neurology of the visual system

Question 1

what are the visual pathway landmarks *

Answer 1

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

Question 2

describe the visual pathway of the retina *

Answer 2

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

Question 3

what is a receptive field in the eye *

Answer 3

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

Question 4

describe the convergence of the receptive field *

Answer 4

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

Question 5

what are on-centre ganglion cells *

Answer 5

stimulated by light at the centre of the receptive field

inhibited by liught at the periphery

Question 6

what are off centre ganglion cells *

Answer 6

inhibited by light at the centre of the receptive field

stimulated by light at the periphery

Question 7

what is the importance of having on and off centre ganglion cells *

Answer 7

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

Question 8

describe the anatomy of the optic chiasma *

Answer 8

53% fibres decussate here - from nasal retina - responsible for temporal visual field

uncrossed fibres are from the temporal retina - responsible for nasal field

Question 9

what does the fact we have 2 eyes mean

Answer 9

we can do depth perception

Question 10

effect of lesion anterior to optic chiasma *

Answer 10

affect vision from 1 eye - monoocular blindness

Question 11

effect of lesion posterior to the opyic chiasma *

Answer 11

affect both eyes

r sided lesion = l homonymous hemianopia in both eyes

L - R homonymous hemianopia in both eyes

Question 12

effect of lesion at the optic chiasma *

Answer 12

damages crossed fibres in both eyes = temporal field deficit in both eyes - bitemporal

eg because of a pituitary tumour

Question 13

what are the disorders of the visual pathway *

Answer 13

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

Question 14

describe the primary visual cortex *

Answer 14

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

Question 15

what is the function of the primary visual cortex *

Answer 15

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

Question 16

describe macula sparing homonymous hemianopia *

Answer 16

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

Question 17

what is the extrastriate cortex *

Answer 17

area around the primary visual cortex in the occipital lobe

converts basic visual information, orientation and position into complex info

Question 18

what is the dorsal pathway *

Answer 18

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

Question 19

what is the ventral pathway involved in *

Answer 19

primary visual cortex to inferiotemporal cortex

object representration, face recognition, detailed fine central vision and colour vision

damage may = cerebral achromatopsia

Question 20

what is the pupillary function *

Answer 20

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

Question 21

describe the pupillary reflex *

Answer 21

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

Question 22

describe direct V consensual reflex *

Answer 22

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

Question 23

describe affernet defects to the pupillary reflex *

Answer 23

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

Question 24

describe efferent defects to the pupillary light reflex *

Answer 24

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

Question 25

describe the touch swinging test *

Answer 25

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

Question 26

why is eye movement essential *

Answer 26

for getting and tracking visual stimuli

Question 27

define:

duction

vergence

version

convergence

Answer 27

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

Question 28

what is saccade *

Answer 28

short fast movement - reflexive, scanning, predictive (to track movements), memory guided

Question 29

what is smooth persuit*

Answer 29

slow movement up to 60 degrees/s

driven by motion of a moving target across the retina

Question 30

what are the muscles of the orbit and what are their function and attachments *

Answer 30

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

Question 31

describe the innervation of the extraoccular muscles *

Answer 31

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

Question 32

describe eye movement testing *

Answer 32

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

Question 33

describe 3rd nerve palsy *

Answer 33

affected eye is down and out - unopposed superior oblique and lateral rectus

droopy eyelid

Question 34

describe 6th nerve palsy *

Answer 34

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)

Question 35

4th nerve palsy *

Answer 35

when affected eye look medial - it is raised

Question 36

describe the oscillatory nystagmus reflex *

Answer 36

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