Visual pathway Flashcards
pathway from eyes to primary visual cortex
- the eye
- optic nerve (ganglion nerve f)
- optic chiasm (where half of the fibres decussate)
- optic tract (where ganglion fibres synapse in the lateral geniculate nucleus)
- optic radiation (4th order)
- PVC/striate cortex
light pathway to receptive ganglion cell
- first order: rod and cone photoreceptor
- second order: retinal bipolar
- third order: retinal ganglionic
what proportion of fibres decussate at the optic chiasm?
53% of the retinal ganglionic fibres decussate at the optic chaise then synapse at the lateral geniculate nucleus (LGN) in the thalamus
what is the LGN, where is it located?
lateral geniculate nucleus
in the thalamus
size of receptive field with distance from fovea
receptive field size increases with distance from the fovea
what is convergence?
compare between cones and rods
rods use high convergence while cones use low convergence
- convergence: number of lower order neurones synapsing at the same high order neurone
- cones tend to be 1:1 i.e. lower order of convergence while in the rods, there are usually many to one e.g. 1:4:8
central and peripheral retinal convergence comparison
central has lower convergence
what is the result on the receptive field
visual acuity
and light sensitivity
as a result of LOW convergence?
small receptive field
fine visual acuity
low light sensitivity
what is the result on the receptive field
visual acuity
and light sensitivity
as a result of HIGH convergence?
large receptive field
course visual acuity
high light sensitivity
what are the two groups of retinal ganglionic cells?
what are they important for?
on centre and off centre ganglionic cells
important for:
contrast sensitivity
edge detection
difference between on centre and off centre ganglionic cells
on centre: stimulated by light at the CENTRE of the receptive field, therefore inhibited by light on the edge of the field
off centre: stimulated by light on the EDGE of the receptive field, therefore inhibited by light in the centre of the field.
what lesion affects one eye only?
anterior (to optic chiasm) lesions
what lesion affects both eyes?
posterior (to optic chiasm) lesions
which part of the retina produce crossed fibres at the optic chiasm?
what is it responsible for?
nasal retina
temporal visual field (light from the sides)
what sort of fibre originates from the temporal retina?
what is the temporal retina used for?
uncrossed fibres
nasal visual field
what is the result of lesion at the optic chiasm? why?
bitemporal hemianopia
- both eyes’ nasal retinas are affected as they are the crossing retgang fibres
- nasal retina receives temporal visual field
what is the result of damaging the crossing RGC fibres in a optic chiasm lesion?
bitemporal hemianopia
as both sets of nasal retina fibres (responsible for temporal visual field) are damaged in the lesion
what is the result of a lesion posterior to the optic chiasm?
right lesion—> left homonymous hemianopia
left lesion–> right homonymous hemianopia
what is another cause of bitemporal hemianopia, in addition to an Optic Chiasm lesion?
enlargement of a pituitary gland tumour causing its compression
what can cause homonymous hemianopia?
stroke
in which disorder is central acuity retained? what is actually affected?
macular sparing disorder
peripheral vision is lost
what condition causes a loss of horizontal plane vision?
glaucoma
what plane do neurological problems often affect?
vertical plane of vision
along which fissure is the PVC located?
along the calcarine fissure
Characterised by a distinct stripe derived from the myelinated fibre of the optic radiation projection.
why is the macula disproportionately presented as a large area?
due to a higher density of RGCs (lower convergence)
where relative to the calcarine fissure is the superior visual field?
projection is below the fissure
what is located above the calcarine fissure?
inferior visual field
in which direction do the left and right semi-field project
right and left (opposite) respectively
how is the PVC organised in the brain in terms of light sensitivity?
In columns with unique sensitivity to the visual stimulus of a particular orientation
alternation of right and left eye columns
what is the common cause of macular sparing homonymous hemianopia?
damage to PVC often due to stroke (of the PCA)
macula sparing due to dual blood supply by PCAs
why is the macula spared in contralateral homonymous hemianopia (with macula sparing)?
due to the dual supply received by the macula
what is the area that surrounds the visual cortex?
extrastriate cortex
- assists the striate cortex with position and orientation ,makes use of the dorsal and ventral pathways
- the striate cortex is part of the visual cortex that processes visual information from the lateral geniculate nucleus (thalamus)
what is the function of the extrastriate complex?
convert basic visual info, orientation and position into complex info
where does the dorsal pathway go from the PVC?
into the posterior parietal cortex via extrastriate cortex
what is the dorsal pathway from the PVC responsible for?
motion detection
visually-guided action
what is the result of damaging the dorsal pathway from the PVC?
motion blindness
where does the ventral pathway from the PVC go?
to the inferiotemporal cortex via the extrastriate cortex
what is the ventral pathway from the PVC responsible for?
object representation
facial recognition
detailed fine central vision
colour vision
what is the result of damage to the ventral pathway from the PVC?
- prosopagnosia (facial agnosia)
- cerebral achromatopsia (partial or total absence of color vision)
what is the function the pupillary function in light?
- decreases spherical aberrations and glare,
- increases depth of field
- reduces bleaching of photopigments
what innervation and nerve mediated constriction in the reflex?
CN III (PNS nerve branch)
what is the function of the pupillary reflex in the dark?
let in more light
what innervation mediates dilation?
SNS nerve
Outline the afferent pathway of the pupillary reflex from the photoreceptor
- light hits photoreceptors
- bipolar cell
- ganglionic cell
- optic nerve
- optic chiasm
- optic tract (exit at posterior third and enter LGN)
- synapse at Pretectal Area
- synapse at EW nucleus (both ipsilateral and contralateral)
which nucleus does the afferent pathway of the pupillary reflex first synapse to, in the brainstem?
pretecal nucleus
then the Edinger-Westphal
both are located in the midbrain
efferent pathway of the pupillary reflex from the Edinger Westphal nucleus?
- EW nucleus
- CN III nerve efferent
- synapse at ciliary ganglion
- via short posterior ciliary nerve (from ganglion)
- pupillary sphincter
what does a stimulus in the afferent pathway have on the efferent?
light from one eye stimulates the efferent pathway in both eyes
direct vs consensual light reflex
direct: constriction in the pupil of light stimulated eye
consensual: constriction of the other eye
what is the effect of a right afferent of the pupillary light reflex (i.e. damage to CN II)
no pupil reflex in both eyes
what is the effect of a defect on one side, e.g. right, efferent (i.e. damage to CN III) ?
no right pupil constriction at all
left one constricts when either eye is stimulated
what can be done to demonstrate the weakness of afferent pathway?
result on constriction in normal and defected pathway?
swinging torch test, determine incomplete or relative damage to afferent pathway
intact pathway will have both eye constrict with light stimulation
in afferent pathway weakness, both eye paradoxically dilate due to reduced drive to constrict both eyes
how many extra ocular muscles are there?
how many cranial nerves innervate them?
6 muscles (4 straight, 2 oblique)
innervated by 3 cranial nerves
4 types of movement of the eye
1) duction (one eye)
2) version (eyes in the same direction)
3) vergence (outward)
4) converge (inward)
what does duction mean?
movement in one eye
version definition (eye movement)
movement of both eyes in the same direction
left and right version of eyes prefix
right = dextroversion
left =levoversion
vergence definition (eye movement)
movement of both eyes in opposite directions
convergence definition (eye movement)
simultaneous adduction (inwards towards midline) movement in both eyes when viewing a near object
fast speed eye movement
examples of types
saccade
e. g. reflexive saccade
e. g. scanning saccade
e. g. predictive saccade
e. g. memory-guided saccade
slow speed eye movement
smooth pursuit
rate in saccade
900 deg per second
short, fast burst of movement
rate in smooth pursuit
60 deg per second
what drives smooth pursuit eye movement
motion of moving target across the retina
what are the 4 straight muscles of the eye?
superior rectus= up movement
inferior rectus= down movement
medial rectus
lateral rectus
which straight muscle(s) are innervated by CN III
superior, inferior and medial rectus
which straight muscle(s) are innervated by CN VI
lAteral rectus (abducens)
what are the 2 oblique muscles?
superior oblique
inferior oblique
attachment of superior oblique muscle
movement of eye?
high on temporal side of eye
passes under superior rectus
moves eye down and out
clinical test: in and down
attachment of inferior oblique muscle
movement of eye?
low on nasal side of eye
passes over inferior rectus
moves eye up and out
clinical test: move in and up
innervation of superior oblique
CN IV (trochlear)
innervation of inferior oblique
CN III (oculomotor)
3 nerves that innervate the eye muscles
which muscles do they innervate?
CN III- inferior oblique, superior rectus, inferior rectus, medial rectus
CN IV- superior oblique
CN VI- lateral rectus
testing movement of lateral rectus
abduction (away from midline)
testing movement of medial rectus
adduction (towards the midline)
testing movement of superior rectus
abduct then elevate (out and up)
testing movement of inferior rectus
abduction and depression (out and down)
testing movement of inferior oblique
move eye medially and elevate (in and up)
testing movement of superior oblique
depression and adduction
presentation of 3rd nerve palsy
- eye droops and abducts (down and out)
- eyelid droop
- lateral rectus and superior oblique take over
unopposed lateral rectus (at the medial rectus is gone) takes over by abducting and the superior oblique also takes over (as inferior oblique is gone)
presentation of 6th nerve palsy
Diplopia:
- affected eye unable to abduct and deviates to midline (adduct)
- therefore double vision- worsens on gazing to side of affected eye
what speeds of eye movement are present in optokinetic nystagmus reflex?
smooth pursuit and fast paced reset saccade
how is visual acuity tested in pre-verbal children
presence of nystagmus movement observed (test for reflex)
if reflex is present, the child has the visual acuity to perceive motion
what is the effect of cold and warm water in the caloric reflex test?
COWS
cold–> opposite
- cold water imitates stimulation of the opposite ear (head is turning the opposite direction) so the eyes in response look to the ipsilateral ear (ear receiving water) whilst nystagmus occurs to the opposite ear
warm–> same
- warm water imitates stimulation of the same ear (head is turning the same direction as ear getting water) so the eyes look to the contralateral ear whilst nystagmus is towards the same ear (ear receiving water)
what effect does warm water have on nystagmus?
nystagmus to ipsilateral ear
- warm water causes endolymph in the horizontal canal to rise
- mimic a head tilt to the ipsilateral side
- causing an increasing firing of the afferent nerve
- the eyes will therefore move to the contralateral eye
- horizontal nystagmus to the ipsilateral ear
what effect does cold water have on nystagmus?
nystagmus to contralateral side
- endolymph falls
- decreased afferent firing
- mimic head turn to the contralateral side
- eye moves to ipsilateral side
- nystagmus to contralateral ear
COWS
applies to the direction of head turn and the resulting NYSTAGMUS (not the normal reflex movement of eyes)
what is RAPD?
Relative Afferent Pupillary Defect
- afferent damage leads to weakening of the pupillary reflex (not complete loss)
- BOTH eyes show a weakened constriction response
what is seen with the swinging torch test in RAPD?
- when going from the healthy eye to the affected eye, there will be paradoxical dilation
- it initially constricted when the other healthy eye had stimulation
- now the light is in the defected afferent eye, the pupil doesn’t respond (as if there were no stimulus) and it dilates back to normal position despite the light being there
