Visual pathways

Question 1

Which fibres under anatomical decussation & where?

Answer 1

Nasal retinal fibres.

At the optic chiasm.

Question 2

Explain the basic visual pathway after the optic chiasm?

Answer 2

Optic chiasm > LGN > optic radiations > primary visual cortex.

Question 3

What are the two divisions of the optic radiations known as?

Answer 3

Upper division = Baums loop.

Lower division = Meyers loop.

Question 4

What retinal quadrants input into the upper divisions of the optic radiation? What quadrants of the visual field are they from?

Answer 4

Superior retinal quadrants - inferior quadrants of the visual field.

Question 5

What retinal quadrants input into the lower divisions of the optic radiation? What quadrants of the visual field are they from?

Answer 5

Inferior retinal quadrants - superior quadrants of the visual field.

Question 6

What would a lesion to the upper divisions of the optic radiations cause?

Answer 6

Homonymous inferior quadrantanopia.

Question 7

What would a lesion to the lower divisions of the optic radiations cause?

Answer 7

Homonymous superior quadrantanopia.

Question 8

What are the two sets of muscles in the iris & what do they do?

Answer 8

Sphincter pupillae muscles - pupil constriction.

Dilator pupillae muscles - pupil dilation.

Question 9

Describe the pathway involved in the pupillary light reflex?

Answer 9

Light shone into eye > depolarisation of retinal ganglion cells > retinal gangion cell axons converge to form the optic nerve > optic nerve synapses with pretectal nucleus (on ipsilateral side) > pretectal nucleus axons synapse on the Edinger-Westphal nucleus on both sides of the brain > Axons from the Edinger-Westphal nucleus (occulomotor nevre) synapse on ciliary ganglion neurones > short ciliary nerves from the ciliary ganglion innervate the sphincter pupillae muscle > pupil constriction.

Question 10

What nerve is involved in the afferent limb of the pupillary light reflex?

Answer 10

Optic nerve.

Question 11

What nerve is involved in the efferent limb of the pupillary light reflex?

Answer 11

Occulomotor nerve.

Question 12

What is RAPD and what does it signify?

Answer 12

Relative afferent pupillary defect - lesion in the optic nerve anterior to the optic chiasm.

Question 13

What does RAPD allow a diagnosis of?

Answer 13

Retrobulbar neuritis.

Question 14

What three actions are involved in pupil accommodation?

Answer 14

Pupil constriction.
Lens accommodation.
Convergence.

Question 15

What is convergence and how does it occur?

Answer 15

Adduction of the eyes nasally.

It occurs due to excitation of the occulomotor nucleus > contraction of the medial rectus muscles.

Question 16

What is lens accommodation and what does it achieve?

Answer 16

Increased curvature of the lens > increased refractive power.

Question 17

Explain how lens accommodation occurs?

Answer 17

Contraction of the ciliary muscles (due to innervation by the short ciliary nerve) > decreased tension in the zonule of zinn > increased lens curvature.

Question 18

What is dorsal midbrain syndrome & what are its signs?

Answer 18

Lesion of the dorsal midbrain resulting in:
> Argyll Robertson pupil - bilateral small pupils that only constrict in response to accommodation & not light.
> Paralysis of upgaze.
> Convergence-retraction nystagmus - on attempting upgaze the eyes converge & retract.

Question 19

What are the causes of dorsal midbrain syndrome?

Answer 19

Pineal tumours.

Neurosyphilis.