Download The Oculomotorl Nerve HO

The Third Cranial Nerve
The Oculomotor
By Prof. Dr. Imran Qureshi
The oculomotor nerve is a purely motor
nerve. It supplies somatic motor input to
levator palpebrae superioris, superior
rectus, medial rectus, inferior rectus, and
inferior oblique muscles. (SE)
The Nucleus of this nerve is located in
the Mid brain at the level of Superior
Colliculus. At this level, the nuclei of the
two sides form a single complex that lies
in the central grey matter, ventral to the
cerebral aqueduct.
It
also
carries
preganglionic
parasympathetic axons for the ciliary
muscle and constrictor pupillae. (GVE)
Figure 3: Oculomotor nerve passing between the
Posterior cerebral & Superior cerebellar arteries
The proprioceptive fibers from these
extraocular muscles run back in the
ophthalmic division of trigeminal heading
toward cell bodies in the trigeminal
ganglion.
After the oculomotor nerve exits the
midbrain, it passes forward between the
superior cerebellar and posterior
cerebral arteries to reach the roof of the
cavernous sinus slightly anterior to the
posterior clinoid process. Here, the nerve
pierces the roof of the cavernous sinus
and runs forward applied to the inner
surface of upper part of its lateral dural
wall.
Figure 2: Cross section of Mid brain at the level of
superior colliculi
Figure 4: Oculomotor nerve passing through the
cavernous sinus alongwith the trochlear & ophthalmic
nerves
Figure 1: Dorsal view of the Brain stem
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After emerging from the front of the sinus,
the oculomotor nerve divides into
superior and inferior divisions that pass
into the orbit through the middle
compartment of the superior orbital
fissure.
upward to synapse in the ganglion.
Figure 7: Short ciliary nerves supplying the ciliary muscle
and sphincter pupillae
Figure 5: Oculomotor nerve superior &
inferior divisions & the muscles supplied by
them
The superior division supplies the levator
palpebrae superioris and the superior
rectus, while the inferior division supplies
the two inferior extraocular muscles and
sends a branch below the optic nerve to
the medial rectus.
The postganglionic axons leave the front
of the ganglion through two or three short
ciliary nerves that, branch a few times
and pierce the sclera in a circle around
the optic nerve. Here they are
accompanied with the short posterior
ciliary arteries.
The short ciliary nerves then run forward
deep to the sclera to reach the ciliary
muscle and constrictor pupillae, which
they supply.
Clinical Considerations
Damage to the oculomotor nerve has
effects, which are due both to interruption
of its somatic motor and visceral motor
fibers.
Figure 6: Ophthalmic artery crossing the optic
nerve & the site of ciliary ganglion
Just anterior to the site where the
ophthalmic artery crosses the optic
nerve,
there
is
a
clump
of
parasympathetic
ganglion
cells
sandwiched between the lateral surface
of the optic nerve and the lateral rectus
muscle. This clump is called the ciliary
ganglion. The inferior division of the
oculomotor nerve passes forward just
below the ciliary ganglion, and, while
doing so it sends a bundle carrying
preganglionic parasympathetic axons
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Figure 8: Complete Ptosis
Since the oculomotor nerve supplies the
levator palpebrae superioris, which is
largely responsible for maintaining the
eyes open while awake. Damage to the
nerve causes the upper lid to droop
down, almost to the point of closure
(Ptosis).
Because the two eyes do not point in the
same direction, double vision (diplopia) is
present.
Interruption of the parasympathetic input
to the constrictor pupillae leads to an
unusually wide pupil that does not narrow
either when light is shown into the eye or
when the eye focuses on a close object.
Figure 8: Physician opening the eye of patient with third
nerve palsy to check for strabismus
No conscious effort can produce
elevation of the lid. In compensation, the
patient will try to elevate the upper lid
indirectly by pulling up on the eyebrow
with the frontalis.
Figure 9: Partial Ptosis with lateral strabismus
The elevation of the eyebrow and
resultant creasing of the forehead are
usually obvious.
Damage to the oculomotor nerve leads to
a paralysis of most of the extraocular
muscles that actually insert on the
eyeball, leaving only the lateral rectus
and superior oblique intact. Thus, the
eyeball is essentially immobile.
Because of the unopposed pull of the
lateral rectus, the eye assumes an
abducted position, which is also known
as a lateral squint or lateral strabismus.
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The ciliary muscle is also paralyzed,
with resulting inability to accommodate.