Abducens Nerve

Overview

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Figure 6-1a. Abducens nerve and the lateral rectus muscle.

Has only a somatic motor (general somatic efferent) component.

Somatic motor: innervates the lateral rectus muscle of the ipsilateral orbit.

The lateral rectus muscle is one of the six extraocular muscles responsible for the precise movement of the eye for visual tracking or fixation on an object.

See the CN III section (occulomotor nerve) for a discussion of eye movements and the interaction between the three nuclei and nerves that innervate the extraocular muscles.

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The abducens nerve innervates the lateral rectus muscle of the ipsilateral orbit.

The lateral rectus muscle is responsible for lateral gaze (its contraction causes the eye to be abducted):

Origin and Central Course

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Figure 6-4. Origin and central course.

The fibers of the abducens nerve originate from the abducens nucleus located in the caudal pons at the level of the facial colliculus.

The nucleus is located just ventral to the fourth ventricle near the midline. Axons of CN VII (facial nerve) loop around the abducens nucleus and give rise to a bulge in the floor of the fourth ventricle - the facial colliculus.

Fibers leaving the abducens nucleus travel ventrally to exit the brainstem at the border of the pons and medullary pyramids.

Intracranial Course

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Figure 6-4. Intracranial course of abducens nerve.

Upon exiting the brainstem the abducens nerve climbs superiorly along the ventral surface of the pons. On reaching the apex of the petrous portion of the temporal bone the nerve makes a sharp turn anteriorly to enter the cavernous sinus. The abducens nerve travels along the lateral wall of the cavernous sinus with CNS III, IV, and V.

Intracranial Course and Final Innervation

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Figure 6-5. Final innervation of abducens nerve.

From the cavernous sinus the abducens nerve enters the orbit through the superior orbital fissure.

CN VI passes through the tendinous ring of the extraocular muscles and innervates the lateral rectus muscle on its deep surface:

Coordination of Lateral Rectus and Medial Rectus Muscles

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 horizontal plane, the lateral rectus muscle of one eye and the medial rectus muscle of the other eye must work precisely together.

The actions of these muscles is coordinated by the lateral gaze center located in the pontine reticular formation.

Inputs from higher centers of the brain synapse in the lateral gaze center, which then sends simultaneous signals to the ipsilateral abducens nucleus and to the contralateral occulomotor nucleus via the medial longitudinal fasciculus.

The abducens nucleus sends signals via CN VI to the lateral rectus muscle of the ipsilateral orbit to command that eye to be abducted. Simultaneously, the occulomotor nucleus generates a command via CN III to contract the medial rectus muscle of the contralateral orbit resulting in adduction of that eye.

The end result is that both eyes precisely fixate on or track the same object: