1b// Visual System Flashcards
when are tears produced by the lacrimal gland?
basal, reflex and emotional responses
Label.
Label.
What are the layers of the eye and describe them?
Sclera - hard and opaque aka white of eye
Choroid - pigmented and vascular
Retina - neurosensory tissue
What is the role of the sclera and what does it have a lot of?
tough, opaque tissue that serves as the eye’s protective outer coat
high water content
What is the vascular coat of the eyeball called and where does it lie?
Choroid and lies between the sclera and retina.
Waht are the parts of the uvea?
Composed of three parts – iris, ciliary body and choroid.
Intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree.
What is the role of thhe retina?
Very thin layer of tissue that lines the inner part of the eye.
Responsible for capturing the light rays that enter the eye. Much like the film’s role in photography.
These light impulses are then sent to the brain for processing, via the optic nerve.
What is the role of the optic nerve?
transmits electrical impulses from the retina to the brain
connects to the back of the eye near the macula
visible portion is called the optic disc
What is the macula and what is it’s role? And what is the centre of the macula?
Located roughly in the centre of the retina, temporal to the optic nerve
A small and highly sensitive part of the retina responsible for detailed central vision
The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.
What is the “blind spot”?
Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot
What is the corresponding anatomic landmark for the physiological blind spot?
Optic Disc
what is the cornea?
transparent, dome shaped window covering front of eye
low water content
provides 2/3 of focusing power
what is the choroid?
lies between retina and sclera
layers of blood vessels
what is the iris?
muscular to dilate and constrict the pupil size
controls light levels inside the eye
what is the role of the fovea?
appreciates detail and focuses central vision
What are the types of vision? (2)
central and peripheral
What does the fovea have high concentration of?
Fovea has the highest concentration of cone photoreceptors
what is central vision?
detail day/colour vision
reading, facial recognition
by fovea of macula
what is the fovea? what does it have a high concetration of?
most sensitive part of retina - centre of macula
highest concentration of cones, low concentration of rods
what is peripheral vision?
shape, movement, navigation and night vision
how is central vision tested?
visual acuity assessment
loss of foveal vision= poor visual acuity
how is peripheral vision tested?
visual field assessment
what happens with loss of central vision?
poor visual acuity e.g reading
what happens with loss of peripheral vision?
inability to navigate environment, patient may need white stick even with perfect visual acuity
what is the overall structure of the retina?
outer - photoreceptors (1st order neuron)
middle - bipolar cells (2nd order)
inner - retinal ganglion cells (3rd order)
what is the function of retinal photoreceptors?
detection of light
what is the function of bipolar cells of the retina?
local signalling processing to improve contrast sensitivity and regulate sensitivity
transmits from photoreceptors to retinal ganglion cells
what is the function of retinal ganglion cells of the retina?
transmission of signal from eye to brain
what are the classes of photoreceptors?
rods and cones
what are rods? and what do they contain?
long outer segment
contains photo-sensitive pigment
slow response to light and responsible for NIGHT vision (scotopic vision)
100x more sensntive to light than cones
which is more abundant, rods or cones?
rods 120mil to 6 mil cones
compare cone cells to rod cells.
cones are…
shorter outer segment
less sensitive to light as rods but faster response
day, fine vision and colour (photopic vision)
which cones detect blue wavelengths?
s cones
which cones detect green light?
m cones
which cones detect red light?
L cones
What idea is refraction based on?
Refraction is based on the idea that light is passing through one medium into another.
As light goes from one medium to another, the velocity CHANGES
what is the index of refraction (n) and how is it calculated?
ratio of the speed of light before and after hitting a boundary
speed of light in vaccum (air)/speed of light in new medium
what happens when light meets a boundary?
some is reflected, some refracts through the boundary into the new medium
what are the two types of lenses and how do they differ?
convex - takes light rays at brings them to a point
concave - takes light rays and spreads them outwards
what is emmetropia?
basically normal, clear vision
adequate correlation between axial length and refractive power
parallel light rays fall on the retina - no accomodation
What is a refractive error of vision called? and what is it?
Ametropia
mismatch between axial length and refractive power
parallel rays dont fall on retina
what are the types of ametropia?
myopia (near sightedness)
hyperopia (far sighted)
presbyopia
what is myopia?
near-sightedness (far objects are blurry, close objects in focus)
Parallel rays converge at a focal point anterior to the retina
what are the causes of myopia?
excessive long globe (axial myopia) more common
excessive refractive power (refractive myopia)
What is the etiology of myopia?
not clear, genetic factor
what are the symptoms of myopia?
blurred distance vision
squint in an attempt to improve uncorrected visual acuity
headaches
how is myopia treated?
negative/diverging (concave) lenses
contact lenses
removal of lens
what is hyperopia? in terms of rays?
far sightedness (close objects blurry, distance fine) Parallel rays converge at a focal point posterior to the retina
what are the causes of hyperopia?
excessive short globe (axial hyperopia)
insufficient refractive power (refractive hyperopia)
What is the etiology of hyperopia?
not clear, inherited
what are the symptoms of hyperopia?
nature of blur ranges from inability to read fine print to clear near vision but is suddenly and intermittently blurry
bad reading vision
blurred vision is more noticeable if person is tired, printing is weak or light inadequate
asthenopic symptoms: eyepain, headache in frontal region, burning eyes
blepharoconjunctivitis, amblyopia if uncorrected
What is the near response triad?
adaptation for near vision
- Pupillary Miosis (Sphincter Pupillae) to increase depth of field
- Convergence (medial recti from both eyes) to align both eyes towards a near object
- Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision
how is hyperopia treated?
positive/converging lens (convex)
contact lenses
positive lens + cataract extraction
intraocular lens surgery
what is astigmatism?
parallel rays come to focus in two focal lines
hereditary condition
what are the causes of astigmatism?
non-spherical refraction media (cornea)
what are the symptoms of astigmatism?
headache, eye pain
blurred vision
distortion of vision
head tilting and turning
how is astigmatism treated?
cylindrical lenses, surgery
irregular astigmatism treated by rigid cylindrical lenses, surgery
What is naturally occurting loss of accomodation called? And what is its onset? And what is its vision like?
presbyopia
Naturally occurring loss of accommodation (focus for near objects)
Onset from age 40 years
Distant vision intact
how is presbyopia treated? and why?
Corrected by reading glasses (convex lenses) to increase refractive power of the eye
what are the drawbacks of contact lenses?
careful daily cleaning and disinfection needed
expense
risk of complications
what are the complications of contact lenses? (4)
infectious keratitis
giant papillary conjunctivitis
corneal vascularisation
severe chronic conjunctivitis
Which statement is false for Myopia?
* A) May be associated with large globe
* B) Light ray converges behind the retina
* C) May be associated with increased corneal curvature
* D) Unable to see objects clearly at distance without glasses or other optical correction
- B) Light ray converges behind the retina
In accommodation, which one of the following events does not take place?
* A) Relaxation of Circular Ciliary Muscle
* B) Relaxation of Zonules
* C) Thickening of Lens
* D) Increase of Lens Refractive Power
- A) Relaxation of Circular Ciliary Muscle
What are zonules?
the elaborate system of extracellular fibers that centers the lens in the eye
Where does the visual pathway transmit signals?
from eye to visual cortex
What are the visual pathway landmarks? (7)
eye
optic nerve
optic chiasm
optic tract
lateral geniculate nucleus
optic radiation
primary visual cortex or striate cortex
what retinal cells make up the optic nerve?
ganglion nerve fibres
What happens at the optic chiasm?
Optic nerves from both eyes converge at the optic chiasm, 53% decussate to contralateral optic tract
What happens at the optic tract?
Ganglion nerve fibres continuation
what is the role of the lateral geniculate nucleus?
(relay centre within thalamus)
– Ganglion nerve fibres synapse
where do the retinal ganglion cell fibres decussate?
53% decussate in the optic chiasm (nasal retina)
What does optic radiation form?
forms 4th order neuron, relay signal from the Lateral Geniculate Ganglion, to the Primary Visual Cortex
Where is the primary visaul/ striate cortex? And what happens there?
withinthe Occipital Lobe, relays to extra-striate cortex (higher visual processing)
What are teh 1st, 2nd and 3rd order neurones in the retina?
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second Order Neurons – Retinal Bipolar Cells
Third Order Neurons –Retinal Ganglion Cells
What is the visual pathway for the retina?
- Optic Nerve (CN II)
- Partial Decussation at Optic Chiasm– 53%of ganglion fibres cross the midline
- Optic Tract
- Destinations
- Lateral Geniculate Nucleus (LGN) in Thalamus – synapse with optic radiations (Fourth Order Neurons) – relay visual information to visual cortex
Where do crossed and uncrossed fibres in the optic chiasm originate from?
Crossed Fibres – originating from nasal retina,
responsible for temporal visual field
Uncrossed Fibres – originating from temporal
retina, responsible for nasal visual field
how do lesions anterior to the optic chiasm present?
affect visual field in one eye only
how do lesions posterior to the optic chiasm present?
affect visual field in both eyes
which fibres decussate at the optic chiasm?
nasal retina aka temporal visual field (due to image flipping)
which fibres don’t cross at the optic chiasm?
temporal retina aka nasal visual field (due to image flipping)
how does a lesion at the optic chiasm present?
temporal field deficit in both eyes - bitemporal hemianopia
damages crossed ganglion fibres from nasal retina in both eyes
how does a right sided lesion posterior to the optic chiasm present?
left homonymous hemianopia in both eyes
how does a left sided lesion posterior to the optic chiasm present?
right homonymous hemianopia in both eyes
what are the causes of bitemporal hemianopias?
enlargement of pituitary gland tumour
pituitary gland sits under optic chiasm
What are possible disorders of visual pathway? (6)
monocular blindness
bitemporal hemianopia
(R/L) nasal hemianopia
homonymous hemianopia
quadrant-anopia
macular sparing
what are the causes of homonymous hemianopia?
stroke (cerebrovascular accident)
what is the cause of homonymous hemianopia with macular sparing? And what happens in that area?
damage to primary visual cortex eg stroke (appears contralaterally to hemisphere damaged)
Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides
what blood vessel supplies the part of the visual cortex which is responsible for representing the macula?
posterior cerebral arteries (dual blood supply from both sides arteries)
therefore macula is likely to be spared in strokes
how does pupillary constriction occur and when?
In light: pupil constriction
- decreases glare
- increases depth of field– see Near Response Triad
- Pupillary constriction mediated by parasympathetic nerve (within CN III)
What happens in the dark to the eye?
pupil dilatation
* increases light sensitivity in the dark by allowing
more light into the eye
* pupillary dilatation mediated by sympathetic nerve
Describe the afferent pathway of the pupillary reflex?
- A small sub-section of retinal ganglion cells participate in the Pupillary Reflex Pathway.
- Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus
- Synapses at brainstem pretectal nucleus
- Afferent (incoming) pathway from each eye synapses on
Edinger-Westphal Nuclei on both sides in the brainstem
(red and green line)
Describe the efferent pathway of the pupillary reflex.
- Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent
- Synapses at Ciliary ganglion
- Short Posterior Ciliary Nerve -> Pupillary Sphincter
(blue line)
What are the 2 types of pupillary reflexes?
direct and consensual
What is direct light reflex?
Constriction of Pupil of the light-stimulated eye
What is the consensual light reflex? And what is the neurological basis for it?
Constriction of Pupil of the other (fellow) eye
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
describe the direct light pupillary reflex
optic nerve receives light signals from retina
synapses in pretectal nucleus and then Edinger-Westphal in midbrain
oculomotor nerve carries impulse through motor branches to the ciliary ganglion, short ciliary nerves constrict the sphincter pupillae
how does the consensual light reflex occur?
the optic nerve activates the efferent pathway in both eyes therefore
optic nerve receives light signals in one eye
synapses in pretectal nucleus and then Edinger-Westphal in midbrain for both eyes
oculomotor nerve carries impulse through motor branches to the ciliary ganglion, short ciliary nerves constrict the sphincter pupillae of both eyes
what is a right afferent defect?
no pupil constriction in both eyes when right eye stimulated with light
normal pupillary response in both eyes when left eye stimulated
how do right afferent defects occur?
damage to optic nerve
what is a right efferent defect?
no right pupil constriction whether left or right eye stimulated
left eye still constricts when right eye is stimulated
how do right efferent defects occur?
damage to oculomotor nerve
What is the unilateral afferent defect?
Difference response pending on which eye is stimulated
What is the unilateral efferent defect?
Same unequal response between left and right eye irrespective which eye is stimulated
what is a swinging torch test used for?
to test relative afferent pupillary defects
what are relative afferent pupillary defects? And how is it ellicited?
partial pupillary response still present when damaged eye stimulated (so semi-damage to optic nerve)
Elicited by the swinging torch test – alternating stimulation of right and left eye with light
Both Pupils constrict when light swings to left undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
Why is eye movement necessary? And how is it facilitated?
Voluntary or involuntary of movement of eyes
Necessary for acquiring and tracking visual stimuli
Facilitated by the six extraocular muscles innervated by the three cranial nerves (III, IV and VI)
how would you test the back of someones eyes
fundoscopy
what are all of the types of eye movements (4)
duction - one eye movement
version - both eyes move in same direction
vergence - bot eyes move in opposite directions
convergence - simultaneous adduction of both eyes
what are the 2 speeds of eye movement
saccade - short fast bursts (reflex/predictive)
smooth pursuit - sustained slow movement (tracking(
what is the optokinetic nystagmus reflex
when following a moving object, and it moves out of the field of vision, eye will snap back to the original viewing position
smooth pursuit then eyes reset to the middle using the fast reflex saccade
what are the movements of the eye muscles (6)
Superior Rectus - eye up Inferior Rectus - eye down Lateral Rectus - abducts eye Medial Rectus - (adducts eye) eye nasally Inferior Oblique- diagonally up and out Superior Oblique - diagonally down and out
appearance of 3rd nerve palsy
affected eye down and out (unopposed SO and LR) droopy eyelid (loss of levator palpebrae superioris) dilated pupil
appearance of 6th nerve palsy
affected eye unable to adduct
on relaxation is deviated inwards
double vision on gazing to the side of affected eye
appearance of 4th nerve palsy
when looking towards unaffected eye, affected eye moves upwards (IO takes over from LR)
what are the pupillary changes in the near response triad mediated by
sphincter pupillae contracts (circular smooth muscle) stimulated by parasympathetic nerves travelling with the oculomotor nerve
what are the lens changes in the near response triad mediated by
ciliary muscle attached to the lens via suspensory ligament contracts
reduces tension on the suspensory ligaments, so the lens relaxes and becomes thicker, causing greater refractive power
how does pupillary dilation occur
sympathetic stimulation to radial muscles causes them to contract
increases sensitivity to light
describe the visual pathway with polarisation involved
photoreceptors depolarise
-synapse to bipolar cells depolarise
- synapse to retinal ganglion fibres and travel out of eye as optic nerve
reach optic chiasm where nasal retinal fibres cross over, temporal dont, to form optic tracts
then synapse at lateral geniculate nucleus in thalamus
then travel via optic radiation to the primary visual cortex in occipital lobe
where does the right visual field travel to in the brain
left hemisphere primary visual cortex
includes nasal retina from right eye and temporal retina from left eye
how does the swinging torch test work
distinguishes relative and complete afferent defects
shine light in one eye, both pupils constrict, then quickly swing to other eye and hold it there
both pupils should constrict - if not, theres a relative afferent pupillary defect in the second eye tested
complete would be tested by eyes in isolation - not quickly changing lights
What muscles does CN3 innervate?
superior branch:
Superior Rectus
Levator palpebrae superioris - raises eyelid
Inferior Branch:
Inferior Rectus
Medial Rectus
Inferior Oblique
(Parasympathetic Nerve – constricts pupil)
What muscles does the CN4 innervate?
Superior Oblique
What muscles does CN6 innervate?
Lateral Rectus
What is cranial nerve palsy?
A palsy is a lack of function of a nerve. A cranial nerve palsy may cause a partial weakness or complete paralysis of the areas served by the affected nerve
