Central Visual Pathways, CNII

Question 1

Where does the optic tract fibres go?

Answer 1

• Superior colliculus (forms part of tectum/roof of midbrain)
  o Visual reflex centre
  o Receives medial root of optic tract (10% of fibers); this forms the brachium of superior colliculus
• Hypothalamus → the suprachiasmatic nucleus
  o This mediates cicardian rhythms (sleep/wake etc)
  o Accounts for why you can feel jetlagged
• Reticular formation → part of the brain stem that is difficult to define; it runs through midbrain (tegmentum), pons & medulla, and has both fibers and small groups of nuclei
  o Regulates levels of arousal
• Pre-tectal nucleus → mediates the pupillary reflex
• Lateral geniculate nucleus – majority of optic tract fibers terminate here. (part of the thalamus)

Question 2

Optic tract ->superior colliculus -> ?

Answer 2

o Via tectospinal tract to ventral root of the cervical spinal cord –orienting head to visual stimuli
• synapses with motor neurons which innervate musculature of the neck

o Via tectobulbar tract to motor nuclei of cranial nerves – orienting eyes to visual stimuli
- Some fibers from the superior colliculus project to thalamus (pulvinar & lateral geniculate nucleus), then to visual association cortex

o Receives input from visual cortex, somatosensory input

Question 3

Describe the path of the pupillary light reflex

Answer 3

SENSORY RECEPTOR -> optic tract -> PRETECTAL NUCLEUS -> EDINGERWESTPHAL NUCLEUS (bilateral projection) in rostral midbrain at the level of superior colliculus) -> OCCULOMOTOR NERVE (CNIII) -> Ciliary ganglion -> SHORT CILIARY NERVES -> Pupillary constrictor

Question 4

LGN (where most of optic tract terminates) has 6 layers named ventral to dorsal: What do layers 1& 2 do?

What do layers 3-6 do

Answer 4

• Layers 1 & 2: magnocellular, input involves movement & contrast (ie spatial orientation)
• Layers 3-6: parvocellular, input involves form & colour (ie shape)

Question 5

From optic tract -> LGN -> ???

Answer 5

-> Optic radiation = GENICULCOCALCARINE TRACT

Question 6

The optic radiation has two different tracts- describe them and the process they carry

Answer 6

SUPERIORLY PROJECTING FIBRES
Carry info/processes from the inferior visual field (which projects to the superior part of the retina
o Terminate on the superior bank of calcarine sulcus

INFERIORLY PROJECTING FIBRES = MEYERS LOOP
o Fibers run alongside (loop around) the posterior horn of lateral ventricle
o Fibers are processing info from the superior half of the visual field
o Terminate in inferior bank of calcarine sulcus

• Macular (foveal) fibers occupy a broad middle area
  o Fibers running more or less in the midline

Question 7

What would a pituitary tumour cause?

Answer 7

BITEMPORAL HEMIANOPIA

Question 8

WHAT would injury to one optic tract cause?

Answer 8

Contralateral homonymous hemianopia

Question 9

What would aneurysm of ICA cause?

Answer 9

Ipsilateral nasal hemianopia

Question 10

What would a lesion in MEYERS loop cause?

Answer 10

Contralateral homonymous upper quadrantopia

Question 11

What would a lesion to the visual cortex cause?

Answer 11

Contralateral homonymous hemianopia with macular sparing

Question 12

Describe the blood supply to the optic radiation

Answer 12

Majority of laterally located fibres & Meyers loop = Middle cerebral artery

Medially located & visual cortex = posterior cerebral arter