Abducent nerve

Question 1

Where is the abducens nucleus located?

Answer 1

mid-pons, beneath floor of upper part of fourth ventricle, close to midline beneath facial colliculus

Question 2

In which midbrain structure can the abducens nucleus be found, and at what level?

Answer 2

mid pons, beneath floor of upper part of fourth ventricle

Question 3

What is the course of abducens fibres from the nucleus to exiting the brainstem?

Answer 3

fibres pass ventrally (anteriorly) and caudally to emerge on the lower border of the pons in the pontomedullary junction near the midline

Question 4

What is the intracranial course of the abducens nerve after it has emerged from the brainstem? 7 steps

Answer 4

1. courses upwards in pontine cistern between prainstem and clivus, either side of basilar artery
2. crossed or ‘bound down’ to brainstem close to origin by anterior inferior cerebellar artery
3. may pierce dura early in upward course upon clivus close to inferior petrosal sinus (2cm below posterior clinoid process)
4. reaches upper border of apex of petrous temporal bone + crosses ifnerior petrosal sinus medially to laterally
5. changes direction sharpy from vertical to horizonal course
6. runs forward beneath petrosphenoidal ligament (of Gruber) and superior petrosal sinus
7. passes forward within cavernous sinus

Question 5

Between which structures does CNVI pass when coursing upwards in the pontine cistern?

Answer 5

between brainstem and clivus, either side of basilar artery

Question 6

What structure crosses or binds down CNVI to the brainstem?

Answer 6

anteiror inferior cerebellar artery

Question 7

Where may CNVI pierce the dura?

Answer 7

early in upward course upon the clivus, close to inferior petrosal sinus, 2cm below posterior clinoid process

Question 8

Where does CNVI change direction sharply from vertical to horizontal?

Answer 8

as it crosses inferior petrosal sinus from medial to lateral

Question 9

What 2 structures is CNVI beneath immediately before passing into the cavernous sinus?

Answer 9

1. petrosphenoidal ligament (of Gruber)
2. superior petrosal sinus

Question 10

What is the course of CNVI within the cavernous sinus?

Answer 10

passes forward within cavernous sinus, surounded by venous spaces and suspended by fine connective tissue trabeculae

lies lateral to asending portion of ICA and then inferolateral to its horizontal portion

Question 11

What surrounds CNVI on its course in the cavernous sinus?

Answer 11

surrounded by venous spaces and suspended by fine connective tissue trabeculae

Question 12

What are the relations of CNVI to the ICA in the cavernous sinus?

Answer 12

lies lateral to ascending portion, then inferolateral to horizontal portion

Question 13

How does the abducens nerve enter the orbit?

Answer 13

enters intraconal space of orbit by passing wihtin the tendinous ring

Question 14

What is the length of the intracranial portion of CNVI like compared with other nerves?

Answer 14

has the longest intracranial course of any cranial nerve

Question 15

Why is CNVI very susceptible to damage in head injuries e.g. basal skull fractures / expanding cerebral lesion?

Answer 15

long intracranial course

Question 16

What may happen to CNVI if the brainstem is displaced downwards (as result of raised ICP)?

Answer 16

nerve may be compressed against inferior cerebellar artery or severed where it bends sharpy over apex or crest of petrous temporal bone

Question 17

Why is CNVI more suspectible than other intracavernous nerves in the cavernous sinus?

Answer 17

not protected by dura of lateral wall

Question 18

What is a type of pathology in the cavernous sinus that can affect CNVI alone?

Answer 18

atheromatous changes in ICA may compress CNVI

Question 19

What are 2 symptoms of lesions of abducens nerve/

Answer 19

paralysis of lateral rectus:

1. patient unable to abduct eye
2. suffers esotropia (internal strabismus) due to unopposed action of medial rectus

Question 20

Where are cell bodies of proprioreceptive fibres in EOMs located?

Answer 20

in mesencephalic nucleus of trigeminal nerve

Question 21

What are 3 sites the mesencephalic nucleus receives proprioreceptive terminals from?

Answer 21

1. extraocular muscles
2. neck muscles
3. face musculature

Question 22

How is simultaneous movement of head and eyes coordinated?

Answer 22

dependent on conjugation of sensory (proprioceptive) information from musculature of neck and eyes, and input from cerebellar oculomotor centres