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Neuroscience and Mental Health > 13. Neurology of the visual system > Flashcards

13. Neurology of the visual system Flashcards

1
Q

Describe the path of a signal from the eye to the visual cortex (i.e. the visual pathway)

A
  • Optic nerve - ganglion nerve fibres
  • Optic chiasma - nasal half cross over
  • Optic tract
  • Lateral geniculate nucleus - ganglion nerve fibres synapse
  • Optic radiation - 4th order neurone
  • Primary visual cortex (aka Striate Cortes) - in occipital lobe
  • Extra-striate cortex - helps with vision and other related functions
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2
Q

Where are the cell bodies of the optic nerve?

A

Retina

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3
Q

Where do the ganglion nerve fibres from the retina synapse with the next order?

A

Lateral geniculate nucleus (relay centre in the thalamus)

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4
Q

What are the first and second order neurones in the visual pathway?

A
  • First - rod and cone retinal photoreceptors

* Second - retinal bipolar cells

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5
Q

What are the third order neurones in the visual pathway?

A 
Retinal ganglion cells
• Optic nerve (II)
• Partial decussation at optic chiasma (53% of fibres)
• Optic tract
• LGN
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6
Q

When do the fibres in the visual pathway become myelinated?

A

After entering the optic nerve

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7
Q

What is the receptive field of a neurone?

A

Retinal space within which incoming light can alter the firing patter of a neurone

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8
Q

What is ‘convergence of receptive field’?

A

Number of lower order neurones field synapsing on the same higher order neurone
i.e. • low convergence: cone= bipolar cell = ganglion cell
• high convergence: cones > bipolar cells > retinal ganglion cells

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9
Q

How is convergence different in the cone system and rod system?

A

Rod system convergence > cone system convergence

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10
Q

How is the convergence different in the central and peripheral retina?

A

Central retina convergence > peripheral retina convergence

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11
Q

Describe the receptive field, visual acuity and sensitivity of low convergence?

A

(Cones or peripheral)
• small receptive field
• fine visual acuity
• low light sensitivity

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12
Q

Describe the receptive field, visual acuity and sensitivity of high convergence?

A

(Rods or centre)
• large receptive field
• coarse visual acuity
• high light sensitivity

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13
Q

What are on-centre ganglion cells?

A
  • Stimulated by light at the centre of the receptive field

* Inhibited by light on the edge of the receptive field

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14
Q

What are off-centre ganglion cells?

A
  • Inhibited by light at the centre of the receptive field

* Stimulated by light on the edge of the receptive field

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15
Q

What are on and off-centre ganglion cells important for?

A
  • Contrast sensitivity

* Enhanced edge detection

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16
Q

How many eyes do lesions, anterior and posterior to the optic chiasm, affect?

A
  • Anterior - one eye

* Posterior - both eyes

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17
Q

Which visual field are crossed fibres (at the optic chiasm) responsible for?

A
  • Originate from nasal retina

* Responsible for temporal visual field

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18
Q

Which visual field are uncrossed fibres (at the optic chiasm) responsible for?

A
  • Originate from temporal retina

* Responsible for nasal visual field

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19
Q

Which hemisphere of the brain does information of the right visual field go to?

A

Left hemisphere

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20
Q

What visual disorder does a lesion at the optic chiasma cause?

A
  • Damaged crossed ganglion fibres (nasal retina - temporal field)
  • Bitemporal hemianopia
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21
Q

What does a (right/left sided) lesion posterior to the optic chiasma cause?

A
  • Right sided - left homonymous hemianopia in both eyes

* Left sided - right homonymous hemianopia in both eyes

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22
Q

What usually causes a bitemporal hemianopia?

A

Enlargement of pituitary gland tumour (sits under optic chiasma)

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23
Q

What usually causes a homonymous hemianopia?

A

Stroke

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24
Q

What is a quadrantanopia?

A

Visual field loss in same quadrant of both eyes

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25
Q

What is macular sparing?

A

Visual field loss that preserves vision in the centre of the visual field

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26
Q

Where is the primary visual cortex located?

A
  • Along the calcarine sulcus, within the occipital lobe
  • Characterised by a distinct stripe derived from the myelinated fibre of the optic radiation projecting into the visual cortex
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27
Q

How are the different visual fields represented in the primary visual cortex?

A
  • Disproportionately large area representing the macula
  • Area above the calcarine fissure represents the inferior visual field
  • Area below the calcarine fissure represents the superior visual field
  • Right hemifield from both eyes projects onto Left PVC
  • Left hemifield from both eyes projects onto right PVC
28
Q

How is the primary visual cortex organised (at a more microscopic level)

A
  • Functional columns
  • Each column is sensitive to visual stimuli of a particular orientation
  • Right and left dominant columns
29
Q

What does the brain compare information from the left and right eye for?

A

Understand depth

30
Q

What usually causes a macula sparing homonymous hemianopia and how?

A
  • Stroke
  • Leads to contralateral homonymous hemianopia (loss on both sides) with macula sparing
  • Area representing macula receives dual blood supply from PCA from both sides
  • Good protection and blood supply allows sparing
31
Q

What is the extra-striate cortex and its function?

A
  • Area around the PVC within the occipital lobe
  • Converts basic visual information (position + orientation) into complex human precepts (motion + object representation)
32
Q

What is the dorsal pathway involved with?

A
  • aka the How Pathway
  • Relays information from the PVC to the posterior parietal lobe
  • Deals with motion detection
33
Q

What is the ventral pathway involved with?

A
  • aka the What Pathway
  • Relays information from the PVC to the temporal visual cortex
  • Handles detailed object representation and face recognition
34
Q

What could damage to the ventral pathway lead to?

A

Cerebral Achromatopsia - inability to perceive colour despite normal functioning cone photoreceptors

35
Q

How does pupil constriction change vision and how is it mediated?

A
  • Decreases spherical aberrations and glare
  • Increases depth of field and reduces amount of light entering the eye
  • Reduces bleaching of photo-pigments

• Parasympathetic nerve (in CN III)

36
Q

How does pupil dilatation chance vision?

A

• Increases light sensitivity in the dark

  • Radial muscle contracts
  • Mediated by sympathetic nerve
37
Q

Describe the afferent pathway of the pupillary reflex

A
  • Rod and cone photoreceptors => bipolar cells => retinal ganglion cells
  • Pupil-specific ganglion cells exit at the posterior third of optic tract before entering the LGN
  • Synapses at dorsal brain stem (Pretectal Nucleus)
  • Synapses on Edinger-Westphal nuclei on both sides in the brainstem
  • Nerves from left side of each eye go to left half of brainstem
38
Q

Describe the efferent pathway of the pupillary reflex

A
  • Edinger-Westphal Nucleus => Oculomotor nerve efferent
  • Synapses at ciliary ganglion (anterior to the optic chiasm)
  • Short posterior ciliary nerve => pupillary sphincter

(parasympathetic)

39
Q

What is direct and consensual light reflex?

A
  • Direct - refers to the constriction of the pupil of the light-stimulated eye
  • Consensual - refers to the constriction of the pupil of the fellow eye
40
Q

What is the neurological basis of the consensual light reflex?

A

Afferent pathway on either side alone will stimulate the efferent pathway on both sides

41
Q

How will a right afferent defect affect pupil constriction?

A
  • When right eye is stimulated with light, no pupil constriction
  • When left eye is stimulated with light, normal pupil constriction in both eyes
42
Q

How will a right efferent defect affect pupil constriction?

A
  • No right pupil constriction at all

* Left pupil constricts, whether right or left eye is stimulated with light

43
Q

How can you test for pupillary reflex damage?

A

Swinging torch test

44
Q

What is duction?

A

Eye movement in one eye (without making any reference to the movement of the fellow eye)

45
Q

What is version?

A

Movement of both eyes in the same direction
• Gazing to the right: dextroversion
• Gazing to the left: levoversion

46
Q

What is vergence?

A

Movement of both eyes in opposite directions simultaneously

47
Q

What is convergence?

A

Simultaneously adduction (inward) movement in both eyes when viewing a near object

48
Q

What is saccade and what are the different types?

A 
Short fast burst movement (up to 900deg/sec)
• reflexive saccade
• scanning saccade
• predictive saccade (track objects)
• memory-guided saccade
49
Q

What is smooth pursuit?

A
  • Sustained slow movement (up to 60deg/sec)
  • Driven by motion of a moving target across the retina
  • Used for hunting to locate and follow the object more clearly
50
Q

What are the 6 extraocular muscles?

A
  • Superior rectus
  • Inferior rectus
  • Lateral rectus
  • Medial rectus
  • Superior oblique
  • Inferior oblique
51
Q

Which general parts of the orbit do the extraocular muscles come out from?

A
  • 5/6 from the back

* Inferior oblique comes in nasally

52
Q

What kind of motion do the vertical rectus muscles produce when the eye is adducted and abducted?

A
  • Adducted (towards midline) - anterior-posterior axis not aligned, torsion motion produced
  • Abduction - anterior-posterior axis is aligned, maximal elevation and depression
  • Small amount of rotation nasally due to angle of attachment of vertical rectus muscles
53
Q

Where is the superior oblique muscle attached and what movement does it produce?

A
  • Travels through the trochlea
  • Attached high on the temporal side of the eye
  • Passes under the superior rectus
  • Down + out
54
Q

Where is the inferior oblique muscle attached and what movement does it produce?

A
  • Attached low on the lateral side of the eye
  • Passes over the inferior rectus
  • Moves the eye in a diagonal pattern - up + out
55
Q

What does the oculomotor nerve (III) control in the eye?

A
  • Superior rectus
  • Lid levator - raises eyelid
  • Inferior rectus
  • Medial rectus (adducts eye)
  • Inferior Oblique (elevates and abducts)
  • Parasympathetic nerve - constricts pupil
56
Q

What does the trochlear nerve (IV) control in the eye?

A

Superior oblique (depresses and abducts)

57
Q

What does the abducens nerve (VI) control in the eye?

A

Lateral rectus (abducts eye)

58
Q

What is supraversion?

A

Elevation of both eyes i.e. simultaneous right and left eye supraduction

59
Q

What is infraversion?

A

Depression of both eyes i.e. simultaneous right and left eye infraduction

60
Q

What is the difference between supraduction and supraversion?

A
  • Supraduction - one eye moving up

* Supraversion - both eyes moving up

61
Q

What is torsion?

A

Rotation of the eye around the anterior-posterior axis eye

62
Q

What is third nerve palsy?

A
  • Oculomotor nerve palsy
  • Only muscles not innervated by CN III are working - lateral rectus + superior oblique muscle
  • Eye moves down and out
  • Droopy eyelid (ptosis)
63
Q

What is sixth nerve palsy?

A
  • Abducens nerve palsy
  • Deficit in abduction of affected eye (by lateral rectus)
  • Affected eye deviates inwards
  • Double vision worsens on gazing to the side of the affected eye
64
Q

What is optokinetic nystagmus?

A
  • Form of physiological nystagmus, triggered by the presentation of a constantly moving grating pattern
  • Smooth pursuit + fast phase reset saccade
65
Q

What is the optokinetic nystagmus reflex useful for testing?

A
  • Visual acuity in pre-verbal children
  • Observing presence of nystagmus movement in response to moving grating patterns of various spatial frequencies
  • Presence of optokinetic nystagmus in response to moving grating signifies that that patient has sufficient visual acuity to perceive the pattern

Neuroscience and Mental Health (17 decks)

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