ICL 7.2: CN II, Visual System, Pathways And Field Deficits Flashcards
what is the first structure involved in the visual pathway and what is its function?
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
what are the components of the ocular media of the eye?
- tear-film
- cornea
- anterior chamber
- lens
- posterior-chamber vitreous (jelly like substance in the eye)
what structures help to focus light?
- tear-air interface and cornea contribute more to the focusing of light onto the fovea than the lens
- ciliary muscles adjust lens shape in order to focus light optimally from varying distances upon the retina= accommodation
- pupils regulate the amount of light reaching the retina
how are visual images projected onto the retina?
visual images are projected upside-down and backwards on to the retina
so everything will be criss-crossed and flipped
what is accommodation?
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
what is the near synkinesthetic response?
the triad required for complete accommodation of the eyes
- accommodation = thickening lens to increase focusing power
- miosis = reduce pupil diameter to increase depth of focus
- 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
what is emmetropia?
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
what is hyperopia?
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!
how do you fix hyperopia?
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
what is myopia?
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!
how do you fix myopia?
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
what is astigmatism?
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
how do you correct astigmatism?
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
what is presbyopia?
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
how do you fix presbyopia?
presbyopia = loss of lens elasticity
a convex lens replaces the lost accommodative power, allowing light rays to focus on target
what is the blood supply of the eye?
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
what are the important branches of the ophthalmic artery?
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
what is the retina?
a mosaic of photoreceptors connected by interneurons to long-range transmitters, the retinal ganglion cells
what is vertical transmission of the retina?
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
what are Mueller cells?
glial cells of the eye that protect signal transmission by buffering extracellular ions and neurotransmitters
what are the 2 types of photoreceptors in the eyes?
- rods
2. cones
what are characteristics of rods?
photoreceptors in the eyes
- low light
- low resolution
- slow motion detection
- they outnumber the cones 20:1
- they are absent from the fovea; they have the highest concentration in the parafoveal region
what are characteristics of cones?
- color
- fine-detail detection
- heavily concentrated in the fovea
why does the fovea have the highest visual acuity?
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
what is the function of the retinal pigment epithelium?
it provides metabolic support to the rods and cones!
how is phototransduction achieved in a rod in the dark?
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
how is phototransduction achieved in a rod in the light?
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
what are the two important ganglion cells in the retina?
- parvocellular = small receptive field, color and form
- 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
what is the retinal nerve fiber layer?
axons of the retinal ganglion cells are what make up the retinal nerve fiber layer
what are the signs/symptoms of a central retinal artery occlusion?
- sudden painless loss of vision
- cherry-redspot in the fovea which indicates that the fovea has been spared since it’s nourished by the choroidal artery, not retinal arteries
- milky appearance of the retina
- afferent pupil defect
note: often seen in Tay Sachs
what is a central retinal vein occlusion and what are the signs/symptoms?
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
what often causes central retinal vein occlusions?
- HTN
- DM2
- hyper coagulable states (SLE)
what is a retinal detachment? what can cause it?
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
- trauma
- surgery cataract removal
what is a scotoma?
an area of vision loss
an island of decreased vision surrounded by normal vision
what are the structures involved in the visual pathway?
visual pathways receive, relay, and ultimately process visual information through:
- eyes
- optic nerves
- optic chiasm/tracts
- lateral geniculate nucleus of the thalamus
- optic radiations
- striate cortex
- extra striate association cortices
what are the two parts of the retina concerning visual fields?
- nasal retina
- 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
what is the optic nerve?
it’s made up of ~1.2 million retinal ganglion cell axons
vs. the acoustic nerve only has ~31,000 axons
what is the optic disc?
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
which cell type myelinated the optic nerve?
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!
what are the 4 parts of the optic nerve?
- intraocular = optic nerve head
- intraorbital = from back of the globe to the optic canal)
- intracanalicular = traverse optic canal
- intracranial
slide 17
what is the pathway of the optic nerve?
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
what is the arterial circle of zinn-haller and what are its components?
it’s the arterial supply specifically to the optic nerve head
- posterior ciliary artiers
- pail arteriole plexus
- peripapillary choroid
what is the first sign of MS?
optic neuritis = painful loss of vision
what is dyschromatopsia?
loss of color testing that is out of proportion to their vision
how do you test for a relative afferent pupillary defect?
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!!
what is the optic chiasm?
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
where is the optic chiasm located?
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
what is the function of the lateral geniculate nucleus?
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
what is the blood supply of the lateral geniculate nucleus?
- anterior choroidal artery (proximal branch of ICA) supplies medial and lateral horns of LGN
- posterior choroidal arteries (PCA branches) supply the hilum of the LGN
from which eye does the lateral geniculate nucleus receive information?
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
where are the optic tracts located?
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
what are the optic radiations?
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
what is the pathway of the optic radiation in the parietal lobe?
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
what is the pathway of the optic radiation in the temporal lobe?
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
localization of visual field defect
slide 31
seriously go look and make sure you understand
what is hemianopia?
blindness in ½ the visual field
what does homonymous mean?
the same field of vision is involved in each eye
if this is happened, then it’s a lesion behind the chiasm!
what things could cause prechiasmatic lesion?
- optic nerve sheath meningioma
- optic neuritis
- retinal detachment
- optic glioma
- malignant, metastatic and pseudotumors
- scotoma
- peripheral vision loss
what things could cause a chiasmatic lesion?
sellar or suprasellar mass that’s >10 mm
this will cause a bitemporal hemianopsia!
which arteries surround the optic chiasm?
- internal carotid artery
- anterior cerebral arteries
- anterior communicating artery
- posterior communicating arteries
where is the lesion if you see homonymous defects?
retrochiasmatic!
what could cause a right superior quadrantanopia?
a left temporal lobe lesion involving fibers from the superior field causes a “pie in the sky” deficit = contralateral superior quadrantanopia
what would a left occipital lobe stroke cause?
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
