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

Question 1

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

Answer 1

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

Question 2

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

Answer 2

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

Question 3

what structures help to focus light?

Answer 3

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

Question 4

how are visual images projected onto the retina?

Answer 4

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

so everything will be criss-crossed and flipped

Question 5

what is accommodation?

Answer 5

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

Question 6

what is the near synkinesthetic response?

Answer 6

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

Question 7

what is emmetropia?

Answer 7

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

Question 8

what is hyperopia?

Answer 8

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!

Question 9

how do you fix hyperopia?

Answer 9

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

Question 10

what is myopia?

Answer 10

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!

Question 11

how do you fix myopia?

Answer 11

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

Question 12

what is astigmatism?

Answer 12

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

Question 13

how do you correct astigmatism?

Answer 13

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

Question 14

what is presbyopia?

Answer 14

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

Question 15

how do you fix presbyopia?

Answer 15

presbyopia = loss of lens elasticity

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

Question 16

what is the blood supply of the eye?

Answer 16

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

Question 17

what are the important branches of the ophthalmic artery?

Answer 17

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

Question 18

what is the retina?

Answer 18

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

Question 19

what is vertical transmission of the retina?

Answer 19

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

Question 20

what are Mueller cells?

Answer 20

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

Question 21

what are the 2 types of photoreceptors in the eyes?

Answer 21

1. rods

2. cones

Question 22

what are characteristics of rods?

Answer 22

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

Question 23

what are characteristics of cones?

Answer 23

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

Question 24

why does the fovea have the highest visual acuity?

Answer 24

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

Question 25

what is the function of the retinal pigment epithelium?

Answer 25

it provides metabolic support to the rods and cones!

Question 26

how is phototransduction achieved in a rod in the dark?

Answer 26

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

Question 27

how is phototransduction achieved in a rod in the light?

Answer 27

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

Question 28

what are the two important ganglion cells in the retina?

Answer 28

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

Question 29

what is the retinal nerve fiber layer?

Answer 29

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

Question 30

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

Answer 30

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

Question 31

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

Answer 31

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

Question 32

what often causes central retinal vein occlusions?

Answer 32

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

Question 33

what is a retinal detachment? what can cause it?

Answer 33

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 2. trauma 3. surgery cataract removal

Question 34

what is a scotoma?

Answer 34

an area of vision loss an island of decreased vision surrounded by normal vision

Question 35

what are the structures involved in the visual pathway?

Answer 35

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

Question 36

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

Answer 36

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

Question 37

what is the optic nerve?

Answer 37

it's made up of ~1.2 million retinal ganglion cell axons vs. the acoustic nerve only has ~31,000 axons

Question 38

what is the optic disc?

Answer 38

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

Question 39

which cell type myelinated the optic nerve?

Answer 39

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!

Question 40

what are the 4 parts of the optic nerve?

Answer 40

1. intraocular = optic nerve head 1. intraorbital = from back of the globe to the optic canal) 3. intracanalicular = traverse optic canal 4. intracranial slide 17

Question 41

what is the pathway of the optic nerve?

Answer 41

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

Question 42

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

Answer 42

it's the arterial supply specifically to the optic nerve head 1. posterior ciliary artiers 2. pail arteriole plexus 3. peripapillary choroid

Question 43

what is the first sign of MS?

Answer 43

optic neuritis = painful loss of vision

Question 44

what is dyschromatopsia?

Answer 44

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

Question 45

how do you test for a relative afferent pupillary defect?

Answer 45

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!!

Question 46

what is the optic chiasm?

Answer 46

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

Question 47

where is the optic chiasm located?

Answer 47

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

Question 48

what is the function of the lateral geniculate nucleus?

Answer 48

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

Question 49

what is the blood supply of the lateral geniculate nucleus?

Answer 49

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

Question 50

from which eye does the lateral geniculate nucleus receive information?

Answer 50

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

Question 51

where are the optic tracts located?

Answer 51

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

Question 52

what are the optic radiations?

Answer 52

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

Question 53

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

Answer 53

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

Question 54

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

Answer 54

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

Question 55

localization of visual field defect

Answer 55

slide 31 seriously go look and make sure you understand

Question 56

what is hemianopia?

Answer 56

blindness in ½ the visual field

Question 57

what does homonymous mean?

Answer 57

the same field of vision is involved in each eye if this is happened, then it's a lesion behind the chiasm!

Question 58

what things could cause prechiasmatic lesion?

Answer 58

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

Question 59

what things could cause a chiasmatic lesion?

Answer 59

sellar or suprasellar mass that's >10 mm this will cause a bitemporal hemianopsia!

Question 60

which arteries surround the optic chiasm?

Answer 60

1. internal carotid artery 2. anterior cerebral arteries 3. anterior communicating artery 3. posterior communicating arteries

Question 61

where is the lesion if you see homonymous defects?

Answer 61

retrochiasmatic!

Question 62

what could cause a right superior quadrantanopia?

Answer 62

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

Question 63

what would a left occipital lobe stroke cause?

Answer 63

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