism?

A

distorted vision from irregularly shaped cornea and as a result you have blurry vision

the corneal curvature is warped so light in 2 different planes does not come to a point focus on retina

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13
Q

how do you correct astigmatism?

A

astigmatism = the corneal curvature is warped so light in 2 different planes does not come to a point focus on retina

cylindrical lens refracts light in only one plane (horizontal), thus light rays can focus on retina

so for example, you can bend the light all in the horizontal direction so then they meet up and focus on the retina in the same place

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14
Q

what is presbyopia?

A

loss of lens elasticity

lens protein stiffens, so that even with ciliary muscle contraction (and loosening of the zonules), the lens cannot assume its natural shape

all of us will get this, you see your parents pulling the newspaper farther away until they have to get glasses because they can’t see stuff at reading distance –> so basically everybody becomes far sighted

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15
Q

how do you fix presbyopia?

A

presbyopia = loss of lens elasticity

a convex lens replaces the lost accommodative power, allowing light rays to focus on target

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16
Q

what is the blood supply of the eye?

A

ophthalmic artery which is the 1st branch of the internal carotid artery in the cavernous sinus

it enters the optic canal and travels with the optic nerve

it then gives rise to two groups of vessels: those who supply the globe vs. those supplying other orbital structures

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17
Q

what are the important branches of the ophthalmic artery?

A

the ophthalmic artery is the 1st branch of the internal carotid artery in the cavernous sinus

it then gives rise to two groups of vessels: those who supply the globe vs. those supplying other orbital structures

the important one in those supplying the globe is the central retinal artery because it’s clinically really important – if you see a cherry red spot it’s usually due to loss of blood supply from the CRA

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18
Q

what is the retina?

A

a mosaic of photoreceptors connected by interneurons to long-range transmitters, the retinal ganglion cells

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19
Q

what is vertical transmission of the retina?

A

there is a network of horizontal cells and amacrine cells which enhance contrast between signals derived from adjacent retinal segments

on the top of the retina are the nerve fibers layer and the bottom is the rods and cones – then in-between there are the interconnections of the horizontal cells and amacrine cells that allow for communication between these two layers = vertical transmission

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20
Q

what are Mueller cells?

A

glial cells of the eye that protect signal transmission by buffering extracellular ions and neurotransmitters

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21
Q

what are the 2 types of photoreceptors in the eyes?

A
  1. rods

2. cones

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22
Q

what are characteristics of rods?

A

photoreceptors in the eyes

  1. low light
  2. low resolution
  3. slow motion detection
  4. they outnumber the cones 20:1
  5. they are absent from the fovea; they have the highest concentration in the parafoveal region
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23
Q

what are characteristics of cones?

A
  1. color
  2. fine-detail detection
  3. heavily concentrated in the fovea
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24
Q

why does the fovea have the highest visual acuity?

A

the fovea has a lot of the highest concentration of cones which are responsible for color and fine detail !

that’s why you’re trying to concentrate light onto the fovea

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25
Q

what is the function of the retinal pigment epithelium?

A

it provides metabolic support to the rods and cones!

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26
Q

how is phototransduction achieved in a rod in the dark?

A

Ca-Na channel is opened in the rod and cGMP is generated which creates an intracellular potential of -40 mV

this leads to glutamate being released

27
Q

how is phototransduction achieved in a rod in the light?

A

rhodopsin is converted to all-trans retinoid acid and opsin = aka bleaching

as a result, cGMP is converted to GMP and the Ca-Na channel is closed and the intracellular potential becomes more negative, -70 mV, and less glutamate is released

28
Q

what are the two important ganglion cells in the retina?

A
  1. parvocellular = small receptive field, color and form
  2. magnocellular = large receptive field, motion and depth

these are all or nothing responses!!

these are what communicate with rods and cones via interneurons aka the amacrine cells and horizontal cells

29
Q

what is the retinal nerve fiber layer?

A

axons of the retinal ganglion cells are what make up the retinal nerve fiber layer

30
Q

what are the signs/symptoms of a central retinal artery occlusion?

A
  1. sudden painless loss of vision
  2. cherry-redspot in the fovea which indicates that the fovea has been spared since it’s nourished by the choroidal artery, not retinal arteries
  3. milky appearance of the retina
  4. afferent pupil defect
    note: often seen in Tay Sachs
31
Q

what is a central retinal vein occlusion and what are the signs/symptoms?

A

it’s a blockage of flow in the vein that drains the inner retina

you’ll get a “blood and thunder” appearance in the eye from the blood pooling back into the eye = flame-shaped hemorrhages

32
Q

what often causes central retinal vein occlusions?

A
  1. HTN
  2. DM2
  3. hyper coagulable states (SLE)
33
Q

what is a retinal detachment? what can cause it?

A

detachment of the retina proper from the underlying pigmented choroid and you’ll end up with a monocular scotoma

you end up with a blood filled eye….

often caused by:
1. myopia = near sightedness

  1. trauma
  2. surgery cataract removal
34
Q

what is a scotoma?

A

an area of vision loss

an island of decreased vision surrounded by normal vision

35
Q

what are the structures involved in the visual pathway?

A

visual pathways receive, relay, and ultimately process visual information through:

  1. eyes
  2. optic nerves
  3. optic chiasm/tracts
  4. lateral geniculate nucleus of the thalamus
  5. optic radiations
  6. striate cortex
  7. extra striate association cortices
36
Q

what are the two parts of the retina concerning visual fields?

A
  1. nasal retina
  2. temporal retina

the fibers of the nasal retina cross through the optic chiasm but the fibers of the temporal retina do NOT

the left temporal retina is responsible for seeing things in the right visual field while the left nasal retina is responsible for seeing things in the left visual field

37
Q

what is the optic nerve?

A

it’s made up of ~1.2 million retinal ganglion cell axons

vs. the acoustic nerve only has ~31,000 axons

38
Q

what is the optic disc?

A

it’s the raised disk on the retina at the point of entry of the optic nerve

it lacks visual receptors and so creating a blind spot!!

it’s located 4 mm nasal to the fovea

39
Q

which cell type myelinated the optic nerve?

A

oligodendrocytes since it’s part of the CNS

the other CN are myelinated by Schwann cells so this tells you CN 2 is truly a structure of the brain!

40
Q

what are the 4 parts of the optic nerve?

A
  1. intraocular = optic nerve head
  2. intraorbital = from back of the globe to the optic canal)
  3. intracanalicular = traverse optic canal
  4. intracranial

slide 17

41
Q

what is the pathway of the optic nerve?

A

the optic nerve exits the eye, passes through the common tendinous ring and then through the optic canal to exit the orbit

it then passes through the optic chiasm which is located immediately superior the sella turcica of the sphenoid bone

42
Q

what is the arterial circle of zinn-haller and what are its components?

A

it’s the arterial supply specifically to the optic nerve head

  1. posterior ciliary artiers
  2. pail arteriole plexus
  3. peripapillary choroid
43
Q

what is the first sign of MS?

A

optic neuritis = painful loss of vision

44
Q

what is dyschromatopsia?

A

loss of color testing that is out of proportion to their vision

45
Q

how do you test for a relative afferent pupillary defect?

A

swinging flashlight test

light source to affected eye perceived as “relatively” dimmer compared to healthy eye, thus rather than constricting to light source, pupils dilate

when you flash a light on a pupil, you get constriction and a consensual response in the other eye –> if you have an abnormal eye where CN 2 if elected, it doesn’t perceive the eye the same; so when you flash a light in a good eye then to a bad eye, you’ll get constriction in the healthy eye abut then with the effected CN 2, the pupil will actually dilate!!

46
Q

what is the optic chiasm?

A

where the two optic nerves converge!

it is at the chiasm that more than ½ of the nasal fibers of the retina cross to reach the contralateral optic tract

47
Q

where is the optic chiasm located?

A

it’s located in suprasellar cistern, 10mm above pituitary gland in sella turcica!!

so a pituitary tumor would have to be pretty big to start effecting the optic nerve since it would have to be over 10 mm large; so there are plenty of times where there is a pituitary tumor that doesn’t have any associated visual field defects

48
Q

what is the function of the lateral geniculate nucleus?

A

it’s part of the thalamus that is a relay of the afferent pathway prior to arrival at the primary visual cortex –> most fibers from optic tracts synapse in ipsilateral LGN

49
Q

what is the blood supply of the lateral geniculate nucleus?

A
  1. anterior choroidal artery (proximal branch of ICA) supplies medial and lateral horns of LGN
  2. posterior choroidal arteries (PCA branches) supply the hilum of the LGN
50
Q

from which eye does the lateral geniculate nucleus receive information?

A

most fibers from optic tracts synapse in ipsilateral LGN

so the left temporal hemiretina sends uncrossing fibers to the left lateral geniculate nucleus

but the right nasal hemiretina sends crossing fibers to the left lateral geniculate nucleus

51
Q

where are the optic tracts located?

A

the optic tracts extend from the optic chiasm posterolateral around hypothalamus and cerebral peduncles to the lateral geniculate nuclei of the thalamus

then the optic radiations are formed by the axons of the LGN projecting to the occipital lobe for visual processing

52
Q

what are the optic radiations?

A

the optic radiations are formed by the axons of the lateral geniculate nucleus of the thalamus projecting to the occipital lobe for visual processing

there are two bundles of the optic radiation in the parietal lobe and temporal lobe

53
Q

what is the pathway of the optic radiation in the parietal lobe?

A

it starts in the lateral geniculate nucleus and and takes a DIRECT non-looping course over the top of the lateral ventricles and then travel posteriorly through the parietal white matter to the superior calcarine cortex

the superior calcarine cortex subserves the INFERIOR visual fields

calcarine cortex = visual cortex

54
Q

what is the pathway of the optic radiation in the temporal lobe?

A

it starts in the lateral geniculate nucleus and course anteriorly before turning posteriorly toward the temporal pole in a sharp (Meyer’s) loop

the course continues around the lateral wall of the inferior horn of the lateral ventricle to the inferior calcarine cortex that subserves the SUPERIOR visual fields

calcarine cortex = visual cortex

55
Q

localization of visual field defect

A

slide 31

seriously go look and make sure you understand

56
Q

what is hemianopia?

A

blindness in ½ the visual field

57
Q

what does homonymous mean?

A

the same field of vision is involved in each eye

if this is happened, then it’s a lesion behind the chiasm!

58
Q

what things could cause prechiasmatic lesion?

A
  1. optic nerve sheath meningioma
  2. optic neuritis
  3. retinal detachment
  4. optic glioma
  5. malignant, metastatic and pseudotumors
  6. scotoma
  7. peripheral vision loss
59
Q

what things could cause a chiasmatic lesion?

A

sellar or suprasellar mass that’s >10 mm

this will cause a bitemporal hemianopsia!

60
Q

which arteries surround the optic chiasm?

A
  1. internal carotid artery
  2. anterior cerebral arteries
  3. anterior communicating artery
  4. posterior communicating arteries
61
Q

where is the lesion if you see homonymous defects?

A

retrochiasmatic!

62
Q

what could cause a right superior quadrantanopia?

A

a left temporal lobe lesion involving fibers from the superior field causes a “pie in the sky” deficit = contralateral superior quadrantanopia

63
Q

what would a left occipital lobe stroke cause?

A

right macular sparing homonymous hemianopia

so this is a posterior cerebral artery occlusion that caused a left occipital lobe stroke and the macula was spared because it has another blood supply

Neuro Week 4 (6 decks)

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