neurology of the visual system Flashcards
what are the visual pathway landmarks *
eye
optic nerve - ganglion nerve fibres
optic chiasm - half of the nerve fibres cross here
optic tract - ganglionic nerve fibres leave as optic tract
lateral genticulate nucleus - ganglionic nerve fibres synapse here
optic radiation - 4th order neuron
primary visual cortex or striate cortes - within the occipital lobe
extrastriate cortex
describe the visual pathway of the retina *
first order neurons - rod and cone retinal photoreceptors - transfer photons to electrons
second order neurons - retinal bipolar cells - modulate the signal
third order neurons - in the optic nerve (CN2) - partial decussation at chiasma (53%) - form the optic tract - go to lateral genticulate nucelus in the thalamus to relay visual info to the visual cortex
what is a receptive field in the eye *
retinal space where incoming light can change the firing rate of the neuron
some of the ganglion cells are switched on when get impulse, and surrounding ones switch off - and vice verca to provide contrast because eyes are working all the time
describe the convergence of the receptive field *
convergence - number of lower order neurons synapsing on the same higher order neuron
in the centre (cone fed circuits) the ratio of photoreceptors to ganglion cells is 1:1:1 - low convergence
in the periphery (red-fed circuits) many photoreceptors feed into 1 ganglion cell - high convergence
high - large receptive field, course visual acuity, high light sensitivity - easier to detect a signal because any of the photons can be activated to result in a response
low - small receptive field, fine visual acuity, low light sensitivity
what are on-centre ganglion cells *
stimulated by light at the centre of the receptive field
inhibited by liught at the periphery
what are off centre ganglion cells *
inhibited by light at the centre of the receptive field
stimulated by light at the periphery
what is the importance of having on and off centre ganglion cells *
important for contrast sensitivity and enhanced edge detection - ie tell one part of the retina is stimulated because the rest is not firing
not absolute perception of stimulusn - instead it is ratio of perception that is important
for spatial and object recognition
describe the anatomy of the optic chiasma *
53% fibres decussate here - from nasal retina - responsible for temporal visual field
uncrossed fibres are from the temporal retina - responsible for nasal field
what does the fact we have 2 eyes mean
we can do depth perception
effect of lesion anterior to optic chiasma *
affect vision from 1 eye - monoocular blindness
effect of lesion posterior to the opyic chiasma *
affect both eyes
r sided lesion = l homonymous hemianopia in both eyes
L - R homonymous hemianopia in both eyes
effect of lesion at the optic chiasma *
damages crossed fibres in both eyes = temporal field deficit in both eyes - bitemporal
eg because of a pituitary tumour
what are the disorders of the visual pathway *
monoocular blindness
bitemporal hemianopia
R nasal hemianopia - pressing on temporal fibres
homonymous hemianopia - damage to all the fibres from the L field of vision - from stroke
quadrantopia - defect further back in brain - fibres further apart so lesion have less of an effect
macular sparing = central region is the biggest represented in occipital cortex so some is spared
describe the primary visual cortex *
situated along calcarine’s sulcus within occipital lobe
also known as striate cortex
distinct stripe from myelinated fibre of optic radiation projecting into the visual cortex
disproportionately large area represenst the macula
superior visual field projects to below the calcarine fissure
inferior visual field projects to above calcarine fissure
R hemifield from both eyes projects to l cortex
L hemifield projects to R cortex
what is the function of the primary visual cortex *
organised as columns with unqiue sensitivity to stimuli from specific orientations
R and L dominant columns will be close - location of these helps with depth perception
describe macula sparing homonymous hemianopia *
damage to primary visual cortex - stroke
contralateral homonymous hemianopia with macula sparing
because area representuing macula has a dual blood supply from posterior cerebral artery from both sides
also if central artery of retina occlusion, some people have superior retianl artery - passes though retina to fovea - entrance behind the central retinal artery in eye globe - occlusion doesnt affect it = macula is spared
what is the extrastriate cortex *
area around the primary visual cortex in the occipital lobe
converts basic visual information, orientation and position into complex info
what is the dorsal pathway *
from primary visual cortes to posterior parietal cortex
involved in motion perception and visually guided action
damage = motion blindness - lack connection between striate and extrastriate cortex to distinguish between vision and movement
what is the ventral pathway involved in *
primary visual cortex to inferiotemporal cortex
object representration, face recognition, detailed fine central vision and colour vision
damage may = cerebral achromatopsia
what is the pupillary function *
regulates light input into the eye
in light pupil constriction - decreases spherical aberrations and glare, increases depth of field, reduces bleacing of photo-pigments, mediated by PNS
in dark pupil dilation - increase light sensitivity by allowing more light into the eye, mediated by SNS
describe the pupillary reflex *
afferent pathway - photoreceptors -> bipolar cells -> retinal ganglion cells -> exit at possterior 1/3 of optic tract -> enter lateral geniculate nucleus -> synapse at brainstem (pretectal nucleus) -> synapse at Edwinger-Westphal nucleus at both sides of the brainstem
efferent - endinger-westphal nucleus ->oculomotor nerve efferent -> synapses at ciliary ganglion -> short posterior ciliary nerve -> pupillary sphincter
shine light in blind pupil - no constriction
shine light in good eye - both constrict
describe direct V consensual reflex *
direct light reflex - constriction of the pupil of the stimulated eye
consensual light reflex - constriction of pupil of other eye
this happens because afferent pathway of either eye will stimulate efferent pathway on both sides
describe affernet defects to the pupillary reflex *
r afferent defect eg damage to r optic nerve - no contraction whhen light in r eye, normal when light in l
R
unilateral affernet - different response depending on what eye is stimulated
describe efferent defects to the pupillary light reflex *
R efferent eg damage to R 3rd nerve - no R constriction regardless of stim, L eye constricts regardless of stim
unilateral - same unequal response - regardless of stim
describe the touch swinging test *
when there is an afferant pupillary defect
partial pupillary response still present when the damaged eye is stimulted
touch test - alternating stim of R and L eye
both pupils constrict when light on normal eye
both paradoxically dilate when on bad eye - because it goes back to the intermediatry level because no affernet connections from eye
why is eye movement essential *
for getting and tracking visual stimuli
define:
duction
vergence
version
convergence
duction - eye movement in 1 eye
vergence - simultanteous movement of both eyes in opp dirns
version - simultaneous movement of both eyes in samee dirn - medially/laterally
convergence - simultaneous adduction in both eyes when viewing a near object
what is saccade *
short fast movement - reflexive, scanning, predictive (to track movements), memory guided
what is smooth persuit*
slow movement up to 60 degrees/s
driven by motion of a moving target across the retina
what are the muscles of the orbit and what are their function and attachments *
superior rectus - attacheed at 12 o clock - moves eye up
inferior rectus - attached at 6 o clock - down
lateral rectus - attach at temporal side - move eye to tempooral side of head
medial rectus - attached on nasal side - move eye to nasal side
superior oblique - attached high on temporal side of eye - passes under superior rectus and through trochlear - move eye in diagonal position down and in
inferior oblique - attached on low nasal side of the eye, pass over inferior rectus - move eye up and out
describe the innervation of the extraoccular muscles *
superior branch of CN 3 - superior rectus, lid levator
inferior branch of CN3 - inferior rectus, medial rectus, inferior oblique. paired with PNS nerve - constricts pupil
CN4 - superior oblique
CN 6 - lateral rectus
describe eye movement testing *
isolate the muscle to be tested by maximising its action, and minimising the action of the other muscles
abduction - lateral rectus
adduction - medial rectus
elevated and abducted - superior rectus
depressed and abducted - inferior rectus
elevated and adducted - inferior oblique
depressed and adducted - superior oblique
to test:
supraduction - 1 eye
supraversion - both
infraduction - 1
infraversion - both
R and l adduction and abduction
torsion - rotation of eye around entire posterior axis of eye
describe 3rd nerve palsy *
affected eye is down and out - unopposed superior oblique and lateral rectus
droopy eyelid
describe 6th nerve palsy *
affected nerve unable to abduct and deviates in
double vision bad on gaxing to side of affected eye (affected eye cant look in this dirn)
4th nerve palsy *
when affected eye look medial - it is raised
describe the oscillatory nystagmus reflex *
nystagmus - oscillatroy eye movement
optokinetic nystagmus - smooth persuit and fast phase reset saccade
useful to test in children who cnat speak
presence of response indicates they have a sufficient visual accuity to percieve the grating pattern
