visual system Flashcards
what is the little red trinagle on corner of eye called?
caruncle
what are the edges of the eye (top lid meets bottom) called?
medial and lateral canthus
what is the border between cornea and sclera?
limbus
what is the antero-posterior diameter of the eye? (mm)
24 mm in adults
3 layers of eye from out to in BACK OF EYE
sclera, choroid, retina
sclera structure, qualities and function
(its the white) high water content, hard and opaque, protective outer coat
choroid qualities
pigmented and vascular
retina - what type of tissue, function, where, describe it structurally.
neurosensory tissue: responsible for capturing the light ,
back of eye (“inner part”),
thin
what is uvea
ALL AROUND THE WHOLE EYE: Vascular coat of eyeball and lies between the sclera and retina.
3 components of uvea
iris, ciliary body (front) and choroid (back)
what is cornea and what is choroid and what is sclera (EASY TO MIX UP)
cornea: OUTER LAYER IN FRONT OF IRIS
choroid- middle layer of 3 LAYERS
sclera: white- outer layer
what usually happens when theres disease in one component of uvea and why?
spreads to the other components too (not always same extent) because they are very intimately connected
list structures front of eye from out to in
cornea
iris
pupil
lens
where is suspensory ligament
bellow lens and attatched to it
where is cilliary body
over lens and attached to it
where and what is macula? what is at its centre?
a little section of the retina at the back centre
highly sensitive- crucual for detailed central vision (ex. required in reading)
fovea at centre
where does the optic nerve connect to RETINA?
near macula at back of eye, at optic disc
what is the visible (in an optic exam) portion of the optic nerve called
optic disc
what is special about the spot where optic nerve meets retina? (optic disc)
there are no light sensitive cells. It is a blind spot
what does central vision entail
-detail day vision,
-colour vision
-reading
-facial recognition
where is the highest concentration of cone photoreceptors
fovea
what assessment is used for central vision
visual acuity (Sharpness) assessment
when do you have poor visual acuity?
loss of foveal vision
what features are distinguished by peripheral vision
shape, movement, night vision, navigation vision
what activity becomes difficult when theres loss in visual field?
navigation (patient may need white stick even with perfect visual acuity)
layers of retina in the order that visual input is processed
BE CAREFUL, processing of visual input- light signals starts at the most POSTERIOR layer of the retina (- cones and rods= first order neurons ) EVEN THOUGH light comes from anterior side. see slide 16.
1st order neuron:
photoreceptor - detction of light
second order neuron: bipolar cells
local signal processing to improve contrast sensitivity
3rd motor neuron:
retinal ganglion cells, transmission of signal from eye to brain
two classes of photoreceptors
rods and cones
are rods or cones more light sensitive (how much) , fast, many and and why?
rods 100x more light sensitive but slower so also more numerically than cones
(120 mil rods, 5 mil cones)
because rods: NIGHT VISION, cones: DAY- FINE vision and colour
what is refraction?
change in DIRECTION of any wave when it changes MEDIUM
WHY does direction of wave change in refraction?
because velocity changes
what are the 2 types of lenses and what happens to wave of light when it meets them
CONVex (CONVerge) -brings rays to a point
concave (diverge) -spreads rays outward
what is the point called where all the rays meet after converging in a convex?
focal point
what is emmetropia? where do parallel light rays fall?
refractive state in a healthy eye where theres adequate correlation between axial length and refractive power
parallel light rays on the retina
what is ametropia
term used to describe general Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina
includes myopia, (near-sightedness)
hyperopia ( farsightedness)
presbyopia
where do parallel rays converge in myopia and hyperopia
myopia: focal point anterior to the retina
hyperopia: .. behind retina
what leads to myopia hyperopia
excessive long globe (axial myopia)
or
excessive refractive power: refractive myopia
excessive short globe (axial hyperopia)
insufficient refractive power (refractive hyperopia)
is axial or refractive myopia/ hyperopia more common?
axial
why do people get long/ short globe or ref power prob?
not clear , genetic factor
visual and asthenopic symptoms of myopia
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
visual and asthenopic symptoms of hyperopia
visual acuity at near tends to blur relatively early
nature of blur vary:
1) inability to read fine print 2) near vision is clear but suddenly and intermittently blurry
blurred vision is more noticeable if person is tired , printing is weak or light inadequate
asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes
what system is problematic in presbyopia?
near response triad
what are the 3 muscles involve din near response triad
sphincter pupillae
medial recti
circular ciliary muscle
what do each of the muscles in nrt do
Pupillary Miosis (Sphincter Pupillae) to increase depth of field (think of miosis: polaplasiazei depth)
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
What is presbyopia? (specifically- why/ how it occurs)
Naturally occurring loss of accommodation (focus for near objects)
what lens type is in the glasses of poeple with presbyopia
(convex lenses) to increase refractive power of the eye
onset age of presbyopia
40 yrs
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 (- increase refractive powers)
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
lens thickness, cilliary muscle state and pupil state when looking near
contracted ciliary : pushes lens towards centre: thicker
also constricted pupil
name of ganglion nerve fibers before optic chiasm (closer to face/ eyes) and after (closer to back of head) chiasm. also what order neurons are these and why
before: optic nerve
after: optic tract
this is 3rd order neuron bc 1st and second were IN retina- very small
what fibres decussate at chiasm
nasal (53%) - the rest just converge (narrow) there
where is lateral geniculate nucleus and what happens there
after optic chiasm in brain, ganglion fibres synapse
what is optic radiation and what does it do
4rth order neyuron relay signal form lateral geniculate ganglion to primary visual cortex
what comes after primary visual cortex
Primary Visual Cortex or Striate Cortex – within the Occipital Lobe, relays to extra-striate cortex (higher visual processing)
what happens to ligh impulses after theyve been captured by retina
sent to brain via optic nerve
what field are the fibres that originate from the nasal visual field responsible for?
temporal visual field
lesion where in relation to chiasm causes lesion in only one eye
in front of chiasm
lesion where in relation to chiasm affect both eyes?
behind chiasm
what happens when lesion at optic chiasm
Damages crossed ganglion fibres from nasal retina in both eyes
Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia
what happens when right side lesion posterior to optic chiasm
Left Homonymous Hemianopia in Both Eyes (bc right fibres- light from left field)
what lesion is right nasal hemianopia referring to?
lesion in only right optic nerve in right eye
what lesion is quadrantanopia
lesion behind Lateral Geniculate Nucleus
what happens is macular sparing
because damage to primary visual cortex is usually due to stroke, AND
the area representing the macula in the eye recieves dual blood supply from posterior cerebral arteries form both sides,
macular vision (central) remains intact while theres contralateral homonymus hemianopia
homonymous hemianopia most common cause
stroke (cerebrovascular accidnet)
3 things that happen in pupil cosntriction in light
(glare? field depth? nerve?)
decreases glare
increases depth of field – see Near Response Triad
Pupillary constriction mediated by parasympathetic nerve
what is glare
Glare refers to the visual discomfort or impairment caused by excessive and uncontrolled brightness or contrast in the field of view.
what CN is parasymp nerve part of?
3
In dark: pupil dilatation, WHTA/ WHY HAPPENS?
increases light sensitivity in the dark by allowing more light into the eye
pupillary dilatation mediated by sympathetic nerve
which cells participate in the pupillary reflex?
A small sub-section of retinal ganglion cells participate in the Pupillary Reflex Pathway.
where dos the pathway of the pupillary reflex ganglion cells FIRST deviate form the normal retinal ganglion cells?
Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus
what are the synapses involved in the pupillary reflex? (afferent AND efferent)
1) primary neurones
synapse with secondary at pretectal neucleus
2) secondary neurones
synapse with tertiary: efferent - PATHWAY FROM EACH EYE HAS SPLIT IN 2 and synapses at the Edinger Westphal nuclei ON BOTH SIDES of the brainstem
3) tertiary neurones: oculomotor nerve efferent
synapses with quaternary neurons at ciliary ganglion
4) quaternary neuron = short posterior ciliary nerve
ends up at effector: pupillary sphincter
what is direct and consensual pupillary reflex?
Direct Light Reflex – Constriction of Pupil of the light-stimulated eye
Consensual Light Reflex – Constriction of Pupil of the other (fellow) eye
why does the consensual reflex happen?
Neurological Basis
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
(See ipad notes picture or slide 41-2 ppt)
what happens when you shed light on right and left eye if theres damage to right optic nerve
No pupil constriction in both eyes when right eye is stimulated with light
Normal pupil constriction in both eyes when left eye is stimulated with light
what happens when right and left eye shed with light in Right Efferent Defect
No right pupil constriction (neither eyes) whether right or left eye is stimulated with light
Left pupil constricts (both eyes) whether right or left eye is stimulated with light
what does the swinging torch test aim to identify/ diagnose?
when damage to afferent pathway is (usually) incomplete, or relative (meaning more than the other eye)
officially called: Relative Afferent Pupillary Defect
what do you do in the winging torch test? what do you expect to see if theres a relative afferent pupillary defect?
YOU SHED LIGHT on undamaged eye first then on damaged - aim is to compare bc normal eye will constrict and so will the semidamaged eye, just less.
thats why you need to do swinging test, bc if you did separately ud see constriction and think its ok but you wnat to identify the LESS constriction.
when you swing the torch, to you it looks like there has been dilation, this is called “paradoxical dilation.”
what are the muscles that move the eye called and how many are there?
extraocular muscles: six
what nerves are the six extraocular muscles innervated by?
CN 3 (occulomotor) , 4 trochlear 6 abducents
what muscles in the eye are striated and which are smooth
extraoccular: striated
smooth: pupil control
what are the areas where striated muscles linked with the periosteum called medially and laterally?
medial and lateral enthesis
name the 6 extraocular muscles
superior and inferior oblique
and 4 straight muscles:
superior , inferior, lateral and medial rectus
what direction does superior oblique move eye to
down and out (think trochlea is in nose and muscle attaches relatively back of eye not very front)
what direction does inferior oblique move eye to
up and out
how do you test superior/ inferior oblique? (tricky)
1)FIRST move eye to look nasally
then
2) ask person to look down for superior and up for inferior obliques
what is the medial and lateral sides of eye also clalled?
nasal and temporal
where does the superior oblique attach on eye?
high on temporal side
relation of superior oblique to superior rectus in space
oblique passes UNDER the superior rectus
attatchment of inf oblique on skull
Attached low on the nasal side of the eye.
relation of inf oblique to inf rectus
OVER inf rectus
what eye muscles does the third cranial nerve innervate?
(2 clear inf/ superior in each of branches and then 2 extras in inf.)
superiorr branch:
superior rectus (raises eye)
levator palpebrae superioris (raises eyelid)
inf branch:
inf rectus
inf oblique
(+ extras:)
parasymp nerve - constricts pupil
medial rectus: adducts eye
what does fourth cranial nerve innervate
trochlear: superior oblique
sixth cranial nerve innervation
lateral rectus
how do we test superior/ inf rectus muscles?
up AND OUT for sup and down AND OUT for inf (the out part is important bc in is for obliques)
