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visual pathways Flashcards

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1
Q

describe where the optic nerve originates and its continuation

A

Eye –> Optic nerve –> optic chiasm: axons pass into optic tract

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2
Q

what does the optic chiasm lie above?

A

the pituitary gland

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3
Q

what can pituitary tumours cause?

A

bilateral loss of peripheral receptive fields

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4
Q

explain the lateral geniculate-occipital cortex pathway

A
  • 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)
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5
Q

what is Meyer’s loop?

A

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

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6
Q

what does damage to the meyer’s loop on the LHS of the brain cause?

A

blindness in the top right hand side of the field of both eyes

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7
Q

where do the optic radiation fibres mediating vision from the upper quadrants travel?

A

through the meyer’s loop - loop more anteriorly around the side of the lateral ventricle

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8
Q

where do the optic radiation fibres mediating vision from the lower quadrants travel?

A

travel more directly back to the visual cortex

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9
Q

which pathway mediates visual perception?

A

the lateral geniculate-occipital cortex pathway

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10
Q

how can the lateral geniculate-occipital cortex pathway get damaged? what does this cuase?

A

by an infarct in the posterior cerebral artery - can cause a person to be blind

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11
Q

why are the superior colliculi needed?

A

to track a moving object

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12
Q

what do the superior colliculi attach to?

A

medial longitudinal fasciculi

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13
Q

what do the MLF link and synchronise?

A

oculomotor nuclei

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14
Q

what is another name for the superior colliculi?

A

optic tectum

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15
Q

describe the path of fibres involved in the pupillary light reflex

A
  • 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
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16
Q

what is the left visual field?

A

everything to the left of your fixation point in both eyes – seen by the right visual cortex

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17
Q

explain how both eyes contribute to both visual fields

A
  • 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
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18
Q

what does a partial optic nerve lesion cause?

A

macula loss - ipsilateral scatoma

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19
Q

what is a scatoma?

A

patch of blindness in the visual field of one eye

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20
Q

what does a complete optic lesion lead to?

A

blindness in that eye

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21
Q

what does an optic chiasm lesion cause?

A

lose extremes of vision

bitemporal hemianopia - tunnel vision

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22
Q

what causes an optic chiasm lesion?

A

pituitary tumour pressing on the optic chiasm

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23
Q

what does an optic tract lesion cause?

A

loss of same half of the visual field in both eyes

Known as homonymous hemianopia

24
Q

what does damage to the meyer’s loop lead to?

A

damage to temporal branch – lose upper quadrant

Known as homonymous upper quadrant anopia

25
Q

what does an optic radiation lesion lead to?

A

homonymous hemianopia. Foveal sparing occurs.

26
Q

what does a visual cortex lesion lead to?

A

lateral portion is not damaged (only medial) so there is macular sparing: no scotoma in centre.
• Also: homonymous hemianopia

27
Q

what is macular sparing?

A

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

28
Q

what is anton-babinski syndrome?

A

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)

29
Q

what do visual pathways to the brainstem mediate?

A

visual reflexes

30
Q

which nerves and muscles mediate visual reflexes?

A

pupillary sphincter muscles and the III, IV, & VI cranial nerves

31
Q

what is the input and output for the pupillary light reflex

A

input - cranial nerve 2

output - cranial nerve 3

32
Q

what is the pupillary light reflex?

A

Consensual reflex where stimulation of one eye leads to constriction in both eyes

33
Q

what drug blocks the pupillary light reflex?

A

Atropine (muscarinic antagonist drug) blocks the pupillary light reflex

34
Q

what is the accommodation reflex?

A

Reflex in response to focusing on a near object then looking at a distant object (and vice versa)

35
Q

what are the 3 components of the accommodation reflex?

A

pupillary constriction, thickening of the lens and convergence

36
Q

what is the input and output for the accommodation reflex?

A

Input from descending projections from frontal eye fields in the premotor cortex cause constriction
output - cn 3

37
Q

what are the frontal eye fields and what do they control?

A

specialized parts of the premotor area dedicated to the motor control of the extra-ocular eye muscles

control pupillary constriction

38
Q

what does damage to FEFs cause?

A

inability to direct gaze from one object to another + loss of fast phase of nystagmus

39
Q

explain what happens to the lens and how when viewing a distant object

A
  • 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
40
Q

explain what happens to the lens and how when viewing a close object?

A

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

41
Q

what happens to the lens during the accommodation reflex?

A

the ciliary muscle contracts and the lens rounds to focus on close objects

42
Q

what controls convergence of the eyes?

A

controlled from frontal eye fields

43
Q

what mediates convergence of the eye?

A

oculomotor nerve bilaterally

44
Q

which muscles are simultaneously activated in the convergence of the eye?

A

medial rectus muscles of both eyes

45
Q

what can disturb the convergence of the eyes?

A

can be disturbed by fatigue, trauma, alcohol and drugs

46
Q

what can incorrect or absent convergence of the eye lead to?

A

diplopia - double vision

47
Q

when does the vestibulo-ocular reflex occur?

A

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

48
Q

what is the input and output of the vestibulo-ocular reflex?

A
  • Input (afferent) arc is the vestibulo-cochlear nerve – receives signals from the semicircular canals
  • Output is the abducens and oculomotor nerves
49
Q

how is the vestibulo-ocular reflex tested?

A

• 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

50
Q

what nerves are involved in the vestibulo-ocular reflex?

A

involves oculomotor nerve in one eye (the one moving inwards) and the abducens in the eye moving outwards

51
Q

explain how the caloric stimulation test works

A
  • 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
52
Q

in a normal subject what happens in the caloric stimulation test with warm water?

A

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

53
Q

in a normal subject what happens in the caloric stimulation test with cold water?

A

mimics a head turn to the contralateral side. Eyes turn slowly towards the ipsilateral ear with horizontal fast flicks towards the non-irrigated ear

54
Q

what is nystagmus?

A
  • 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
55
Q

what gives the direction of the nystagmus?

A

the direction of the fast flick

56
Q

what is the purpose of the blink reflex?

A

protects eye from foreign bodies

57
Q

what is the input and output to the blink reflex?

A
  • 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.

brain and behaviour (24 decks)

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