Medial longitudinal fasciculus

The medial longitudinal fasciculus (MLF) is a bundle of paired nerve fibers, each located on either side of the brainstem's midline. Positioned adjacent to the midline, it runs anterior to the cerebral aqueduct and the fourth ventricle.

The MLF plays a crucial role in coordinating eye movements through its connections between the ipsilateral oculomotor and trochlear nerve nuclei and the contralateral abducent nerve nucleus. This connectivity enables conjugate eye movements, where both eyes move in a synchronized and parallel manner. Such movements include saccades, smooth pursuit eye movements, and components of the vestibulo-ocular reflexes.

The abducent nerve nucleus not only activates the lateral rectus muscle but also functions as the horizontal gaze center. It sends signals via the MLF to the contralateral oculomotor nucleus, leading to the simultaneous contraction of the medial rectus muscle on the opposite side. For example, when a person looks to the right, the right abducent nerve nucleus activates the right lateral rectus muscle, while the left oculomotor nucleus activates the left medial rectus muscle, ensuring both eyes move to the right. A lesion in the MLF, such as in neurodegenerative diseases like multiple sclerosis, can result in internuclear ophthalmoplegia, causing horizontal gaze diplopia.

Beyond coordinating conjugate eye movements, the MLF integrates eye movements with head, neck, and body movements. It connects the medial and superior vestibular nuclei on the same side with the extraocular motor nerve nuclei to mediate vestibulo-ocular reflex. The MLF also serves as a conduit for the medial vestibulospinal descending fibers, connecting medial vestibular nuclei with the cervical spinal cord, coordinating head and neck movements with eye movements to maintain gaze on objects of interest.

Thus, the MLF is vital in the interconnection between vestibular nuclei, extraocular motor nuclei, and the cervical spinal region, coordinating a wide range of movements involving the eyes, head, neck, and upper torso.