Overview
Movement
of the eye is important for pursuit movements, and is coordinated by
connections between the cranial nerve nuclei i.e. the medial longitudinal
fasciculus. The oculomotor nerve is the third cranial nerve, and originates
from the midbrain, inferior to the mammillary bodies. It innervates 4 of the 6
extraocular eye muscles and also constricts the pupils.
Gross Anatomy
The
oculomotor nerve is also known as cranial nerve 3. It arises from the
oculomotor nerve of the brainstem and passes forwards to leave the skull via
the superior orbital fissure. Its name suggests its function, which is to move
the eyeball. It divides into a superior branch and an inferior branch once it
enters the orbit. The superior branch passes across the optic nerve, and
supplies the superior rectus (depresses the eye), and the levator palpebrae
superioris. The inferior branch is larger, and supplies the medial rectus
(adducts the eye), inferior rectus (elevates the eye), inferior oblique (elevates
and adducts the eye) and the lower part of the ciliary ganglion. These branches
of the inferior division all enter the muscles on their ocular surface, except
inferior oblique, where the nerve enters the inferior oblique at its posterior
border. It also, which is the muscle that elevates the upper eyelid. It also
innervates constrictor pupillae, via the short ciliary nerves that run on its
outer surface, with the motor component of the nerve in the centre.
The
Edinger-Westphal nucleus is the other oculomotor nuclei, and sends
parasympathetic innervation via the ciliary ganglion. The short ciliary nerves
innervate the sphincter pupillae, which constricts the pupil. The long ciliary
nerves also arise from the ciliary ganglion, and provide sensation to the
surface of the eye and cornea. They also provide sympathetic innervation to the
dilator pupillae muscle. The
medial longitudinal fasciculus connects the oculomotor nucleus of the midbrain
(superior to the superior colliculus) to the trochlear and abducens nucleus on
the contralateral side. Hence, our eyes are able to move in the same direction
when we look at something.
The light reflex involves an
afferent and efferent pathway. When light is shone onto one eye it follows the
afferent pathway i.e. he optic nerve, optic chiasm, optic tract or Pretectal
nucleus (which in the dorsal midbrain). The efferent pathway consists of the
Edinger-Westphal nucleus, third cranial nerve and ciliary ganglion (which
constricts the pupil).
Clinical Anatomy
Third Nerve palsy- Palsy
of the nerve affects the superior, medial and inferior rectus, as well as the
inferior oblique and levator palpebrae superioris. The constrictor pupillae
muscle (innervated by the parasympathetic component of the nerve) also is
affected. The loss of innervation to the muscles results in a down and out
position of the eye. The lack of innervation to the levator palpebrae
superioris results in ptosis of the upper eyelid, and lack of innervation to
constrictor pupillae results in a dilated pupil.
Relative afferent pupillary
defect/Marcus Gunn pupil- This is elicited by the swinging
light test, where a torch is swung between each eye. Normally, when one eye
constricts (due to light), so does the other eye. In RAPD this occurs, but to a
lesser extent in one eye when compared to the other. RAPD results in the other
eye (the consensual reflex) not constricting as much as the eye being lit by
the torch. The condition is characterised by a reduced amount of neural
information being sent from one optic pathway to another i.e. a failure to
transmit fully the light information to the other eye. The condition is due to
optic nerve pathology.
Quick Anatomy
Key Facts
Developmental precursor- Basal plate of the embryonic midbrain
Muscles-
Superior rectus, medial rectus, inferior rectus, inferior oblique, levator
palpebrae superioris.
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
The
oculomotor nerve is the third cranial nerve, and originates from the midbrain,
inferior to the mammillary bodies. It innervates 4 of the 6 extraocular eye
muscles and also constricts the pupils.
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
5.
The Definitive Neurological Surgery Board Review
By Shawn P. Moore,
2005
6.
Human Neuroanatomy
By James R. Augustine,
2008