Neuro - Cranial Nerves CN III, IV, and VI

Question 1

Identify the location of the cell body of origin and the course of the abducens nerve.

Answer 1

cell body of origin in the pontine tegmentum -> rootlets exit anteriorly -> longest intracranial course -> lateral rectus muscle -> abducts eye

Question 2

What structure is innervated by the abducens nerve?

Answer 2

lateral rectus muscle

Question 3

List the signs and symptoms seen following a lesion of the abducens nerve.

Answer 3

• diplopia (double vision) at rest since affected eye looks medially
• diplopia (double vision) when asked to look lateral on the lesioned side
• compensatory head-turn away from side of lesion

Question 4

Identify the location of the cell body of origin and the course of the trochlear nerve.

Answer 4

cell body of origin in the midbrain tegmentum -> rootlets exit posteriorly -> superior oblique muscles

Question 5

Name the structure innervated by the trochlear nerve.

Answer 5

superior oblique muscle

Question 6

List the signs and symptoms seen following a lesion of the trochlear nerve.

Answer 6

• extorsion of eye (top of eye is tipped laterally)
• accompanying vertical double vision (diplopia)
• weakness during attempted downward medial gaze
• compensatory head-tilt toward intact shoulder

Question 7

Identify the location of the cell body of origin of both components and the course of the oculomotor nerve.

Answer 7

• GVE cell body of origin in the midbrain tegmentum -> rootlets exit into interpeduncular fossa -> Edinger-Westphal nucleus -> ciliary ganglion
• SE cell body of origin in the midbrain tegmentum -> rootlets exit into interpeduncular fossa -> oculomotor nucleus -> all extraocular muscles and levator palpebrae muscle except SO and LR

Question 8

Name the structure(s) innervated by the oculomotor nerve.

Answer 8

• intrinsic muscles of the eye

- medial rectus, inferior oblique, inferior rectus, superior rectus, and levator palpebrae muscles

Question 9

List the signs and symptoms seen following a lesion of the oculomotor nerve.

Answer 9

• SE functional component
  
  • ptosis - paralysis of levator palpebrae muscle makes eyelid droop
  • eye position - down and out
• GVE functional component (parasympathetic)
  
  • dilated, fixed pupil
  • paralysis of accommodation

Question 10

How does ptosis due to a lesion of the oculomotor nerve differ from that seen in Horner syndrome?

Answer 10

in Horner’s syndrome, the drooping of the eyelid is much less severe (?)

Question 11

What is the significance of unequal pupil size?

Answer 11

it could be used to signal if a person has an uncal herniation since the parasympathetic fibers of CN III (which are one of the controls of pupil size) are unmyelinated and easily damaged by an uncal herniation (?)

Question 12

Define diplopia. How can the patient minimize diplopia?

Answer 12

• diplopia - double vision

- can be compensated for by tilting the head so that both eyes align

Question 13

Name the neuroanatomical structure that corresponds with the center for lateral gaze.

Answer 13

abducens nucleus controls eye movements in the horizontal plane (center for lateral gaze); loss of the abducens nucleus results in paralysis of lateral gaze

Question 14

Name the tract that contains the axons that arise from the internuclear neurons of the abducens nucleus.

Answer 14

medial longitudinal fasciculus

Question 15

Name the source of the axons that terminate in the ciliary ganglion.

Answer 15

Edinger-Westphal nucleus in the midbrain -> ciliary ganglion

Question 16

Name the muscle innervated by the oculomotor nerve that holds the upper eyelid up.

Answer 16

levator palpebrae muscle

Question 17

Describe the anatomical basis of paralysis of lateral gaze.

Answer 17

• abducens nucleus controls the lateral rectus muscle
• abducens nucleus is also connected to the oculomotor nuclear complex via internuclear neurons
• abducens nucleus controls eye movements in the horizontal plane (center for lateral gaze)
• loss of the abducens nucleus results in paralysis of lateral gaze

Question 18

Describe the anatomical basis of internuclear ophthalmoplegia.

Answer 18

• paralysis of the contralateral medial rectus muscle because the internuclear neuron (neuron connecting abducens and oculomotor nuclei) is cut
• medial rectus does not contract on attempted ipsilateral horizontal gaze
• medial rectus does not contract during accomodation
• nystagmus (quivering) in abducting eye contralateral to lesion

Question 19

Describe the pupillary light reflex.

Answer 19

• if you shine a light in one eye, the afferent component comes back to the pretectum and measures the amount of light coming in
• efferent fibers go out and affect both eyes
• direct response - response of the eye into which the light was shined
• consensual response - the response of the opposite eye in which the light was not shined