Neuro 13 - Neurology of the Visual System

Question 1

Describe the visual pathway anatomy

Answer 1

Landmarks:

Eye –> optic nerve –> optic chasm –> optic tract —> lateral geniculate nucleus (ganglion fibres synapse here) —> optic radiation (4th order neuron) —> primary visual cortex (also extra striate cortex)

Question 2

Where is the primary visual cortex located?

Answer 2

Occipital lobe

Question 3

Describe the visual pathway in the retina

Answer 3

1. First order neurones = rod and cone retinal photoreceptors
2. 2nd order neurones = retinal bipolar cells
3. 3rd order neurones = retinal ganglion cells (i.e. Optic nerve - which partially decussates at optic chiasm (53% crossing midline), optic tract)

Question 4

Describe the retinal field of a neuron

Answer 4

Retinal space within which incoming light can alter firing pattern of a neurones

Photoreceptors feed into bipolar cells which feed into ganglion cells (+/- signals). Retinal ganglion cells get input from neighbouring photoreceptors (convergence)

Question 5

What is convergence

Answer 5

Number of lower order neurones fields synapsing on same higher order neuron

Question 6

Cones have lower convergence than rods. Central convergence is …. than peripheral convergence

Answer 6

Central convergence < peripheral convergence

Question 7

Contrast high and low convergence

Answer 7

Low convergence (cones):

1. Small receptive field
2. Fine visual acuity
3. Low light sensitivity

High convergence (rods):

1. Large receptive field
2. Coarse visual acuity
3. High light sensitivity

Question 8

Describe the 2 types of retinal ganglion cells

Answer 8

On centre:

1. Stimulated by light at centre of receptive field
2. Inhibited by light on edge of receptive field

Off centre:

1. inhibited by light in centre of receptive field
2. stimulated by light on edge of receptive field

On/off centre ganglion cells are important for contrast Sensitivity and enhanced edge detection

Question 9

Describe crossing and uncrossing of fibres in the optic chiasm

Answer 9

Crossing fibres = originate from nasal retina, responsible for temporal visual field

Uncrossing fibres = originate from temporal retina, responsible for nasal visual field

Question 10

Describe how lesions may affect the visual field

Answer 10

Lesions anterior to optic chiasm = affects visual field in one eye only

Lesions posterior to optic chiasm = affects vision in both eyes

Question 11

When might one get left anopia (i.e. blindness in left side)

Answer 11

If left optic nerve damaged/lesioned

Question 12

When might one get bitemporal hemianopia (i.e. outside temporal visual fields not present)

Answer 12

If lesion at optic chiasm

Question 13

When might one get right homonymous hemianopia (i.e. can’t see in Lt nasal and Rt temporal fields)

Answer 13

Left sided lesion posterior to optic chiasm

Question 14

Homonymous hemianopia is typically due to?

Answer 14

Stroke

Question 15

Bitemporal hemianopia is typically due to?

Answer 15

Pit gland tumour - most common

Question 16

When might right nasal hemianopia occur

Answer 16

If only half of the optic chiasm is affected

Question 17

Most visual field defects affect the vertical line and not the horizontal line (except in quadrant anopia). What eye condition can affect the horizontal field line?

Answer 17

Glaucoma

Question 18

Where is the primary visual cortex located

Answer 18

Along the cal calcarine sulcus in the occipital lobe (has distinct stripe from myelinated fibre of optic radiation projecting into visual cortex)

Question 19

How is the primary visual cortex represented

Answer 19

1. Disproportionately large area for macula
2. Superior visual field –> projects BELOW calcarine fissure
3. Inferior visual field –> projects ABOVE calcarine fissure
4. Rt hemifield (stuff seen from R side of both eyes) —> L primary visual cortex
5. Lt hemifield (stuff seen from L side of both eyes) —> R primary visual cortex

Question 20

Describe the arrangement of the primary visual cortex in relation to its function

Answer 20

Arranged in columns —> R eye and L eye dominant columns intersperse each other

Each ganglion goes to a specific nucleus. This is what makes rewiring difficult.

Each column has unique sensitivity to visual stimulus of a particular orientation

Question 21

How does macular sparing homonymous hemianopia arise

Answer 21

1. Damage to primary VC due to stroke
2. This leads to contralateral homonymous hemianopia (with macula sparing)
3. The macula is spared because the area representing the macula receives dual blood supply from posterior cerebral arteries of both sides

Question 22

Describe the extra striate cortex pathways

Answer 22

(Extrastriate cortex = area around primary VC in occipital lobe - it converts basic visual info, orientation fn position into complex information)

2 pathways-

1. Dorsal pathway - Primary VC to posterior parietal cortex. Needed for motion detection, visually guided action. Damage results in motion blindness
2. Ventral pathway - Primary VC to inferotemporal cortex. Needed for object representation, face recognition, detailed fine central vision and colour vision. Damage may lead to cerebral achromatopsia

Question 23

Pupillary function is to regulate light input into the eye (less than 2 log unit change). How does this differ in light and dark

Answer 23

In light: constriction (circular)

• Decreases spherical aberrations and glare, increases depth of field, reduces bleaching of photo-pigments (less light enters eye)
• Constriction mediated by PNS (CN3)

In dark: dilatation (radial)

• increases light sensitivity by allowing more light into eye
• mediated by SNS

Question 24

Describe the pathways of the pupillary reflex

Answer 24

Afferent:

1. Rod and cone photoreceptors synapse on bipolar cells, which synapse on retinal ganglion cells
2. Retinal ganglion cells exit at posterior third of optic tract–> then enter lateral geniculate nucleus
3. Synapses at brainstem (in pretectal nucleus)
4. Synapses on Edinger-Westphal nuclei on both sides in brainstem

Efferent pathway:
1. Edinger-westphal —> oculomotor nerve —> synapses at ciliary ganglion —-> short posterior ciliary nerve —> pupillary sphincter

Question 25

What is the neurological basis for the consensual light reflex

Answer 25

Afferent pathway on either side stimulates efferent on both sides

Question 26

Contrast defects in (right) afferent and efferents

Answer 26

Right afferent defect (e.g. damage to optic nerve) –> No constriction in either eye when R stimulated, but normal constriction in both when L stimulated

Right efferent defect (e.g. damage to oculomotor nerve) –> No R constriction when R or L stimulated, but L constricts when either eye stimulated

Question 27

What is the swinging torch test used for

Answer 27

Testing relative afferent pupillary defects

Alternating stimulation of R and L eye

Both constrict when light moves to undamaged side

Both pupils (paradoxically) dilate when light swings to damaged side (as relatively reduced drive for pupillary constriction)

Question 28

Define duction, version, vergence, convergence

Answer 28

Duction - eye movement in one eye

Version - simultaneous dual eye movement in same direction (dextroversion - gazing to R, levoversion - gazing to L)

Vergence - simultaneous dual eye movement in opposite direction

Convergence - simultaneous adduction in both eyes (when viewing near object)

Question 29

What are the terms used for different eye movement speeds

Answer 29

1. Saccade - short, fast burst (900 degrees /s) - voluntary or involuntary
   - can be reflexive saccade to external stimuli
   - scanning saccade (e.g. reading)
   - Predictive (to track objects)
   - memory guided saccade
2. Smooth pursuit - sustained slow movement (unto 60 degrees/s)
   - driven by motion of moving target across retina

Question 30

There are 6 extra ocular eye muscles. 5 out of 6 come from cone from back of orbit. Which is the exception and where does it come from

Answer 30

Inferior oblique - comes in nasally

6 extra ocular muscles used for straight and rotary movement

Question 31

Describe the 4 straight muscles of the eye

Answer 31

1. Superior rectus (attached to eye at 12 o’clock, moves eye up)
2. Inferior rectus (6 o’clock, moves eye down)
3. Lateral rectus - attaches high on temporal side of eye - moves eye towards temple
4. Medial rectus - attached on nasal side of eye, moves eye towards nose

Question 32

Describe the 2 rotary muscles of the eye

Answer 32

1. Superior oblique - attached high on temporal side of eye. Passes under superior rectus. Anatomically moves eye down and in. Travels through trochlea
2. Inferior oblique - attached low on nasal side of eye. Passes over inferior rectus. Anatomically, moves eye up and outwards

Question 33

The oculomotor nerve supplies 4/6 extra ocular muscles. Describe the extraocular muscles it innervates and other things it innervates

Answer 33

CN3 divides into superior and inferior branches when it enters orbit

Superior branch - 1. Superior rectus- elevate eye
2. Lid levator - elevates eyelid

Inferior branch -

1. Inferior rectus - depresses eye
2. Medial rectus - adducts eye
3. Inferior oblique - elevates eye
4. PNS nerve - constricts pupil (circular muscle)

Question 34

CN4 innervates which extra ocular muscle?

Answer 34

Superior oblique = trochlear

Superior oblique depresses eye

Question 35

CN6 innervates which extra ocular eye muscle?

Answer 35

Lateral rectus - abducts eye

Question 36

What can a CN3 palsy cause

Answer 36

Affected eye goes down and out (as superior oblique and lateral rectus only muscles not working)

Also causes droopy eyelid (ptosis) –> lid elevator muscle not functioning

Question 37

What may a CN6 palsy cause?

Answer 37

Affected eye unable to abduct (LR not working)

Double vision also made more apparent on gazing to side of affected eye

Question 38

What is optokinetic nystagmus reflex

Answer 38

Optokinetic nystagmus = smooth pursuit + fast-phase reset saccade