Fig. 1. The brain-stem pathway for horizontal gaze. Supranuclear inputs converge on the paramedian pontine reticular formation (PPRF), the premotor center for horizontal eye movements. The innervation for a horizontal eye movement flows from the ipsilateral PPRF to both an abducens motor neuron and an internuclear neuron in the abducens nucleus. The latter internuclear neuron decussates to the contralateral medial longitudinal fasciculus, where it ascends to reach the contralateral medial rectus subnucleus.