Neuro 12: Neurology Of The Visual System Flashcards
Define the visual pathway
Visual pathway = neurological pathway where by vision is converted to neurological impulses —> to be transmitted from the eye to the visual cortex to the posterior part of the brain
The eye is connected to the brain via the ———— nerve
Optic nerve (aka cranial nerve)
What is the optic nerve made out of ?
Made up of myelinated ganglion nerve fibres
With cell bodies originating in the retina
Optic nerves from both eyes converge at the
Optic chiasma
Half of gamglionic nerve fibres Cross at:
While the remaining ganglion nerve fibres:
1/2 - Cross at the optic chiasma then goes along the contralateral optic tract
1/2 - exit along the optic tract on the same side
What does the primary visual cortex do?
- Primary Visual Cortex,
relays visual information to the Extra-striate Cortex,
a region adjacent to the Primary Visual Cortex,
–> for further higher visual processing.
Ganglion nerve fibres originate within:
the retina –>synapse upon the next order neurons,
at the Lateral Geniculate Nucleus.
What is the lateral geniculate nucleus?
a relay centre situated within the Thalamus.
Optic Radiation relaying signal from :
to:
from the Lateral Geniculate Ganglion,
to the Primary Visual Cortex within the occipital lobe,
–> lower visual processing.
the primary visual cortex (striate cortex) relays visual info to:
the extra striate cortex for further higher visual processing
List the visual pathway
eye –> optic nerve –> optic chasm or optic tract –> nerve fibres synapse at lateral geniculate nucleus –> optic radiation –> striate cortex –> extra striate cortex
What are first order neurons?
- rod + cone retinal photoreceptors
What are seconds order neurons?
- retinal bipolar cells
What are third order neurons ?
- retinal ganglion cell
Retinal ganglion fibres terminate at the _____ _____ ______ and synapse upon the ___th order neurons
Retinal ganglion fibres terminate at the LATERAL GENICULATE GANGLION and synapse upon the
4th order neurons
Lesions ______ to optic chiasma affects the visual field in one eye only
anterior
Lesions ______ to optic chiasma affects the visual field in both eyes
posterior
Crossed fibres originate from the _____ ______
nasal retina
What are crossed fibres responsible for?
- responsible for temporal visual field
What are uncrossed fibres responsible for?
- responsible for nasal visual field
uncrossed fibres originate from the _____ ______
temporal retina
What happens upon a lesion at the optic chiasma?
- it damages crossed ganglion fibres from nasal retina in both eyes
- results in temporal field deficit in both eyes
–> bitemporal hemianopia
What happens upon a lesion posterior to optic chiasma?
RHS lesion –> (left homonymous) –> hemianopia in both eyes
LHS lesion –> (right homonymous) –> hemianopia in both eyes
What is bitemporal hemianopia usually caused by?
- enlargement of pituitary gland tumor
WHat is homonymous hemianopia typically caused by?
- typically caused by stroke
Where is the primary visual cortex located at ?
- located along calcimine sulcus in the occipital lobe
What does the primary visual cortex specials in ?
- specialises in processing visual information of static + moving objects
Right hemispherefield from both eyes projects to ____ primary visual cortex
Left hemifield from both eyes projects to the _____ primary visual cortex
Right hemispherefield from both eyes projects to LEFT primary visual cortex
Left hemifield from both eyes projects to the RIGHT primary visual cortex
How is the primary visual cortex organized?
- organised as column with unique sensitivity to visual stimulus of a particular orientation
- right eye + left eye dominant columns are arranged alternatively
Damage to the primary visual cortex due to stroke may result in>
homonymous hemianopia of the contralateral side
with characteristic sparing of the macula central vision.
What is the function of extra striate cortex?
it converts basic visual information such as position + orientation into complex human precepts like motion + object representation
What is the pupillary function?
it regulates light input into the eye
What happens to pupil in light?
pupil constriction
- iris circular muscle contracts
What happens to the pupil in dark?
pupil dilation
why does the pupil constrict in light?
what is this action mediated by?
- reduces amount of light entering the eye
- and so reduces rate of photo-pigment bleaching
mediated by: Parasympathetic nerve
why does the pupil dilate in dark?
what is this action mediated by?
- increases amount of light entering the eye
- increases light sensitivity
mediated by: sympathetic nerve
PUPILLARY REFELX : describe the afferent pathway
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PUPILLARY REFELX : describe the efferent pathway
- provided by parasympathetic nerve which synapses at the ciliary ganglion
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What is the difference between direct and consensual reflex?
Direct: constriction of pupil of light stimulated eye
Consensual: constriction of pupil of the other eye
–> because afferent pathway on either side alone –> will stimulate efferent pathway on both sides.
What would happen in right afferent defect?
- e.g damage to right optic nerve
RIght eye stimulation:
- no pupillary constriction in both eyes when right eye
BUT
Left eye stimulation: normal pupillary response in both eyes
What would happen in right afferent defect?
e.g damage to right 3rd neve
Right eye stimulation:
- no pupillary constriction in right eye
- normal pupillary constriction in left eye
left eye stimulation:
- no pupillary constriction in right eye
- normal pupillary constriction in left eye
What is the difference between unilateral AFFERENT and EFFERENT defect ?
UNILATERAL AFFERENT DEFECT: response is different depending on which eye is stimulated
UNILATERAL EFFERENT DEFECT: unequal response is same no matter if left or right eye is stimulated.
How is the swinging torch test conducted?
- stimulate one eye at a time , alternating between right + left eye
If there is a relative afferent pupillary defect, what would you see in a swinging torch test?
Both pupils constrict,
when light swings to the left eye with intact afferent pathway.
Both pupils will paradoxically dilate,
when the light swings to the right eye with damaged afferent pathway,
as a result of relatively reduced drive for pupillary constriction in both eyes.
What is the definition of Eye movement ?
- voluntary or involuntary movement of the eyes, necessary for tracking visual stimuli
Eye movement = facilitated by _ extra ocular muscles
–> innervated by _ cranial nerves
Eye movement = facilitated by 6 extra ocular muscles
–> innervated by 3 cranial nerves
What is duction?
eye movement in one eye
What is version?
simultaneous movement of both eyes in same direction
What is vergence?
simultaneous movement of both eyes in opposite direction
What is convergence?
simultaneous inward movement in both eyes –> when viewing a near object
What is Saccade
- short, fast burst eye movement
- both voluntary + involuntary
What is the purpose of Saccade?
- useful for acquiring new external target
- for scanning text
- for tracking objects
- or for performing eye movement guided by memory\
- up to 900 deg/sesc
What is smooth pursuit?
- it is a slow sustain movement
- involuntary
- up to 60 deg /s
How many extraocular muscles are there?
6
What does the extra ocular muscle do?
- attaches eyeball to orbit
- provides straight + rotary movement
What are the 4 straight muscle?
- superior rectus
- inferior rectus
- lateral rectus
- medial rectus
What does the superior rectus do?
- attached to eye at 12 o’clock
- moves the eye up
what does the inferior rectus do?
- attached to eye at 6 oclock
- moves the eye down
What does the lateral rectus do?
aka external rectus
- attaches on temporal side of the eye
- and moves the eye towards to outside of the head –> towards the temple
What does the medial rectus do?
aka internal rectus
- attaches on nasal side of the eye
- moves eye towards the middle of the head –> towards nose
where is the superior oblique muscle located at?
- it is attached high on temporal side of the eye
- passes under the superior rectus
What does the superior oblique muscle do?
- it moves the eye in a diagonal pattern
down + in
where is the inferior oblique muscle located at?
- it is attache slow on the nasal side of the eye
- and passes over the inferior rectus
What does the inferior oblique muscle do?
- it moves the eye in a diagonal pattern
up and out
The lateral rectus muscle = innervated by:
The lateral rectus muscle = innervated by: 6th cranial nerve ABDUCENS NERVE
The superior oblique muscle = innervated by:
The superior oblique muscle = innervated by: 4th cranial nerve TROCHLEAR NERVE
apart from lateral rectus muscle + superior oblique muscle, all other extra ocular muscles = innervated by:
apart from lateral rectus muscle + superior oblique muscle, all other extra ocular muscles = innervated by: 3rd cranial nerve OCULOMOTOR NERVE
–> superior oculomotor nerve = superior rectus muscle
–> inferior oculomotornerve = rest
How is lateral rectus muscle action best tested?
in abducted position
How is medial rectus muscle action best tested?
in adducted position
How is superior rectus muscle action best tested?
in elevated + abducted position
How is inferior rectus muscle action best tested?
in depressed + abducted position
How is inferior oblique muscle action best tested?
in elevated and abducted position
How is superior oblique muscle action best tested?
in depressed + abducted position
elevation of one eye =
elevation of both eyes =
elevation of one eye = supraduction
elevation of both eyes = supraversion
depression of one eye =
depression of both eyes =
depression of one eye = infraduction
depression of both eyes = infraversion
dextroversion (right gaze) =
- simultaneous right eye abduction
AND - left eye adduction
Levoersion (left gaze) =
- simultaneous left eye abduction
AND - right eye adduction
what is adduction
- duction movement of one eye
- -> moving towards nose
What is abduction?
- duction movement of one eye
- -> moving AWAY from nose
What happens 3rd nerve palsy?
lateral rectus muscle + superior oblique muscle = not working
- eye adopts down and out position
- droopy eyelid
What happens in 6th nerve palsy?
- affected eye = unable to abduct
- -> and deviates inwards
- when patient is asked to abduct affected eye, affected eye deviates INWARDS
Optokinetic Nystagmus reflex
is an oculomotor reflex closely linked to the vestibular system which contributes to the stabilisation of retinal images.
It is seen when an individual follows a moving object with their eyes, which then moves out of the field of vision at which point their eye moves back to the position it was in when it first saw the object.
what is meant by convergence?
convergence = no. of lower order neurons field synapsing on raw same high order neurone
cone system convergence > rod system convergence
-
differentiate between high and low convergence
low :
- small receptive field
- fine visual activity
- low light sensitivity
high :
- large receptive field
- coarse visual activity
- high light sensitivity
differentiate between on centre ganglion + off centre ganglion
on centre ganglion
- stimulated by light at centre of receptive field
- inhibited by light on the edge of the receptive field
off centre ganglion
- inhibited by light at centre of receptive field
- stimulated by light on edge of receptive field
note:
important for = contrast sensitivity + enhanced edge detection
What are the 2 types of retinal ganglion cells?
- off centre ganglion
- on centre ganglion
humans have 2 eyes anteriorly –> which allows 3D vision (depth) of a particular object
-
What does the extra striate cortex do ?
- area around primary visual cortex within the occipital lobe
- which coverts basic visual info orientation and position into complex info.
what is meant by torsion?
rotation of eye around anterior = posterior axis of the eye
