Overview
The
abducens nerve is also known as cranial nerve 6. Like the other eye related
cranial nerves, the abducens emerges either side of the midline. It leaves the
pontomedullary junction, medial to the facial and vestibulocochlear nerves. As
with all eye related cranial nerves (3,4,6) excluding the optic nerve, the
nerve leaves the cranial vault via the superior orbital fissure.
Gross Anatomy
The
abducens is also known as cranial nerve 6. The name Abducens comes from the
word ‘abduct’ meaning move away from the midline. The nucleus of the abducens
nerve lies in the floor of the fourth ventricle, on the posterior surface of
the pons. The fibers of facial nerve loops around the fibers of the abducens nerve
in the pontine tegmentum. The abducens nerve emerges from the pontomedullary
junction either side of the midline. The nerve then enters the subarachnoid
space and runs in Dorello’s canal, which is bordered superiorly by the
petrosphenoid ligament, and inferior laterally by the petrous portion of the
temporal bone and the clivus medially. It then runs forwards to eventually
reach the cavernous sinus, where it runs close the internal carotid artery in
the medial wall. It passes forwards to leave the skull via the superior orbital
fissure and innervate the lateral rectus muscle (the muscle that abducts the
eye).
The
oculomotor nerve nucleus, trochlear nerve nucleus and the abducens nerve
nucleus lie within in the brainstem. A tract called the medial longitudinal
fasciculus connects them. This ensures that our two eyes move in coordination,
so when we are following a moving object we are able to focus on it. When the
medial rectus contracts in one eye (oculomotor innervation), the lateral rectus
contracts in the other. Hence the eyes move in the same direction.
Clinical Anatomy
Lateral rectus palsy- This
manifests as an inability to abduct the eye beyond the midline in the affected
eye. The eye is able to move to the midline from the adducted position, but is
not able to move beyond. Causes include diabetes, brain tumours, and
demyelinating diseases. The relatively long course of the abducens nerve makes
the nerve vulnerable to injury. Petrous temporal bone fractures can damage the
nerve, as can intracavernous aneurysms of the internal carotid artery. Any
lesion that causes inferior brainstem movement can also damage the nerve by
damaging the point at which the nerve exits the pontomedullary junction, and
the point where it arches over the petrous portion of the temporal bone.
Quick Anatomy
Key Facts
Developmental precursor- Basal plate of the pons (metencephalon- secondary brain
vesicle)
Muscle supplied- Lateral rectus
Aide-Memoire
SO4 LR6
Superior Oblique innervated by the
fourth cranial nerve (trochlear nerve)
Lateral rectus innervated by the 6th
cranial nerve (abducens)
The others are supplied by the
oculomotor nerve.
Summary
Developmental precursor- Basal plate of the pons (metencephalon- secondary brain
vesicle)
Muscle supplied- Lateral rectus
References
1.
Frank H.Netter
MD: Atlas
of Human Anatomy, 5th Edition, Elsevier Saunders, Chapter 1 Head and
Neck
2.
Chummy S.Sinnatamby: Last’s
Anatomy Regional and Applied, 12th Edition, Churchill Livingstone Elsevier
3.
Richard L.
Drake, A. Wayne Vogl,
Adam. W.M.
Mitchell: Gray’s Anatomy for Students, 2nd Edition, Churchill Livingstone Elsevier
4.
Elliiot L.Manchell: Gray's Clinical Neuroanatomy:
The
Anatomic Basis for Clinical Neuroscience
- http://radiopaedia.org/articles/dorello-canal
accessed 23/2/16