Medial longitudinal fasciculus

The medial longitudinal fasciculus (MLF) is a myelinated composite fiber tract found in the brainstem. The MLF primarily serves to coordinate the conjugate movement of the eyes and associated head and neck movements.

Gross Anatomy

Containing both ascending and descending fiber tracts, the MLF is found on each side of the brainstem near the midline, ventral to the periaqueductal grey matter, and ascends to the interstitial nucleus (of Cajal) ¹.

The MLF links the nuclei of the vestibulocochlear nerve (CN VIII) and the three primary nerves controlling the movements of the eye:

• oculomotor nerve (CN III)
• trochlear nerve (CN IV)
• abducens nerve (CN VI)

Ascending fibers are contributed to by the four vestibular nuclei. Descending axons from the medial vestibular nuclei partially decussate and continue as the medial vestibulospinal tract at the level of the spinal cord. The dorsal trapezoid, posterior commissural and lateral lemniscal nuclei all contribute fibers, thereby linking both vestibular and cochlear components of CN VIII to the MLF.

Function

The MLF integrates the information received about the movement of the eyes and the movement of the head and plays a central role in the optokinetic and vestibulo-ocular reflexes. Fibers within the fasciculus connect the abducens nucleus with the contralateral oculomotor nucleus allowing horizontal conjugate lateral gaze and saccadic eye movements. Fibers are also carried from the vestibular nuclei to integrate with the oculomotor and trochlear nuclei, which serve to influence eye movements during movement of the head and neck ².

Related pathology

• lesions to the MLF cause internuclear ophthalmoplegia