visual pathways Flashcards
describe where the optic nerve originates and its continuation
Eye –> Optic nerve –> optic chiasm: axons pass into optic tract
what does the optic chiasm lie above?
the pituitary gland
what can pituitary tumours cause?
bilateral loss of peripheral receptive fields
explain the lateral geniculate-occipital cortex pathway
- retinal ganglion cells supply the axons of the optic nerve
- optic nerve axons project into LGN
- ## LGN cells project axons in the optic radiation to the primary visual cortex (project anteriorly then posteriorly along the side of posterior horn of the lateral ventricle)
what is Meyer’s loop?
the pathway of the fibres in the optic radiation mediating vision from the upper quadrants, where they loop more anteriorly around the side of the lateral ventricle
what does damage to the meyer’s loop on the LHS of the brain cause?
blindness in the top right hand side of the field of both eyes
where do the optic radiation fibres mediating vision from the upper quadrants travel?
through the meyer’s loop - loop more anteriorly around the side of the lateral ventricle
where do the optic radiation fibres mediating vision from the lower quadrants travel?
travel more directly back to the visual cortex
which pathway mediates visual perception?
the lateral geniculate-occipital cortex pathway
how can the lateral geniculate-occipital cortex pathway get damaged? what does this cuase?
by an infarct in the posterior cerebral artery - can cause a person to be blind
why are the superior colliculi needed?
to track a moving object
what do the superior colliculi attach to?
medial longitudinal fasciculi
what do the MLF link and synchronise?
oculomotor nuclei
what is another name for the superior colliculi?
optic tectum
describe the path of fibres involved in the pupillary light reflex
- retinal fibres project to pretectal nuclei then bilaterally to the EW nuclei
- Parasympathetic preganglionic fibres project from the E-W nuclei to the ciliary ganglia in the orbit behind each eye
- Postganglionic fibres then enter the eye and act on sphincter muscles around the pupil to constrict it
what is the left visual field?
everything to the left of your fixation point in both eyes – seen by the right visual cortex
explain how both eyes contribute to both visual fields
- Nasal hemiretina from the left eye projects to the right visual cortex
- Temporal hemiretina from the right eye projects to the same right visual cortex
- Both hemiretina view objects in the left visual field
what does a partial optic nerve lesion cause?
macula loss - ipsilateral scatoma
what is a scatoma?
patch of blindness in the visual field of one eye
what does a complete optic lesion lead to?
blindness in that eye
what does an optic chiasm lesion cause?
lose extremes of vision
bitemporal hemianopia - tunnel vision
what causes an optic chiasm lesion?
pituitary tumour pressing on the optic chiasm
what does an optic tract lesion cause?
loss of same half of the visual field in both eyes
Known as homonymous hemianopia
what does damage to the meyer’s loop lead to?
damage to temporal branch – lose upper quadrant
Known as homonymous upper quadrant anopia
what does an optic radiation lesion lead to?
homonymous hemianopia. Foveal sparing occurs.
what does a visual cortex lesion lead to?
lateral portion is not damaged (only medial) so there is macular sparing: no scotoma in centre.
• Also: homonymous hemianopia
what is macular sparing?
Central parts of the eye (macula and fovea) have a bilateral projection to the visual cortex so a lesion on one side won’t affect central vision
what is anton-babinski syndrome?
when patients can say they can see despite loss of visual cortex (they see they can see when they can’t – they make up their visual surroundings)
what do visual pathways to the brainstem mediate?
visual reflexes
which nerves and muscles mediate visual reflexes?
pupillary sphincter muscles and the III, IV, & VI cranial nerves
what is the input and output for the pupillary light reflex
input - cranial nerve 2
output - cranial nerve 3
what is the pupillary light reflex?
Consensual reflex where stimulation of one eye leads to constriction in both eyes
what drug blocks the pupillary light reflex?
Atropine (muscarinic antagonist drug) blocks the pupillary light reflex
what is the accommodation reflex?
Reflex in response to focusing on a near object then looking at a distant object (and vice versa)
what are the 3 components of the accommodation reflex?
pupillary constriction, thickening of the lens and convergence
what is the input and output for the accommodation reflex?
Input from descending projections from frontal eye fields in the premotor cortex cause constriction
output - cn 3
what are the frontal eye fields and what do they control?
specialized parts of the premotor area dedicated to the motor control of the extra-ocular eye muscles
control pupillary constriction
what does damage to FEFs cause?
inability to direct gaze from one object to another + loss of fast phase of nystagmus
explain what happens to the lens and how when viewing a distant object
- Light rays from distant objects are nearly parallel – don’t need as much refraction to bring them into focus
- Ciliary muscles are relaxed
- Fibres are taught and the lens is under tension from a ring of suspensory ligaments which pull on it to stretch and flatten it
- Lens is at minimum strength for distant vision
explain what happens to the lens and how when viewing a close object?
o Light rays from close objects diverge and require more refraction for focusing
o Ciliary muscles contract, fibres slack and the lens rounds to allow the eye to focus on nearby objects
what happens to the lens during the accommodation reflex?
the ciliary muscle contracts and the lens rounds to focus on close objects
what controls convergence of the eyes?
controlled from frontal eye fields
what mediates convergence of the eye?
oculomotor nerve bilaterally
which muscles are simultaneously activated in the convergence of the eye?
medial rectus muscles of both eyes
what can disturb the convergence of the eyes?
can be disturbed by fatigue, trauma, alcohol and drugs
what can incorrect or absent convergence of the eye lead to?
diplopia - double vision
when does the vestibulo-ocular reflex occur?
when your eyes rotate in opposite direction to turning of the head
Occurs when you turn your head while looking at a distant object. Keeps object in view while youre moving and keeps the eyes steady
what is the input and output of the vestibulo-ocular reflex?
- Input (afferent) arc is the vestibulo-cochlear nerve – receives signals from the semicircular canals
- Output is the abducens and oculomotor nerves
how is the vestibulo-ocular reflex tested?
• Tested by holding someone’s eyes open and gently rotating head from side to side
o Normal: eyes should rotate in the head to keep gaze direction constant
o Absent reflex: eyes stayed fixed in the head
also the caloric stimulation test
what nerves are involved in the vestibulo-ocular reflex?
involves oculomotor nerve in one eye (the one moving inwards) and the abducens in the eye moving outwards
explain how the caloric stimulation test works
- Warm water is irrigated into the external auditory canal with a syringe.
- Temperature difference between body and injected water creates a convective current in the endolymph of the nearby horizontal semicircular canal
in a normal subject what happens in the caloric stimulation test with warm water?
mimics a head turn towards the ipsilateral side. Both eyes turn slowly away from irrigated ear towards the contralateral ear, followed by nystagmus (horizontal fast flick) towards the irrigated ear
in a normal subject what happens in the caloric stimulation test with cold water?
mimics a head turn to the contralateral side. Eyes turn slowly towards the ipsilateral ear with horizontal fast flicks towards the non-irrigated ear
what is nystagmus?
- A form of VOR caused by continuing rotation of fluid in semicircular canals
- Shows as an initial slow rotation followed by a fast flick back
what gives the direction of the nystagmus?
the direction of the fast flick
what is the purpose of the blink reflex?
protects eye from foreign bodies
what is the input and output to the blink reflex?
- Input: sensory nerve endings in cornea or conjunctiva; these are branches of the ophthalmic branch (V1) of the trigeminal (V) nerve.
- Output: motor fibres in the facial (VII) cranial nerve to the obicularis oculi muscle which pushes the eyelids together.
