oculomotor nerve
TRANSCRIPT
OCULOMOTOR -The III CN
By- Sinchana S Kumbar
Intro:The oculomotor nerve is the third of 12
pairs of cranial nerves in the brain. This nerve is responsible for eyeball and
eyelid movement. The oculomotor nerve involves two
separate components, each of which has a distinct function.
somatic motor componentvisceral motor component
Somatic motor component - Supplies four extraocular muscles in the eye and the upper eyelid's levator palpebrae superioris with motor (movement) fibers.
- It controls the muscles that allow for visual tracking and fixation by the eye.
Visceral motor component - controls parasympathetic innervation (nerves related to involuntary actions) of the ciliary muscles and constrictor papillae, aiding ACCOMMODATION and pupillary light reflexes.
Course of III CN
ORIGIN - The oculomotor nerve originates from the anterior aspect of the midbrain.
It moves anteriorly, passing below the posterior cerebral artery, and above the superior cerebellar artery.
The nerve pierces the dura mater and enters the lateral aspect of the cavernous sinus.
Within the cavernous sinus, it receives sympathetic branches from the internal carotid plexus.
These fibres do not combine with the oculomotor nerve – they merely travel within its sheath.
The nerve leaves the cranial cavity via the superior orbital fissure. At this point, it divides into superior and inferior branches.
Once within the orbital cavity, both branches innervate accessory structures of the eye:
a.Superior branch: Motor innervation to the superior rectus and levator palpabrae superioris. Sympathetic fibres run with the superior branch to innervate the superior tarsal muscle.
b.Inferior branch: Motor innervation to the inferior rectus, medial rectus and inferior oblique. Parasympathetic fibres to the ciliary ganglion, which ultimately innervates the sphincter pupillae and ciliary muscles.
1. The oculomotor nerve (CN III) arises from the anterior aspect of midbrain. There are two nuclei for the oculomotor nerve:
• The oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the striated muscle in levator palpebrae superioris and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle.
• The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion, and thus controls the sphincter pupillae muscle (affecting pupil constriction) and the ciliary muscle (affecting accommodation).
Nuclei Of Oculomotor Nerve
Parasympathetic Functions
There are two structures in the eye that receive parasympathetic innervation from the oculomotor nerve:
Sphincter pupillae – Constricts the pupil, reducing the amount of light entering the eye.
Ciliary muscles – Contracts, causes the lens to become more spherical, and thus more adapted to short range vision.The parasympathetic fibres travel in the inferior branch of the oculomotor nerve. Within the orbit, they branch off and synapse in the ciliary ganglion. The fibres are carried from the ganglion to the eye via the short ciliary nerves.
These structures innervates:
SRIRMRIOLPSSphincter pupillaeCiliaris muscle
Cn 3 i
Cn 3 s
Cn 6
Cn 4
It is one of the extraocular muscles.
It is innervated by the superior division of the oculomotor nerve
Functions – Elevation Intorsion Adduction
Superior Rectus
The superior division of oculomotor nerve passes upward lateral to the optic nerve and enters the superior rectus muscle .
Then it pierces and terminates by supplying to the LPS
SR palsy commonly is of congenital origin.
The paralyzed eye is affected primarily in elevation and abduction. Elevation is normal in adduction.
When superior rectus palsy has been present for long periods, elevation from primary position and adduction may also become limited.
The ipsilateral inferior rectus and the contralateral inferior oblique muscles overact, and a small excyclotropia usually is present.
The paralyzed eye is hypotropic in primary position, and Bell’s phenomenon is absent.
Inferior rectus
Innervated by inferior branch of III CN It depresses, adducts, and helps extort
(rotate laterally) the eye
Attachments: Originates from the inferior part of the common tendinous ring, and attaches to the inferior and anterior aspect of the sclera.
The Inferior division of the ouculomotor nerve divides intio 3 branches , which supplies the IR,MR and IO
The branch IR runs forward
The patient fixates with the paralyzed eye. Note right
hypotropia and pseudoptosis in the primary position,
and secondary overaction of the superior oblique and
superior rectus muscles.
The diagnosis is made on the basis of the prism and cover test in the diagnostic positions and on examination of ductions and versions.
The deviation is greatest on attempts to look downward with the affected eye in abduction
Diagnosis
Medial rectus
Attachments: Originates from the medial part of the common tendinous ring, and attaches to the anterio-medial aspect of the sclera.Actions: Adducts the eyeball.
Innervation : the branch to the MR passes medially below the Optic nerve .
An isolated paralysis of the medial rectus muscle without involvement of other muscles supplied by cranial nerve III is very rare.
With this type of paralysis the greatest defect of ocular motility occurs when the affected eye moves into adduction.
Since the action of the antagonistic lateral rectus muscle is unopposed, an exotropia usually is present in primary position.
Right medial rectus paralysis. Exotropia in primary position and overaction of left lateral rectus muscle
The differential diagnosis of an isolated medial rectus paralysis includes internuclear ophthalmoplegia
Diagnosis
Inferior oblique
Attachments: Originates from the anterior aspect of the orbital floor. Attaches to the sclera of the eye, posterior to the lateral rectus.
Actions: Elevates, abducts and laterally rotates the eyeball.
Innervation: Oculomotor nerve (CN III). The branch to the IO , is the longest branch , that passes forward close to the orbital floor and lateral to the IR muscle , the nerve enters the post. Border of the Oblique muscle
The inferior oblique muscle is least likely to become paralyzed.
The onset is usually congenital but trauma has been mentioned as a cause.
In primary position the affected eye may be hypotropic or the unaffected eye hypertropic, depending on whether the patient fixates with the nonparalyzed or paralyzed eye.
Causes
The forced duction test is necessary in making this diagnosis, since the prevalence of Brown syndrome is far greater than paralysis of the inferior oblique muscle and since the defect of ocular motility is clinically similar.
However, with Brown syndrome the involved eye is frequently depressed more severely in adduction than it is with inferior oblique paralysis.
Diagnosis
LPS
Attachment : The levator palpebrae superioris originates on the lesser wing of the sphenoid bone, just above the optic foramen. It broadens and becomes the levator aponeurosis.
Function : Elevates eye lid
Innervation : The sup. Division of the oculomotor nerve passes upward lateral to the optic nerve . By supplying to SR ,the nerve pierces the muscle and terminates by supplying the LPS
Damage to its innervation, can cause Ptosis - The drooping of the eyelid.
Ptosis can also be caused by damage to the adjoining superior
tarsal muscle, or its sympathetic innervation.
Such damage to the sympathetic supply occurs in Horner's syndrome, and presents as a partial ptosis.
Clinical condition:
Parasympathetically III CN
These are postganglionic fibres travel to the ciliary and sphincter pupillae muscles in the short ciliary nerves and innervate two eye muscles:
1.Sphincter pupillae - constricts the pupil, a movement known as Miosis 2.Ciliaris muscle - releasing tension on the Zonular fibres, making the lens more convex, also known as accommodation.
Sphincter pupillae
Muscle contracts the pupil ,and the dilator pupilae muscle dilates it.
Coloured part of the eye Its inner edge forms the margin of the pupil
Functions - In humans, it functions to constrict the pupil in bright light or during accomodation
Innervation – Parasympathetically.
.
Examination
When performing a pupillary exam - illuminate pupils indirectly / directly to see what is happening.
Observe the pupil size and shape at rest, looking for anisocoria (one pupil larger than the other)
Observe the direct response (constriction of the illuminated pupil)
Observe the consensual response (constriction of the opposite pupil)
Check for accommodation (constriction of pupil when viewing a close object)
Pupillary responses & tests…
1.Efferent pupillary defect – (size and shape) the patient has to look at the distant object , then room is darkened and the direct reflex of pupil is then tested with different intensities - if both the pupil gets constricted then There is no efferent pupillary defect .
2.Afferent pupillary defect(Swinging Flash test) – examiner has to indirectly illuminates 1 eye then quickly switches to another eye - 2 pupils are always of EQUAL diameter - afferent is good
Clinical conditions…Anisocoria: > Refers to the asymmetric sizes of pupils > Physiologic anisocoria is very common and a normal variant in up to 20% of the population. The variation should be no more than 1mm and both eyes should react to light normally.
RAPD - is a defect in the direct response. It is due to damage in optic nerve or severe retinal disease.
Its diagnosis…..
Swinging Flashlight Test:> 10-15 times swinging should be done(10sec)
> Swing a light back and forth in front of the two pupils and compare the reaction to stimulation in both eyes. > When light reaches a pupil there should be a normal direct and consensual response. > An RAPD is diagnosed by observing paradoxical dilatation when light is directly shone in the affected pupil after being shown in the healthy pupil. >>> Helps to distinguish decreased vision and RAPD.Some causes of a RAPD include:optic neuritis , ischemic optic disease or retinal disease , severe glaucoma causing , trauma to optic nerve , direct optic nerve damage , retinal detachment , very severe macular degeneration , retinal infection.
1.shown in the healthy pup
Adie's (Tonic) Pupil: Common in women (but also can be present in men)• Either no or sluggish response to
light (both direct and consensual responses)
• Thought to be caused from de- innervation in the postganglionic parasympathetic nerve
Associated with Holmes-Adie syndrome described .
Ciliary muscleThe Ciliary ganglion is a para sympatheticlly innervated structure located in the posterior orbit.
Fibres from Edinger – westpal nucleus , it travels in the oculomotor nerve along its outer edge , and enter the ciliary ganglion .
Function – Accommodation , Trabecular meshwork pore size
Innervation-The ciliary muscle receives only parasympathetic fibers from the short ciliary nerves that arise from the ciliary ganglion. These postganglionic fibers are part of cranial nerve III .
Presynaptic parasympathetic signals that originate in the Edinger-Westphal nucleus are carried by cranial nerve III and travel through the ciliary ganglion.
Parasympathetic activation of the M3 muscarinic receptors causes ciliary muscle contraction, the effect of contraction is to decrease the diameter of the ring of ciliary muscle.
The zonule fibers relax and the lens becomes more spherical, increasing its power to refract light for near vision.
Accommodationchanges in lens shape for light focusing. the ciliary muscle contracts. Converging of eyes occurs.
Trabecular meshwork pore size: Contraction and relaxation of the
longitudinal fibers, which insert into the trabecular meshwork in the anterior chamber of the eye, cause an increase and decrease in the meshwork pore size, respectively, facilitating and impeding aqueous humour flow into the canal of Schlemm.
Clinical condition
Glaucoma - Open-angle glaucoma (OAG) and closed-angle glaucoma (CAG) may be treated by muscarinic receptor agonists , which cause rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork, facilitating drainage of the aqueous humour into the canal of Schlemm and ultimately decreasing intraocular pressure.
References
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