The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) sends excitatory monosynaptic projections to vertical ocular motor neurons and to the intersitial nucleus of Cajal (INC), and receives projections from ipsilateral vertical semicircular canal neurons.

The riMLF is adjacent to the INC and also has a key role in the genesis of vertical and torsional fast eye movements.

Vertical Saccades
Each riMLF projects bilaterally to motoneurons for the elevator muscles (superior rectus and inferior rectus), but ipsilaterally to motoneurons for the depressor muscles (inferior rectus and superior oblique)

Torsional Saccades
The right riMLF is responsible for conjugate clockwise saccades and the left riMLF is responsible for conjugate counterclockwise saccades (from patient’s point of view).

The riMLF produces the pre-motor neural drive for all vertical and for ipsiversive torsional fast eye movements: the riMLF neurons fire in bursts just before any and all fast eye movements that have a vertical or torsional component.
While these riMLF neurons are highly selective for the torsional direction of a fast eye movement (they fire only for ipsiversive torsional fast eye movements), they are not selective for the vertical direction of the fast eye movement. For example, neurons in the right riMLF will fire immediately before any fast eye movement with a clockwise torsional component, whether it has an upward or a downward vertical component. They will not fire at all with fast eye movements that have only a counter-clockwise torsional component.
As with INC stimulation, the eccentric eye position produced during riMLF stimulation is maintained until the next fast eye movement, indicating that the eye velocity signal has been integrated to an eye position signal.

Lesions of the riMLF
Unilateral riMLF lesions :

The following are all preserved:

  1. Vertical fast eye movements
  2. Contralesional torsional fast eye movements
  3. Vertical gaze-holding
  4. Torsional and vertical VOR

Bilateral riMLF lesions (Common because both riMLFs are often supplied by a single posterior thalamic–subthalamic paramedian artery)
Produce total loss of all vertical and torsional fast eye movements with preservation of the vertical and torsional VOR, vertical gaze-holding, and of all horizontal eye movements.

Figure 1. Relationships of the riMLF, INC and the nuclei of CN 3 and 4.

Redrawn from: Bhidayasiri R, Plant GT, Leigh RJ. A hypothetical scheme for the brainstem control of vertical gaze. Neurology. 2000;54(10):1985-1993. doi:10.1212/wnl.54.10.1985

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

References