CN III = oculomotor

Anatomy

Somatic motor component

Parasympathetic motor component

  • Pupillary Light reflex
  • Accommodation Reflex

Eye movement

Visceral motor component = innervates the intrinsic ocular muscles:

  • Constrictor pupillae
  • Ciliary muscle

Somatic motor component = innervates 4/6 extra ocular muscles

Nerve also innervates the levator palpebrae superioris muscle = elevate the upper eyelid

Oculomotor complex consists:

  • Somatic motor subnuclei
  • Edinger-westphal nucleus = visceral motor neurons
  • Located at the superior colliculus

1) Axons of both nuclei course ventrally in the midbrain -> oculomotor nerve
2) Emerges from the interpeduncular fossa in the midbrain
3) Passing between the posterior cerebral and the superior cerebellar arteries

4) Pierces the dura and enters the cavernous sinus (sup to the trochlear nerve)
5) Split into:

  • Smaller superior division
  • Larger inferior division
    and then passes through the superior orbital fissure

Divisions

Inferior division:

  • Medial rectus
  • Inferior rectus
  • Inferior oblique muscles

Superior division:

  • Superior rectus
  • Levator palpebrae superioris muscles

Axons

Visceral motor axons

  • Run with the nerve to the inferior oblique muscle for a short distance, then leave it to terminate in the ciliary ganglion

Postganglionic axons

  • Leave the ciliary ganglion as 6 to 10 short ciliary nerves to enter the eye at the posterior aspect near the exit of the optic nerve

Subnuclei within the oculomotor nucleus supply individual muscles:

  • Lateral subnuclei
  • Medial subnucleus
  • Central subnucleus

Oculomotor nucleus -> tegmentum of the midbrain at the level of the superior colliculus

Medial subnucleus:

  • contralateral superior rectus muscle

Central subnucleus:

  • Levators palpebrae superioris bilaterally

Lateral subnuclei:

  • Ipsilateral inferior rectus
  • Inferior oblique
  • Medial rectus muscles

Actions of the extraocular muscles innervated by CN III are:

Superior rectus muscle: elevation

Inferior rectus muscle: depression

Medial rectus muscle: adduction

Inferior oblique muscle: excyclotorsion

The combination of these muscles plus the superior oblique (CN IV) and lateral rectus (CN VI) muscles enable eye movements around the three axes of movement

The actions of two muscles are antagonistic: one muscle must relax while the other contracts to execute eye movements

Edinger-Westphal (visceral motor) nucleus

  • Located in midbrain
  • Preganglionic visceral motor axons leave the nucleus and course ventrally
  • The parasympathetic and somatic axons together constitute CN III

Eyelids

upper eyelid = palpebra superioris

  • Elevated by elevator palpebra superioris
  • Levator and the superior rectus muscles are innervated by the superior division of the oculomotor nerve

lower eyelid = palpebra inferioris

  • Does not have its own muscle, instead = tendon of the inferior rectus muscle -> Inferior tarsus
  • Both eyelids also moved by smooth muscle called "Muller's muscles" or sup and inf tarsal muscles
  • The smooth muscle is innervated by sympathetic nerves
  • The parasympathetic axons are located on the superomedial surface of the nerve
  • The parasympathetic axons branch from the nerve to the inferior oblique muscle and terminate in the ciliary ganglion near the apex of the orbit
  • Postganglionic axons leave the ciliary ganglion as 6 to 10 short ciliary nerves --> terminate in the constrictor pupillae muscle and the ciliary muscle

Control the size of the pupil and the shape of the lens

Pupillary light reflex

Control the amount of light entering the eye

The pupillary light reflex involves two cranial nerves:

  • The optic nerve (CN II): forms the sensory limb by carrying the sensory signal to the brainstem
  • The oculomotor nerve (CN III): forms the motor limb by carrying motor signals to the pupillary constrictor muscle
  1. Light entering the eyes causes signals to be sent to both the ipsilateral and the contralateral pretectal olivary nuclei
  2. The pretectal olivary nucleus cells send signals bilaterally to the Edinger- Westphal nuclei
  3. Visceral motor signals are sent to the ciliary ganglia
  4. Postganglionic axons terminate in the constrictor pupillae muscles of the iris

In the normal reflex, light shone in either eye causes:

  • Constriction of the pupil in the same eye (the direct light reflex)
  • Constriction in the other eye (the consensual light reflex)

Accomodation reflex (adaptation of the eyes for near vision)

Near triad

An increase in the curvature of the lens

  • Suspensory ligament maintains tension on the lens
  • Flat to look far away
  • Curvature of the lens increase to look near

Pupillary constriction

  • Edinger-Westphal nucleus -> pupillary constrictor muscle to contract

Convergence of the eyes

  • Oculomotor nucleus -> send signals to medial rectus muscles -> contraction
  • Eyes to converge in order to keep the object centered on the fovea

Oculomotor nerve damage

Peripheral axons

Nucleus

Trauma, ischemia, or demyelination within the midbrain

Damage to axons in the subarachnoid space may be due to infection, tumor infiltration, or infarction

  • (loss of blood supply, usually caused by diabetes or hypertension)

Compression of axons may be due to aneurysms, most typically in the posterior communicating artery and sometimes in the basilar artery

Compression of axons may be caused by the uncus of the temporal lobe during cerebral herniation if there is raised intracranial pressure

Compression of axons in the cavernous sinus may be due to tumors, inflammation, infection, or thrombosis

Damage may be caused by trauma to the area where axons pass through the superior orbital fissure to enter the orbit

Distinguishing a 3rd nerve palsy due to damage of the oculomotor nucleus from damage to the nerve itself

behavior of the upper eyelid

Nuclear lesion

  • No ptosis
  • No weakness in upward gaze in the ipsilateral eye, but weakness of upgaze in the contralateral eye
  • Ipsilateral weakness of downward gaze
  • Ipsilateral weakness of adduction
  • Dilated unresponsive pupil

Right unilateral nuclear lower motor neuron lesion of cranial nerve III nucleus

Ipsilateral side of lesion

Contralateral side of lesion

  • No ptosis (LPS)
  • Upward gaze intact (SR)
  • Weakness of downward gaze (IR)
  • Weakness of adduction (MR)
  • Dilated unresponsive pupil (PC)
  • No ptosis (LPS)
  • Weakness upward gaze (SR)
  • Downward gaze intact (IR)
  • Adduction intact (MR)
  • Responsive pupil (PC)

Disorders

Peripheral lesions

GSE component lesions will produce lower motor neuron paralysis of the extraocular eye muscles supplied by this nerve

Lateral strabismus

Drooping of the eyelid (ptosis)

The inability to move the eye inward or vertically

LesionsoftheGVEcomponentswill produce the following autonomic effects:

Lossof the pupillary light reflex

Accommodation,which includes the convergence reactions

Central lesions

Lesion located near the ventromedial aspect of the midbrain

  • Such a lesion invariably affects both fibers of the third nerve and corticospinal fibers

both third nerve paralysis as well as upper motor neuron paralysis of the contralateral limbs

Weber’s syndrome or superior alternating hemiplegia