Medial longitudinal fasciculus (MLF)

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Medial longitudinal fasciculus (MLF) is a highly specialized paired neural tract in the brainstem that plays a central role in coordinating eye movements, particularly horizontal conjugate gaze and vestibulo-ocular reflexes. It is a key relay structure linking the nuclei of cranial nerves involved in eye movement.

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Anatomy

Feature	Details
Location	Paired structure in the dorsomedial brainstem, adjacent to the midline; extends from midbrain to upper cervical spinal cord
Relations	Lies ventral to the fourth ventricle in the pons and near the cerebral aqueduct in the midbrain
Connects	Abducens nucleus (CN VI) → contralateral oculomotor nucleus (CN III)
Function	Coordinates horizontal conjugate gaze, vestibulo-ocular reflex, and head-eye coordination

Function of the MLF

A. Horizontal Gaze Coordination

When looking laterally:
- Abducens nucleus on one side activates the ipsilateral lateral rectus.
- Sends signals via the contralateral MLF to activate the contralateral oculomotor nucleus, specifically the medial rectus subnucleus → adduction.
Ensures conjugate eye movement (both eyes move together in the same direction).

B. Vestibulo-Ocular Reflex (VOR)

Coordinates eye movements with head movements by integrating inputs from vestibular nuclei to extraocular muscle nuclei via MLF.

Lesions of the MLF

A. Internuclear Ophthalmoplegia (INO)

Classic sign of MLF lesion.
Disruption of signal from abducens interneurons to contralateral medial rectus.
Features:
- Impaired adduction of the eye ipsilateral to the lesion.
- Abducting nystagmus of the contralateral eye.
- Convergence usually spared (since it uses a different pathway).

B. Bilateral MLF Lesions

Often due to multiple sclerosis (MS).
May result in bilateral INO or Wall-eyed Bilateral INO (WEBINO):
- Bilateral adduction failure
- Primary gaze exotropia