Oculomotor Nerve

Overview

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Consists of two components with distinct functions:

Somatic motor
(general somatic efferent)
Supplies four of the six extraocular muscles of the eye and the levator palpebrae superioris muscle of the upper eyelid.
Visceral motor
(general visceral efferent)
Parasympathetic innervation of the constrictor pupillae and ciliary muscles.

The somatic motor component of CN III plays a major role in controlling the muscles responsible for the precise movement of the eyes for visual tracking or fixation on an object.

The visceral motor component is involved in the pupillary light and accomodation reflexes.

Overview of the Somatic Motor Component

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Overview, somatic motor component of CN III.

There are six extraocular muscles in each orbit.

The somatic motor component of CN III innervates the following four extraocular muscles of the eyes:

  • Ipsilateral inferior rectus muscle
  • Ipsilateral inferior oblique muscle
  • Ipsilateral medial rectus muscle
  • Contralateral superior rectus muscle 

The remaining extraocular muscles, the superior oblique and lateral rectus muscles, are innervated by the trochlear nerve (CN IV) and abducens nerve (CN VI), respectively. 

The somatic motor component of CN III also innervates the levator palpebrae superioris muscles bilaterally. These muscles elevate the upper eyelids.

Actions of the Extraocular Muscles

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Figure 3-3a. Muscles of the orbit.

The actions of all six extraocular muscles are summarized in the table below:

Muscle

Innervation 

Primary action

Secondary action

Tertiary action

Medial rectus

CN III

Adduction

--

--

Superior rectus

CN III

Elevation

Intortion

Adduction

Inferior rectus

CN III

Depression

Extortion

Adduction

Inferior oblique

CN III

Extorsion

Elevation

Abduction

Superior oblique

CN IV

Intorsion

Depression

Abduction

Lateral rectus

CN VI

Abduction

--

--

A knowledge of the origins and points of insertion of the extraocular muscles on the eye relative to the axes of motion of the eye is critical to understanding the actions of these muscles.

Medial Rectus Muscle

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The medial rectus muscle originates from the tendinous ring of the orbit and inserts on the medial border of the eye. Contraction of this muscles leads to adduction of the eye.

Superior Rectus Muscle

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The superior rectus muscle originates from the tendinous ring of the orbit and inserts on the superior surface of the eye slightly medial to the eyes vertical axis of rotation. Due to these factors, contraction of the superior rectus results in elevation, intorsion, and adduction of the eye. The primary action of the superior rectus muscle (elevation of the eye) can be isolated by having the patient look laterally and then upwards.

Inferior Rectus Muscle

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The inferior rectus muscle originates from the tendinous ring of the orbit and inserts on the inferior surface of the eye slightly medial to the eyes vertical axis of rotation. As a result, contraction of the inferior rectus causes depression, extorsion, and adduction of the eye. The primary action of the inferior rectus muscle (depression of the eye) can be isolated by having the patient look laterally and then downwards.

Inferior Oblique Muscle

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Figure 3-4b. Muscles of abduction.

The inferior oblique muscle originates from the floor of the bony orbit and passes laterally and posteriorly to insert on the posterolateral surface of the eye slightly behind its vertical axis of rotation. Contraction of this muscle results in extorsion (outward rotation about the anterior-posterior axis of rotation), elevation, and abduction. The action of the inferior oblique muscle can be isolated by having the patient look medially and then upwards.

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The actions of the superior oblique and lateral rectus muscles are discussed in the modules on CN IV and CN VI, respectively.

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Origin & Central Course

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Figure 3-5. Somatic motor component-origin and central course

The somatic motor component of CN III originates from the oculomotor nucleus located in the rostral midbrain at the level of the superior colliculus.

Like other somatic motor nuclei, the oculomotor nucleus is located near the midline just ventral to the cerebral aqueduct.

In a coronal cross-section of the brainstem the oculomotor nucleus is "V-shaped" and is bordered medially by the Edinger-Westphal nucleus and laterally and inferiorly by the medial longitudinal fasciculus which allows communication between various brainstem nuclei.

Fibers leaving the occulomotor nucleus travel ventrally in the tegmentum of the midbrain passing through the red nucleus and medial portion of the cerebral peduncle to emerge in the interpeduncular fossa at the junction of the midbrain and pons.

Intracranial Course

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Figure 3-6. Somatic motor component - intracranial course.

Upon emerging from the brainstem the oculomotor nerve passes between the posterior cerebral and superior cerebellar arteries and pierces the dura mater to enter the cavernous sinus. 

The nerve runs along the lateral wall of the cavernous sinus just superior to the trochlear nerve and enters the orbit via the superior orbital fissure.

Final Innervation

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Figure 3-7a. Somatic motor component, final innervation, anterior view.

Within the orbit CN III fibers pass through the tendinous ring of the extraocular muscles and divide into superior and inferior divisions. 

The superior division ascends lateral to the optic nerve to innervate the superior rectus and and levator palpebrae superioris muscles on their deep surfaces.

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The inferior division of CN III splits into three branches to innervate the medial rectus and inferior rectus muscles on their ocular surfaces and the inferior oblique muscle on its posterior surface.

Vertical Gaze

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Figure 3-8. Vertical gaze center, CN III & CN IV.

The exact control of eye movements requires input from integration centers in the brain that coordinate the output from the occulomotor, trochlear, and abducens nuclei which control the six extraocular muscles.

For eye movements in the vertical plane, the superior rectus, inferior rectus, inferior oblique, and superior oblique muscles of the eyes must work precisely together.

The actions of these muscles is coordinated by the vertical gaze center which is thought to be located in the periaqueductal grey matter of the midbrain at the level of the superior colliculus (its location has not yet been positively identified).

The vertical gaze center projects to the oculomotor nuclei which control the superior rectus, inferior rectus and inferior oblique muscles as well as to the trochlear nuclei which control the superior oblique muscles.

The center that controls torsional movements of the eye is probably close to, or the same as, the vertical gaze center since all muscles that elevate or depress the eyes also cause them to rotate about their anterior-posterior axis of motion.

The lateral gaze center is discussed in the abducens nerve (CN VI) module.