Organization of the Midbrain

Question 1

Name the six extrinsic muscles of the eye and which midbrain structure innervates each:

Answer 1

Superior Rectus(Oculomotor Nucleus), Lateral Rectus (abducens nucleus),Inferior Rectus (O.N), Nasal Rectus(O.N.), Superior Oblique (Trochlear Nucleus), Inferior Oblique (O.N).

Question 2

Name the two targets of the Abducens Nucleus and the fiber tracts that carry fibers to each destination:

Answer 2

Lateral Rectus (Abducens Nerve), Contralateral Oculomotor Nucleus (Medial Longitudinal Fasciculus)

Question 3

What two sites project to the abducens nucleus?

Answer 3

Vestibular Nuclei, Paramedian Pontine Reticular Formation.

Question 4

Name the syndrome caused by a lesion of the abducens nerve, what are the symptoms?

Answer 4

Lateral Rectus Palsy, ipsilateral eye can’t gaze laterally, Horizontal diplopia.

Question 5

What is the symptom produced by a lesion of the abducens nucleus, what are its symptoms?

Answer 5

lateral gaze paralysis, inability to gaze toward the side with the lesion, forced gaze contralaterally. Paralysis of the Lateral Rectus, paresis of the medial rectus, convergent response remains intact because of other inputs to the oculomotor nucleus.

Question 6

What syndrome would result from damage to the medial longitudinal fasciculus? What are the symptoms?

Answer 6

Lateral Internuclear Opthalmoplegia, paresis of the medial rectus of the contralateral eye, disrupting coordinated eye movements. Nystagmus of the contralateral eye (due to repetitive “move” commands to the abducens nucleus).

Question 7

Describe the pathway of efferent fibers from the trochlear nucleus:

Answer 7

Trochlear Nucleus (embedded in the medial longitudinal fasciculus) sends fibers through the trochlear nerve which cross the midline through the anterior medullary velum and innervate the contralateral superior oblique muscle of the eye.

Question 8

Which syndrome and symptoms result from a lesion of the trochlear nucleus or nerve?

Answer 8

Left superior oblique paralysis, Extorted eye, difficulty climbing down stairs, difficulty reading,

Question 9

An aneurysm of which vessels would produce trochlear nerve compression?

Answer 9

Posterior Cerebral Artery, Superior Cerebellar Artery

Question 10

What is the syndrome and the symptoms of a lesion to the oculomotor nucleus?

Answer 10

External Strabismus (Downward abducted eye because of unopposed lateral rectus and superior oblique), Ptosis, Mydriasis (damage to the Edinger-Westphal nucleus)

Question 11

When light is shined in the right eye, both pupils dilate, when shined in the left eye, neither pupil dilates. What type of damage is implicated?

Answer 11

Damage to the left optic nerve

Question 12

When light is shined in the right eye, both eyes dilate but not to their fullest extent, when shined in the left eye, they dilate even less. What type of damage is implicated?

Answer 12

Damage to the left optic tract

Question 13

When light is shined in either eye, only the right pupil dilates. What damage is implicated?

Answer 13

Damage to the left oculomotor nerve

Question 14

Which midbrain structure is essential to the consensual pupillary light reflex?

Answer 14

Pretectal olivary nuclei, oculomotor nerve

Question 15

Which midbrain structure is essential to performing the sensory computations needed for saccadic and tracking eye movements?

Answer 15

Superior Colliculus

Question 16

Describe the anatomical layout of the superior colliculus:

Answer 16

Upper Layers contain a retinotopic map, Intermediate layers receive auditory and somatosensory input. Lower layers receive inputs from the frontal eye fields and communicate with the Rostral interstitial nucleus of medial longitudinal fasciculus, paramedian pontine reticular formation, and the tectospinal pathway.

Question 17

A stroke in the basilar artery at the level of cranial nerve III may produce which syndrome? Describe the symptoms:

Answer 17

Superior alternating hemiplegia, damage to oculomotor nerve (ipsilateral opthalmoplegia), damage to crus cerebri, corticospinal and corticobulbar fibers(contralateral hemiplegia).