Visual pathways Flashcards
Where does the optic nerve travel to from the eye?
The optic chiasm
Where do axons pass into once they move through the optic chiasm?
The optic tract
Briefly describe where the optic chiasm is positioned in the brain
Immediately above the pituitary
What cells supply the axons that make up the optic nerve?
Retinal ganglion cells
Where do majority of retinal ganglion cells project?
Lateral geniculate nuleus (visual thalamic relay)
Where do lateral geniculate nucleus cells send axons?
Primary visual cortex (area 17) in the occipital lobe
What would happen if the lateral geniculate- occipital pathway is damaged?
The person will be blind
What structure is tested when a patient is asked to follow your finger with their eyes?
Superior colliculus
Where is the superior colliculus located?
Midbrain
What does the superior colliculus connect to, and what is the function of this?
The medial longitudinal fasciculus
Links together and synchronises nuclei
What is the optic tectum?
Another name for superior colliculus
What nucleus controls pupillary light reflex?
Edinger Westphal nucleus
Describe the pathway of axons from cells of LGN to visual cortex
via the optic radiation
project back anteriorly in Meyer’s loop and then run alongside the posterior horn of lateral ventricle to occipital lobe
Define left visual field
Everything to the left of your fixation point in both eyes
What part of the brain ‘sees’ the left visual field
right visual cortex
Why does left visual field project to right visual cortex and vice versa?
Nasal hemiretina from left eye projects to right visual cortex, as does the temporal hemiretina from right eye.
Define scotoma
A patch of blindness
What is the result of partial optic nerve legion?
Ipsilateral scotoma
What is the result of complete optic nerve lesion?
Complete blindness in that eye
What is the result of optic chiasm lesion
Bitemporal hemioanopia
What is the result of an optic tract lesion
Homonymous hemiapopia
What is the result of damage of the Meyer’s loop?
Homonymous upper quadrant anopia
What is the result of optic radiation lesion?
Homonymous hemianopia
What is the result of visual cortex lesion
Homonymous hemianopia
Name the four key visual reflexes
Pupillary eye reflexes
Accomodation reflex
Vestibulo-ocular reflex
Blink reflex
How are cranial nerves III, IV and VI linked?
fibre tract called the medial longitudinal fasciculus
Which cranial nerves are involved in pupillary light reflex
In on II, out on III
Describe the passage of axons in the pupillary light reflex
Axons of CN II go the pretectal nucleus is the superior colliculus. From here, go to Edinger-Westphal nucleus and out on CN III to ciliary ganglion.
This sends axons to pupillary constrictor muscle
Constrictor of this reduces pupil size
What can block the pupillary light reflex?
Atropine
What are the 3 components of the accomodation reflex?
Pupillary constriction
Thickening of lens
Convergence
What is the input for pupillary constriction as part of accomodation reflex?
Comes from descending projections from frontal eye fields in premotor cortex
What is function of pupillary constriction in accommodation reflex?
Focusing on close objects/ depth of field
Can accomodation reflex occur in unconcious person?
No
What part of the brain controls the accommodation reflex?
Frontal eye fields (specialised part of premotor area)
What is the result of damage to FEFs
Inability to direct gaze from one object to another
Describe the shape of the lens at rest
Under tension from ring of suspensory ligaments which flatten it away from relaxed shape
Describe the shape of the lens when the ciliary muscle contracts
Reduces diameter of ring of suspensory ligaments, slackens the tension and lens bulges out into relaxed shape
How is the lens thickened during accomodation and why?
Tension in suspensory ligaments is reduced and lens thickens to normal shape to focus on obkects
Which muscles cause convergence as part of accommodation reflex
Medial rectus muscles
What is the vestibulo-ocular reflex?
If you turn your head to the side whilst looking at distant object, eyes rotate in opposite direction of the head to keep direction of gaze constant and object stays in view
What is the input and output of the vestibulo-ocular reflex?
In: CN VIII, receiving signals from semicircular canals
Out: CN VI and CN III
How is the vestibulo-ocular reflex tested clinically?
Holding someones eyes open and gently rotating the head from side to side
Eyes should rotate in the head to keep direction constant
WHat is nystagmus?
Form of VOR caused by continuing rotation of fluid in the semicircular canals
Function of blink reflex
Projects eye from foreign bodies
Input and output of blink reflex?
V1 branch of trigeminal nerve
Out- facial nerve input to obicularis oculi muscles
