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Y2 LCRS2 Neuroscience and Mental Health > Neurology of the Visual System > Flashcards

Neurology of the Visual System Flashcards

1
Q

Name the visual pathway structures.

A
  • Eye
  • Optic Nerve – Ganglion Nerve Fibres
  • Optic Chiasm - Half of the nerve fibres cross here
  • Optic Tract – Ganglion nerve fibres exit as optic tract
  • Lateral Geniculate Nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus
  • Optic Radiation – 4th order neuron
  • Primary Visual Cortex or Striate Cortex – within the Occipital Lobe
  • Extrastriate Cortex (not within the boundaries of the visual cortex but helps with vision)
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2
Q

What are the first, second and third order neurons of the visual pathway?

A
  • 1st order neurons - rod and cone retinal photoreceptors
  • 2nd order - retinal bipolar cells
  • 3rd order - retinal ganglion cells
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3
Q

Describe the visual pathway after the retinal ganglion cells.

A

Retinal ganglion cells –> optic nerve –> partial decussation of ganglion fibres at optic chiasm to cross the midline –> optic tract –> LGN in thalamus –> visual cortex

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4
Q

What percentage of retinal ganglion fibres decussate at the optic chiasm?

A

53%

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5
Q

What is a receptive field? What is the receptive field of a photoreceptor and of retinal ganglion cells?

A

Retinal space within which incoming light can alter the firing pattern of a neuron.

Photoreceptor - a small circular space surrounding the photoreceptors.

RGC - input from neighbouring photoreceptors through convergence

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6
Q

Why can input from photoreceptors into RGC be both positive and negative?

A

Light has more photons so will produce a positive signals whereas darness has less photons but you would still see it

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7
Q

What is determined by the magnitude of convergence of receptive fields? Do rods or cones have greater convergence?

A

How sharp the image looks - ACUITY

Rod system convergence>cone system convergence

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8
Q

Is convergence greatest at the central or peripheral part of the retina?

A

Peripheral retina convergence > central retina convergence

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9
Q

Compare the features of low and high convergence systems (i.e. rods and cones)

A

Low Convergence
– Small Receptive Field - light needs to hit at a very specific place to stimulate the RGC
– Fine Visual Acuity
– Low Light Sensitivity

High Convergence
– Large Receptive Field - light can hit any of the rods to stimulate RGC
– Coarse Visual Acuity
– High Light Sensitivity

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10
Q

What are the two types of ganglion cells?

A

On centre and off centre

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11
Q

Describe the mechanism of on-centre and off-centre ganglion cells.

A

On-centre Ganglion
– stimulated by light at the centre of the receptive field
– Inhibited by light on the edge of the receptive field

Off-centre Ganglion
– Inhibited by light at the centre of the receptive field
– Stimulated by light on the edge of the receptive field

When both centre and edges are stimulated only a WEAK response is generated in both types of cells. SEE IMAGE BELOW.

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12
Q

What is the function of on and off-centre ganglion cells?

A

Important to have both for:

  1. Contrast sensitivity
  2. Enhanced edge detection
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13
Q

How do lesions anterior and posterior to the optic chiasm affect vision?

A
  • Lesions anterior to Optic Chiasma affect visual field in one eye only
  • Lesions posterior to Optic Chiasma affect visual field in both eyes
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14
Q

Where do the crossed fibres at the optic chiasm usually originate?

A
  • *Crossed Fibres** – originating from nasal retina, responsible for temporal visual field
  • *Uncrossed Fibres** – originating from temporal retina, responsible for nasal visual field
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15
Q

What type of visual defect would damage at the optic chiasm cause?

A
  • Nasal retinal fibres would be damaged in both eyes
  • Temporal field deficit in both eyes would result –> bitemporal hemianopia

Closest points (in the middle) can still be seen because their light rays are being projected to the temporal side of the opposite eye. See below.

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16
Q

What type of visual field defect would result from a lesion posterior to the optic chiasm?

A

– Right sided lesion – Left Homonymous Hemianopia in Both Eyes
– Left sided lesion – Right Homonymous Hemianopia in Both Eyes

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17
Q

List different disorders of the visual pathway and state where the defect would be.

A
  1. Normal vision
  2. Mononuclear blindness - one optic nerve affected
  3. Bitemporal hemianopia - middle of chiasm
  4. Right nasal hemianopia = one half of the chiasm is affected
  5. Homonymous hemianopia = damage on the tract
  6. Quadrant anopia = damage further back
  7. Macular sparing = occurs when the damage is furthest back on the cortex

The further back the damage the smaller the visual field affected.

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18
Q

How can you tell, by looking at a visual field plot, where the defect is located?

A

Neurological field defects usually do not cross the vertical line

Whereas glaucoma doesn’t cross the horizontal line.

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19
Q

What is a common cause of bitemporal hemianopia?

A

Enlargement of a pituitary gland tumour . Pituitary gland sits under the optic chiasm.

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20
Q

What is a common cause of homonymous hemianopia? Describe and explain the visual field.

A

Stroke (cerebrovascular accident) –> visual field loss on the same side of both eyes

  • right half of brain has visual pathways for the left hemifield of both eyes
  • left half of the brain visual pathways for the right hemifield of both eyes
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21
Q

What is the cause of macular sparing in homonymous hemianopia?

A

Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides

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22
Q

What is the result of damage to teh primary visual cortex e.g. in stroke?

A

Contralateral homonymous hemianopia with macular sparing

(so if on right side the the left side of the left and right eye will be affected)

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23
Q

Where is the primary visual cortex situated? What is it also known as?

A
  • Along the Calcarine sulcus within the occipital lobe
  • AKA Striate cortex
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24
Q

What is a distinct characteristic of the striate cortex?

A

It has a distinc stripe derived from the myelinated fibre of the optic radiation projecting into the visual cortex.

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25
Q

Describe the representation of different parts of the visual field/retina on the primary visual cortex.

A
  • Right hemifield from both eyes–> left primary visual cortex
  • Left hemifield from both eyes –> right primary visual cortex
  • Superior visual field –> projects below the Calcarine fissure
  • Inferior visual field –> projects to above the Calcarine fissure

There is a disproportionately large area representing the MACULA

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26
Q

Descirbe the organisation of the primary visual cortex into columns.

A

NB: right of right+right of left –> left primary visual cortex etc. So right and left eye regions are present on each side of visual cortex.

  • Organised into columns which have a unique sensitivity to visual stimuli of a particular orientation (allows depth perception)
  • Right eye dominant and left eye dominant columns intersperse each other

T**his is why optic nerve regeneration isn’t very easy - each ganglion cell projects to a very specific part of the visual cortex.

27
Q

What is the location of the Extrastriate cortex? What is its function?

A

It is an area arround the primary visual cortex within the occipital lobe

  • it converts basic visual information, orientation and position into complex information.
28
Q

What is the function of the dorsal pathway in the brain? Where does it run?

A
  • Motion detection
  • Visually guided action

Location: PVC –> Posterior parietal cortex

29
Q

What is the function of the ventral pathway in the brain? Where does it run?

A
  • object representation
  • face recognition
  • detailed fine central vision
  • colour vision

Location: PVC –> inferiotemporal cortex.

30
Q

What are the consequences of damage to

a) Dorsal pathway
b) Ventral pathway

A

a) Motion blindness
b) Cerebral achromatopsia

31
Q

What is the purpose of pupil constriction in light?

A
  • Decreases spherical aberrations and glare
  • Increases depth of field (see near response triad)
  • Reduces bleaching of photo-pigments
32
Q

What is the purpose of pupil dilation in darkness?

A

Increases light sensitivity in the dark by allowing more light into the eye

33
Q

Which nerves mediate pupil constriction and dilation?

A

Constriction - parasympathetic nerve within CNIII

Dilation - sympathetic nerve

34
Q

What is the afferent pathway of the pupillary reflex?

A

Afferent pathway (Red & Green)

  1. Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells
  2. Pupil-specific ganglion cells exits at posterior third of optic tract before
  3. entering the Lateral Geniculate Nucleus
  4. Synpases at Brain Stem (Pretectal Nucleus)
  5. Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
35
Q

What is the efferent pathway of the pupillary reflex?

A

Efferent pathway (Blue)

  1. Edinger-Westphal Nucleus ->
  2. Oculomotor Nerve Efferent ->
  3. Synapses at Ciliary ganglion ->
  4. Short Posterior Ciliary Nerve ->
  5. Pupillary Sphincter
36
Q

Would a patient in a coma still have a pupillary reflex? Why?

A

Yes because it goes through the brainstem - no primary visual cortex involvement occurs

37
Q

What are the differences between a direct and consentual pupillary reflex? What is the neurological basis?

A
  • Direct light reflex - constriction of pupil in the light stimulated eye
  • Consentual light reflex - constriction of pupil in the other (non-light stimulated) eye

Neurological basis - afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides.

38
Q

Describe the consequences of a right afferent defect in the pupillary reflex.

A

e.g. damage to the optc nerve

  • No pupil constriction in BOTH eyes when RIGHT eye is stimulated with light
  • Normal pupil constriction in BOTH eyes when the LEFT eye is stimulated with light
39
Q

Describe the consequences of a right efferent defect in the pupillary reflex.

A

e.g. damage to the right CNIII

  • No pupil constriction whether right of left eye is stimulated with light
  • Left pupil constriction whether right or left eye is stimulated with light
40
Q

Describe how the magnitude of pupillary response varies in unilateral afferent and efferent defects, depending on which eye is stimulated.

A

Unilateral afferent - difference in response depending on which eye is stimulated

Unilateral efferent - SAME unequal response between the left and right eye irrespective of which eye is stimulated.

41
Q

Descirbe the basis of the swinging torhc test.

A

Used to test relative afferent pupillary defects (RAPD):

  • Partial pupillary response still present when damaged even is stimulated
  • Elicicted by swinging torch test - alternating stimulation of right and left eye with light
  • Both pupils constrict when the light swings to the undamaged side
  • Both pupils paradoxically dilate when the light swings to the damaged side (=positive RAPD of *damaged* eye)
42
Q

Defiine duction.

A

Eye movement in one eye

43
Q

Define version.

A

Simultaneous movement of both eyes in the same direction

44
Q

Define vergence.

A

Simulatneous movement of both eyes in the opposite direction

45
Q

Define convergence in terms of eye movement.

A

Simultaneous adduction movement in both eyes when viewing a near object

46
Q

What are the two speeds of eye movement? Which is faster?

A

Saccade - short fast burst, up to 900 degrees/second

Smooth pursuit - sustained slow movement - up to 60 degrees/second. This is driven by motion of a moving target across the retina.

47
Q

What are the 4 types of saccade eye movement?

A
  1. Reflexive saccade -to external stimuli
  2. Scanning saccade
  3. Predictive saccade
  4. Memory guided-saccade
48
Q

Name the 6 extraocular muscles.

A
  1. Superior rectus
  2. Inferior rectus
  3. Lateral rectus
  4. Medial rectus
  5. Superior oblique
  6. Inferior oblique
49
Q

Describe the attachment of the superior and inferior rectus to the eye and their functions.

A
  • Superior recus: attached at 12o’clock at an angle; moves the eye up.
  • Inferior rectus: attached at 6o’clock; moves the eye down

Note that this is a superior view:

50
Q

Describe the attachment of the lateral and medial rectus to the eye and their functions.

A

Lateral rectus (external rectus) = attaches on temporal side of eye; moves eye outwards

Medial rectus (internal rectus) = attached on nasal side of the eye; moves eye inward

51
Q

Describe the attachment of the superior and inferior oblique to the eye and describe its function.

A

Superior oblique (right eye shown on picture; superior view)

  • comes from back of eye –> travels through the trochlea –> –> passes under superior rectus + attaches to the temporal side of the eye.
  • This moves the eye diagonally - down and in

Inferior oblique

  • Passes over the inferior rectus and attaches to the nasal side of the eye
  • Moves eye diagonally - up and out
52
Q

Name the muscle of the eye that are innervated by the superior and inferior branch of the third cranial nerve.

A

Third Cranial Nerve

Superior Branch

  • Superior Rectus – elevates eye
  • Lid Levator – raises eyelid

Inferior Branch

  • Inferior Rectus – depresses eye
  • Medial Rectus – adducts eye
  • Inferior Oblique – elevates eye
  • Parasympathetic Nerve –constricts pupil
53
Q

Describe the muscles of the eye innervated by the fourth cranial nerve.

A

Superior oblique - depresses the eye

54
Q

Name the muscles of the eye innervated by the sixth cranial nerve.

A

Lateral rectus - abducts the eye

55
Q

CNIII lesion in one eye - will the patient complain of double vision?

A

No because the eye is shut, CNIII also innervates the upper eyelid.

56
Q

Describe how you would test these muscles individually.

  1. Lateral rectus
  2. Medial rectus
  3. Inferior rectus
  4. Superior rectus
A

To test extraocular muscles you need to isolate the mucscle to be tested by maximising its action and minimizing the action of other muscles:

  1. Lateral rectus - abduction
  2. Medial rectus - adduction
  3. Superior rectus - elevation and abduction
  4. Inferior rectus - depression and abduction
57
Q

Describe how you would test the superior oblique and inferior oblique muscles individually.

A

Superior oblique - depression and adductoin

Inferior oblique - elevation and adduction

Testing the superior oblique - once the eyeball has been adducted the eyeball is now parallel with the superior oblique and so only the superior oblique can depress the eye

58
Q

Namen using the correct terminology, the different directions of eye movement for one or both eyes.

A

Up (Elevation)

  • Supraduction – one eye
  • Supraversion – both eyes

Down (Depression)

  • Infraduction – one eye
  • Infraversion – both eyes

Right – Dextroversion

  • Right Abduction
  • Left Adduction

Left – Levoversion

  • Right Adduction
  • Left Abduction

Torsion – rotation of eye around the anteriorposterior axis of the eye

59
Q

What are the signs of third nerve palsy?

A

Affected eye down and out

Droopy eyelid (or completely closed if complete lesion)

  • Unopposed superior oblique innervated by CNIV –> down
  • unopposed lateral rectus action innervated by CNVI –> out
60
Q

What are the signs and symptoms of sixth nerve palsy?

A
  • Affected eye is unable to abduct and deviates inwards
  • Double vision worsens on gazing to the side of the affected eye
61
Q

What is a sign of fourth nerve palsy?

A

NB: LR6SO4

  • Up and in movement of the eye
62
Q

What is the optokinetic nystagmus reflex?

A
  • oscillatory eye movement
  • composed of smooth pursuit tracking and fast phase reset saccade
63
Q

When is the optokinetic nystagmus reflex useful?

A

In testing visual acuity of pre-verbal children and observing the nystagmus movements in response to moving grating patterns on an scree on various spatial frequencies

Presence of optokinetic nystagmus in response to moving grating = patient has sufficient visual acuity to perceive the grating pattern

Y2 LCRS2 Neuroscience and Mental Health (17 decks)

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