Neuroanatomy: Final Common Pathway (ARCHIVE)

Notes

The abducens nucleus of cranial nerve 6 within the pons comprises pools of motoneurons and interneurons.
For example, LEFT CN 6 motoneurons innervate the left eye's lateral rectus muscle, which drives the left eye to the left (laterally).
The left abducens interneurons project fibers across midline that ascend the right medial longitudinal fasciculus and synapse in the RIGHT oculomotor nucleus of cranial nerve 3 in the midbrain.
The right oculomotor nucleus innervates the right eye's medial rectus muscle and drives the right eye to the left (medially).

Injury to the abducens motoneurons causes loss of ipsilateral eye abduction.
Injury to the abducens interneurons causes loss of contralateral eye adduction.
In a complete abducens nuclear injury, there is loss of gaze to the side of the lesion:
- For example, in a left abducens nuclear injury, the eyes are unable to deviate to the left.

When the medial longitudinal fasciculus is injured, the ipsilateral eye is unable to adduct.
This is called an internuclear ophthalmoplegia (or MLF syndrome) - the ipsilateral eye is unable to adduct and the opposite eye has horizontal nystagmus when it abducts.

When both medial longitudinal fasciculus tracts are injured, neither eye can adduct - the right eye can't turn horizontally to the left and the left eye can't turn horizontally to the right.
- This is called bilateral internuclear ophthalmoplegia.

When the left abducens nucleus is injured, there is loss of gaze toward the side of the lesion (left gaze palsy), and when the adjacent medial longitudinal fasciculus is injured, the ipsilateral eye (the left eye) can't adduct.
Thus, when both the abducens nucleus and the adjacent medial longitudinal fasciculus are injured, the only intact movement is right eye abduction (and it has nystagmus from the left medial longitudinal fasciculus injury).
Thus, one-and-a-half of the two complete eye movements are impaired, so the injury pattern is called one-and-a-half syndrome.