57. Oculomotor/Pupil Pathways

Question 1

How does the brain generate saccades?

Answer 1

Superior Colliculi gets input from:

1. Frontal Eye Field (Premotor Area)
2. Posterior Parietal Cortex (Lateral Intraparietal - attention and target selection of eye field)

Question 2

How does the brain generate smooth pursuit eye movements?

Answer 2

Striate Cortex (visual processing) - MT (middle temporal) - Dorsolateral pontine nuclei - cerebellum (flocculus/vermis) + vestibular nuclei

Question 3

What cells of the eye are photosensitive and play a role in pupillary reflexes - how do they work?

Answer 3

melanopsin-containing Retinal Ganglion Cells (3rd type of photoreceptor behind rods/cones)
signal overall light levels - respond to light with PERSISTENT depolarization (no decay/adaptation) with response to light adaptation
maintained response - maintains pupil reflexes (and sustains circadian rhythms)

Question 4

What is the pathway for pupillary constriction in response to light?

What is Adie’s pupil

Answer 4

1o: RGC - around LGN - OLIVARY PRETECTAL AREA
2o: decussate and project BILATERALLY to Edinger Westphal Nucleus (both eyes constrict)
3o: EWN along CNIII to cilary ganglion (parasymp preganglionic fibers)
4o: ciliary ganglion to pupillary constrictor m. (parasymp postganglionic fibers)

Adie’s: damage to ciliary ganglion = slow to constrict

Question 5

What is the pathway for pupillary dilation reflex? What is the stimulus?

Answer 5

Stim: arousal, mood, fright (Limbic System)

1o: hypothalamus - lateral horn of SC (ciliospinal center)
2o: symp preganglionics to superior cervical ganglion
3o: Sup Cer Ganglion - around ICA, along V1 - long/short ciliary nerves - pupillodilator m. in iris

Question 6

Superior Colliculus

• what does it do?
• What are the differences between deep and superficial layers?
• How do these layers interact to perform a fx?

Answer 6

SC commands size/direction of orienting saccade that fixates a stimulus

Deep: neurons specify motor error (same size/direction no matter starting point)
Superficial: visual map of visual fields (input of retina)

Fx: motor response of deep layer matches visual response of superficial layer - focus eye on newly appearing objects in visual field

Question 7

What is the paramedian pontine reticular fomation (PPRF)? What is the path of neurons?

Answer 7

PPRF: horizontal eye movement saccade generator
1o: Pause neuron - tonically firing when awake, pauses for saccade (fx: inhibit burst neurons until you want saccade)
2o: Burst neuron - inhibited by pause n., once disinhibited, fires burst of activity to crease size of eye movement, stimulates oculomotor neuron (LMN) and neuronal integrator
3o: Neuronal Integrator - adds in burst to overall excitatory level, raises response rate in response to burst size
3/4o: Oculomotor N. (LMN) - to extraocular muscle, contains PULSE (from burst neuron - moves eye to new spot) and STEP (from integrator - keeps eye at new spot)

Question 8

Within PPRF, what are the function of internuclear interneurons?
What occurs with PPRF damage?

Answer 8

Internuclear Interneurons: b/w to oculomotor nuclei - carry identical output (to abducens muscle) to oculomotor nucleus of contra eye - decussates along MLF - identical signal to both eyes!

PPRF damage - attacks tonic nerves first - affects STEP - ability of eye to stay in position = GAZE NYSTAGMUS - eye has repeated jumps and drifts due to mild brainstem damage

Question 9

What is Internuclear opthalmoplegia (INO)?

What is 1.5 Syndrome?

Where are vertical eye movements generated?

Answer 9

INO: damage to 1 MLF = one eye cannot do medial saccades but can do convergence

1.5: damage to 1 MLF and PPRF
loss of PPRF: lose ipsilateral gaze to both eyes (can’t look in one direction)
loss of MLF: lose medial gaze to ipsi eye (other direction)
ipsi eye: no horizontal movements (ONE)
contra eye: no medial movements (HALF)