An explanation of eye movements seen in internuclear ophthalmoplegia

Recent findings in experiments on monkeys show that a horizontal saccade is made by a pulse-step of neural activity, part of which rises to medial rectus (MR) motoneurons on the fibers of the medial longitudinal fasciculus (MLF). If inhibitory as well as excitatory fibers run in the MLF, then the data show that each MLF must carry both the excitatory and inhibitory activity required for contralateral, horizontal saccades. Therefore, interruption of these fibers removes not only excitation from the ipsilateral MR but inhibition from the contralateral MR. Such a lesion also disturbs the correct relationship between the pulse and the step and creates abnormal saccades. These facts make it possible to explain why, in internuclear ophthalmoplegia, the eye on the side of the lesion adducts slowly and inadequately, while the opposite eye has nystagmus in abduction.