Overview
We move
our eyes constantly, and need to be able to move them in order to follow moving
objects, read words written on a page and perform any activity that requires
visual pursuit. We have 6 extraocular muscles that move the eyeball and one
that moves the upper eyelid. These muscles are innervated by three cranial
nerves (oculomotor, trochlear, abducens).
Gross Anatomy
There are
six muscles that move the eyeball itself, one muscle that moves the upper eyelid,
and one that shuts the eye tight. The muscles that move the eyeball itself are
as follows: the medial rectus attaches to the medial surface of the eyeball,
adducts the eye and is innervated by the oculomotor nerve. The lateral rectus
attaches to the lateral surface of the eyeball, abducts the eye and is
innervated by the trochlear nerve. The inferior rectus attaches to the superior
surface if the eyeball, elevates the eye and is innervated by the oculomotor
nerve. The superior rectus attaches to the superior part of the eyeball,
depresses the eye and is innervated by the oculomotor nerve. The superior
oblique attaches to the superior part of the eyeball, causes eye intorsion,
moves the eye down and out and is innervated by the trochlear nerve. The inferior
oblique attaches to the inferior part of the eyeball moves the eye up and in
and causes eyeball extorsion, and is innervated by the oculomotor nerve. The
two oblique muscles run through sling like pulleys before they insert onto the
surface of the eyeball, and act as pure elevators/depressors when the eye is
adducted.
The
levator palpebrae superioris (innervated by the oculomotor nerve) is the muscle
that moves the upper eyelid. The orbicularis oculi (innervated by the facial
nerve) is the muscle that shuts the eye tight. The facial nerve also innervates
the lacrimal glands that produce tears.
Clinical Anatomy
3rd nerve palsy- This
presents as a down and out eye (as the lateral rectus and superior rectus are
performing their actions only), with a dilated pupil (constrictor pupillae is
innervated by the short ciliary branches of the ciliary ganglion, which run on
the outside of the oculomotor nerve) and a ptosis (levator palpebrae superioris
is denervated).
6th nerve palsy- This
is abducens nerve palsy. Abducens innervates the lateral rectus muscle, and
hence the patient will not be able to abduct the eye beyond the midline.
Horner’s syndrome- One
third of the innervation to levator palpebrae superioris is sympathetic. Hence,
in Horner’s syndrome when the sympathetic innervation is absent, the upper
eyelid will partially droop i.e. partial ptosis. The pupil will also be dilated
in Horner’s syndrome, as the sympathetic long ciliary nerves of the ciliary
ganglion innervate the dilator pupillae muscle.
Myaesthenia Gravis- This
usually presents as double vision and drooping eyelids, due to
anti-acetylcholine receptor antibodies causing muscle weakness. The eye muscles
are often impacted first.
Quick Anatomy
Key Facts
Developmental precursor- Somitomeres 1-4 (paraxial mesoderm cranial to the occipital
somites).
Muscles-
Superior rectus, inferior rectus, medial rectus, lateral rectus, inferior
oblique, superior oblique.
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
There are
6 extraocular muscles that move the eyeball and one that moves the upper
eyelid. These muscles are innervated by three cranial nerves (oculomotor,
trochlear, abducens).