The Medial Longitudinal Fasciculus is a fiber tract located ventrolaterally to the oculomotor nucleus that connects the trochlear nucleus, oculomotor nucleus, and abducens nucleus. The MLF helps with yoked eye movements. Dysconjugate eye movements are a common symptom of multiple sclerosis due to lesions in the MLF. For example, depending on the severity of the lesion, MLF lesions cause partial to complete adduction failure. Internuclear ophthalmoplegia is the medical term for the condition.
Additionally, the medial longitudinal fasciculus is responsible for transporting fibers that originate in the vestibular nuclei and are destined for the oculomotor, trochlear, and interstitial nuclei of Cajal. These fibers become active whenever there is an activity in the vestibulo-ocular reflex, which occurs when the head is moved.
Medial Longitudinal Fasciculus Functions
The oculomotor, trochlear, and abducent nerves, along with the vestibulocochlear nerve, are controlled by the medial longitudinal fasciculus. The medial longitudinal fasciculus’s function is to coordinate eye and head motion.
The MLF, which is essential for both optokinetic and vestibulo-ocular reflexes, integrates the information about head and eye movement that is received. The abducens nucleus is connected to the contralateral oculomotor nucleus by the fasciculus fibers, which enables horizontal conjugate lateral gaze as well as saccadic eye movements.
The gaze reflex is addressed by the medial longitudinal fasciculus, which descends from the vestibular nucleus. The vestibulocochlear nerve, the eighth cranial nerve, the fastigial nucleus, and the flocculus of the cerebellum provides the input necessary for this to happen. Proprioceptors in the head, neck, and ankle muscle spindles, as well as the fastigial nucleus, all provide data for processing.
Medial Longitudinal Fasciculus Location
The MLF is a pair of highly specialized nerve fibers that are highly myelinated and run in a craniocaudal direction just ventral to the cerebral aqueduct and the fourth ventricle in the tegmentum of the midbrain and the dorsal pons. These nerve fibers are located in the tegmentum of the midbrain and the dorsal pons.
A group of crossed fibers with ascending and descending fibers is known as the medial longitudinal fasciculus and is located in the brainstem. The medial longitudinal fasciculus connects the three major nerves that control eye movements, namely the oculomotor, trochlear, and abducent nerves, along with the vestibulocochlear nerve.
Medial Longitudinal Fasciculus Lesion
Certain clinical signs are manifested in a patient when a lesion affects the heavily myelinated tracts of the MLF. These clinical signs differ according to the location of the lesion.
These signs are clinically classified as:
- Internuclear Ophthalmoplegia (INO).
- Wall-eyed bilateral INO syndrome.
- INO and one and a half syndrome.
- INO and trochlear syndrome.
A lesion of the MLF that causes classic INO manifests as a loss of ipsilateral adduction with preservation of contralateral abduction. When performing a clinical exam, saccadic eye movement is the most effective method for understanding this. INO and trochlear syndrome are both caused by damage to the MLF, which is located in the caudal region of the midbrain, as well as the ipsilateral trochlear nucleus. When the trochlear nerve is damaged, the superior oblique muscle on the affected side weakens, leading to internal rotation of the thigh and hyperextension of the affected leg.
An injury to the medial longitudinal fasciculus (MLF) in the dorsal pontine tegmentum, along with the ipsilateral abducens nucleus (AN) or posterior cingulate reticulum (PPRF), causes unilateral nystagmus (INO) and bilateral lateral gaze palsy (ILP). A lesion of the bilateral MLF causes bilateral INO syndrome, which manifests as “wall-eyed” eyes.
Medial Longitudinal Fasciculus Damage
A medial longitudinal fasciculus lesion causes slowed or absent ipsilateral eye adduction during the contralateral gaze. The condition known as internuclear ophthalmoplegia is typically accompanied by jerky, uncontrollable eye movements of the abducting eye.
Ophthalmoplegic patients frequently experience double vision as well as blurred vision. They are also unable to coordinate the movement of their eyes. Many people also experience a drooping of their eyelids, and some people show difficulty moving both eyes in any direction at the same time.