Neurology of the visual system

Question 1

What are the landmarks of the visual pathway?

Answer 1

Eye.

Optic nerve- myelinated ganglion nerve fibres, with cell bodies originating within the retina.

Optic chiasm- half of the nerve fibres cross here, optic nerves from both eyes converge here just above and in front of brainstem.

Optic tract- ganglion nerve fibres exit as optic tract.

Lateral geniculate nucleus- ganglion nerve fibres synapse at lateral geniculate nucleus, a relay centre in the thalamus.

Optic radiation – 4th order neuron, relaying signal from lateral genicular ganglion to primary visual cortex within occipital lobe for lower visual processing.

Primary visual cortex or striate cortex- within the occipital lobe.

Extrastriate cortex- information relayed for higher visual processing.

Question 2

What are the first order neurons in the retina (visual pathway)?

Answer 2

Rod and cone retinal photoreceptors.

Question 3

What are the second order neurons in the retina (visual pathway)?

Answer 3

Retinal bipolar cells.

Question 4

What are the third order neurons in the retina (visual pathway)?

Answer 4

Retinal ganglion cells.

Question 5

What are receptive fields in the eye?

Answer 5

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

Photoreceptor- a small circular space surrounding the photoreceptor.

Retinal ganglion cell- input from neighbouring photoreceptors (convergence).

Question 6

What is convergence of receptive fields?

Answer 6

Number of lower order neurons field synapsing on the same higher order neuron.

Cone system convergence > rod system convergence.

Central retina convergence > peripheral retina convergence.

Low convergence: small receptive field, fine visual acuity, low light sensitivity.

High convergence: large receptive field, coarse visual acuity, high light sensitivity.

Question 7

What are on-centre retinal ganglion cells stimulated and inhibited by?

Answer 7

Stimulated by light at the centre of the receptive field.

Inhibited by light on the edge of the receptive field.

Question 8

What are off-centre retinal ganglion cells stimulated and inhibited by?

Answer 8

Inhibited by light at the centre of the receptive field.

Stimulated by light on the edge of the receptive field.

Question 9

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

Answer 9

Contrast sensitivity.

Enhance edge detection.

Question 10

What percentage of ganglion fibres cross at the optic chiasma?

Answer 10

53%

Question 11

What determines whether a lesion affects one eye or both eyes?

Answer 11

Lesions anterior to optic chiasma affects visual field in one eye only.

Lesions posterior to optic chiasma affect visual field in both eyes.

Question 12

What is the result of a lesion at the optic chiasma?

Answer 12

Damages crossed ganglion fibres from nasal retina in both eyes.

Temporal field deficit in both eyes- bitemporal hemianopia.

Question 13

What is the result of a lesion posterior to the optic chiasma?

Answer 13

Right-sided lesion- left homonymous hemianopia in both eyes.

Left-sided lesion- right homonymous hemianopia in both eyes.

Question 14

What causes bitemporal hemianopia?

Answer 14

Typically caused by enlargement of pituitary gland tumour.

Pituitary gland sits under optic chiasma.

Question 15

What causes homonymous hemianopia?

Answer 15

Stroke (CVA).

Question 16

Where is the primary visual cortex and what is it characterised by?

Answer 16

Situated along calcarine sulcus within occipital lobe.

a.k.a. striate cortex.

Characterised by a distinct stripe derived from the myelinated fibre of the optic radiation projecting into the visual cortex.

Question 17

What is the function of the primary visual cortex?

Answer 17

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

Right eye and left dominant columns intersperse each other.

Question 18

What is macula sparing homonymous hemianopia?

Answer 18

Damage to primary visual cortex.

Often due to stroke.

Leads to contralateral homonymous hemianopia with macula sparing.

Area representing the macula receives dual blood supply from posterior cerebral arteries from both sides.

Question 19

What is the extrastriate cortex?

Answer 19

Area around primary visual cortex within the occipital lobe.

Converts basic visual information, orientation and position into complex information like motion and object representation.

Question 20

What is the function of the pupil?

Answer 20

Regulates light input to the eye (but less than 2 log unit change) like a camera aperture.

In light- pupil constriction:

• decreases spherical aberrations and glare
• increases depth of field- near response triad
• reduces bleaching of photopigments
• pupillary constriction mediated by parasympathetic nerve (within CN III)

In dark- pupil dilatation:

• increases light sensitivity in the dark by allowing more light into the eye
• pupillary dilatation mediated by sympathetic nerve

Question 21

What is the afferent pathway of the pupillary reflex?

Answer 21

Rod and cone photoreceptors synapsing on bipolar cells synapsing on retinal ganglion cells.

Pupil-specific ganglion cells exit at posterior third of optic tract before entering the lateral geniculate nucleus.

Synapses at brain stem (pretectal nucleus).

Afferent (incoming) pathway from each eye synapses on Edinger-Westphal nuclei on both sides in the brainstem.

Question 22

What is the efferent pathway of the pupillary reflex?

Answer 22

Edinger-Westphal nucleus

Oculomotor nerve efferent

Synapses at ciliary ganglion

Short posterior ciliary nerve

Pupillary sphincter

Question 23

What is the direct light reflex?

Answer 23

Constriction of pupil of the light-stimulated eye.

Question 24

What is the consensual light reflex?

Answer 24

Constriction of pupil of the fellow (other) eye.

Question 25

What is the neurological basis of the direct and consensual pupillary light reflexes?

Answer 25

Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides.

Question 26

What are the consequences of a defect of the right afferent pathway of the pupillary light reflex?

Answer 26

e.g. damage to optic nerve

No pupil constriction in both eyes when right eye is stimulated with light.

Normal pupil constriction in both eyes when left eye is stimulated with light.

Question 27

What are the consequences of a defect of the right efferent pathway of the pupillary light reflex?

Answer 27

e.g. damage to right 3rd nerve

No right pupil constriction whether right or left eye is stimulated with light.

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

Question 28

What is the key difference between a unilateral defect of the afferent vs. efferent pathway of the pupillary light reflex?

Answer 28

Unilateral afferent defect: different response depending on which eye is stimulated.

Unilateral efferent defect: same unequal response between left and right eye irrespective which eye is stimulated.

Question 29

What is the result of a swinging torch test with a relative afferent pupillary defect in the right eye?

Answer 29

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 undamaged side.

Both pupils paradoxically dilate when light swings to the right damaged side.

Question 30

What is duction?

Answer 30

Eye movement in one eye.

Question 31

What is version?

Answer 31

Simultaneous movement of both eyes in the same direction.

Question 32

What is vergence?

Answer 32

Simultaneous movement of both eyes in the opposite direction.

Question 33

What is convergence?

Answer 33

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

Question 34

What are the 2 different speeds of eye movement?

Answer 34

Saccade- short, fast burst, up to 900deg/sec:

• reflexive saccade to external stimuli
• scanning saccade
• predictive saccade to track objects
• memory-guided saccade

Smooth pursuit- sustain slow movement:

• slow movement- up to 60deg/sec
• driven by motion of a moving target across the retina

Question 35

How are the actions of the extraocular muscles tested?

Answer 35

Abduction- lateral rectus

Adduction- medial rectus

Elevated and abducted- superior rectus

Depressed and abducted- inferior rectus

Elevated and adducted- inferior oblique

Depressed and adducted- superior oblique

Isolate muscle to be tested by maximising its action and minimising the action of other muscles.

Question 36

What is the result of third nerve palsy?

Answer 36

Affected eye down and out.

Droopy eyelid.

Unopposed superior oblique innervated by 4th nerve (down).

Unopposed lateral rectus action innervated by 6th nerve (out).

Also accompanied by ptosis on the affected side (drooping) as the upper lid levator muscle is innervated also by the superior branch of the oculomotor nerve.

Question 37

What is the result of sixth nerve palsy?

Answer 37

Affected eye unable to abduct and deviates inwards.

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

Question 38

What is the optokinetic nystagmus reflex?

Answer 38

Nystagmus- oscillatory eye movement.

Optokinetic nystagmus = smooth pursuit + fast phase reset saccade.

Optokinetic nystagmus reflex is useful in testing visual acuity in preverbal children by observing the 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 the subject has sufficient visual acuity to perceive the grating pattern.