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Year 2 LCRS - Neuroscience > Neurology of the Visual System > Flashcards

Neurology of the Visual System Flashcards

1
Q

What is the visual pathway?

A
  1. Eye
  2. Optic nerve – ganglion nerve fibres
  3. Optic chiasm – half of fibres decussate here
  4. Optic tract – ganglion fibres exit as optic tract
  5. Lateral Geniculate Nucleus – ganglion fibres synapse in nucleus
  6. Optic radiation – 4th order neuron
  7. Primary visual cortex OR Striate Cortes (occipital lobe)
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2
Q

What are the different types of neurons in the retina?

A

First order neurons – Rod and Cone retinal photoreceptors

Second order neurons – Retinal bipolar cells

Third order neurons – Retinal ganglionic cells

  • Travel down the length of the optic nerve and have a PARTIAL decussation (53% cross) at the optic chiasm
  • Fibres synapse in the LGN (Lateral Geniculate Nucleus) located within the thalamus
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3
Q

What is the receptive field?

A

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

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4
Q

What is convergence?

A

number of lower order neurons field synapsing on the same higher order neuron

The cone system has a lower order of convergence than the rod system (as the cones tend to be one-to-one whereas many photoreceptors link to a single RGC in the rod system)

Low Convergence

  • Small Receptive Field
  • Fine Visual Acuity
  • Low Light Sensitivity

High Convergence

  • Large Receptive Field
  • Coarse Visual Acuity
  • High Light Sensitivity
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5
Q

What are on centre and off centre ganglion cells?

A

On-centre ganglionic cells:

  • Stimulated by light at the CENTRE of the receptive field
  • Inhibited by light on the EDGE of the receptive field
  • Off-centre ganglionic cells:
  • Inhibited by light at the CENTRE of the receptive field
  • Stimulated by light at the EDGE of the receptive field

This is important for – contrast sensitivity and edge detection

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6
Q

What are crossed fibres?

A

originate from nasal retina, responsible for temporal visual field

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7
Q

What are uncrossed fibres?

A

originate from temporal retina, responsible for nasal visual field

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8
Q

What does a lesion at the optic chiasm cause?

A

damages crossed RGC fibres from nasal retina in both eyes -> temporal field deficit in both eyes – Bitemporal hemianopia.

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9
Q

What would a lesion posterior to the optic chiasma cause?

A

Right = left homonymous hemianopia in both eyes

Left = right homonymous hemianopia in both eyes

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10
Q

What are causes of bitemporal and homonymous heminopia?

A

Bitemporal hemianopia
- usually enlargement of pituitary gland tumour.

Homonymous hemianopia
- stroke

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11
Q

What happens in macula sparing disorder?

A

loss of peripheral vision but you retain central activity

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12
Q

What’s the difference between the consequence of glaucoma and neurological problems on vision?

A

Glaucoma often affects the HORIZONTAL plane of loss of vision

Neurological problems often affect the VERTICAL plane of loss of vision

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13
Q

What is the primary visual cortex?

A

*striate cortex

Situated along the Calcarine fissure

Characterised by a distinct stripe derived from the myelinated fibre of the optic radiation projection

Representation:
- Disproportionately large area representing the macula – due to a higher density of RGCs (lower convergence).
- Visual fields:
> Superior visual field projects BELOW the Calcarine fissure
> Inferior visual field projects ABOVE the Calcarine fissure
> Left hemi-field projects to the right
> Right hemi-field projects to the left

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14
Q

How is the primary visual cortex organised?

A

Organised as columns with unique sensitivity to visual stimulus of a particular orientation

Right and left eye columns intersperse each other

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15
Q

What are common causes of macula sparing homonymous heminopia?

A
  • Commonly due to damage of the primary visual cortex
  • Often due to stroke
  • Contralateral homonymous hemianopia with central macula sparing
  • Macula is spared due to dual blood supply.
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16
Q

What is the extrastiate cortex?

A

the area surrounding the visual cortex

Converts basic visual information, orientation and position into complex information

17
Q

What are the pathways in the extrastriate cortex?

A

Dorsal pathway:

  • Primary visual cortex -> posterior parietal cortex
  • Motion detection, visually-guided action.
  • Damage results in motion blindness

Ventral pathway:

  • Primary visual cortex -> Inferiotemporal cortex
  • Object representation, face recognition, detailed fine central vision and colour vision
  • Damage results in cerebral Achromatopsia
18
Q

What is the function of the pupillary?

A

In light – decreases spherical aberrations and glare, increases depth of field, reduces bleaching of photopigments
- Constriction mediated by PNS nerve within CN III

In dark – allows more light into eye
- Dilation mediated by SNS nerve.

19
Q

What happens in the pupillary reflex?

A
  • look at diagram

Afferent pathway:
Rod and cone photoreceptor -> bipolar cells -> RGCs -> nerves exit at posterior 1/3rd of optic tract and enter LGN -> synapse at brainstem (Pretectal nucleus) -> synapse at Edinger Westphal nucleus

Efferent pathway :
Edinger Westphal -> oculomotor nerve efferent -> synapse at ciliary ganglion -> short posterior ciliary nerve -> pupillary sphincter

  • the afferent pathway on either eye stimulates the efferent pathway in BOTH eyes

** Direct light reflex – constriction of pupil of the light stimulated eye

*** Consensual light reflex – constriction of pupil of the other eye

20
Q

What happens in right afferent defect?

A

damage to CN II

no pupil constriction in both eyes when right is stimulated, pupil constriction in both eyes when left eye is stimulated

21
Q

What happens in right efferent defect?

A

damage to right CN III

no right pupil constriction at all, left eye constricts whether left or right is stimulated

22
Q

What can you do to demonstrate weakness of the afferent pathway?

A

swinging torch test

The damage to the afferent pathway is usually incomplete or relative e.g Relative afferent pupillary defect in right eye

- Partial pupillary response
still present when the
damaged eye is
stimulated
- Elicited by the swinging
torch test – alternating
stimulation of right and
left eye with light
- Both pupils constrict when light swings to left eye with intact pathway
- Both pupils will paradoxically dilate when light swings to the right eye as a result of relatively reduced drive for pupillary constriction in both eyes
23
Q

What happens in unilateral afferent and efferent defects?

A

Unilateral Afferent Defect
- Difference response pending on which
eye is stimulated

Unilateral Efferent Defect
- Same unequal response between left and
right eye irre

24
Q

What are the movements of the eye?

A

Duction – eye movement in one eye

Version – eye movement in both eyes in the same direction, dextro- is to the right, levo- is to the left

Vergence – eye movement in both eyes in opposite directions

Convergence – simultaneous adduction (inwards) movement in both eyes when viewing a near object

25
Q

What are the speeds of eye movement?

A

Saccade – short, fast burst of movement (900deg/second) – reflexive saccade, scanning saccade, predictive saccade and memory-guided saccade

Smooth pursuit – sustained slow movement (60deg/second) – driven by motion of a moving target across the retina

26
Q

What are the muscles of the eyes?

A

4 straight muscles:

  • Superior rectus - moves eye up
  • Inferior rectus - moves eye down
  • Medial rectus - occulomotor innervation (plus all above)
  • Lateral rectus - abducens innervation.

2 oblique muscles:

  • Superior oblique - attached high on temporal side of eye and passes under superior rectus, moves eye down and in - trochlear innervation.
  • Inferior oblique – attached low on nasal side of eye and passes over inferior rectus, moves eye up and out - occulomotor innervation
27
Q

What does each muscle do to the eye?

A

Lateral rectus – abduction (away from midline)

Medial rectus – adduction (towards midline)

Superior rectus – elevated and abducted

Inferior rectus – depressed and abducted

Inferior oblique – elevated and adducted

Superior oblique – depressed and adducted

28
Q

How are the muscles innervated?

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

Fourth Cranial Nerve
Superior Oblique – depresses
eye

Sixth Cranial Nerve
Lateral Rectus – abducts eye

29
Q

What happens in 3rd nerve palsy?

A

The affected eye droops and abducts and the eyelid droops

As the third nerve is affected, the unopposed lateral rectus takes over (abducts) and the superior oblique also takes over

30
Q

What happens in 6th nerve palsy?

A

The affected eye is unable to abduct and deviates to the midline (adducts)

Double vision worsens on gazing to the side of the affected eye

31
Q

What are the directions of eye movement?

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 anterior- posterior axis of the eye

32
Q

What happens in the optokinetic nystagmus reflex?

A

Nystagmus – oscillatory eye movement

Optokinetic nystagmus – smooth pursuit + fast paced reset saccade

This reflex is used to test visual acuity in pre-verbal children by observing presence of nystagmus movement

The reflex being present is physiological and a sign the child has the visual acuity to perceive motion

Year 2 LCRS - Neuroscience (17 decks)

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