Brainstem Ocular Motor Machinery

Seen here is a sagittal view of the brainstem. The medulla has a significant role in gaze-holding, and the nucleus prepositus hypoglossi (NPH, along with the medial vestibular nucleus ) is the horizontal neural integrator. The abducens (6th) nucleus is located in the dorsal pons, and sends off the 6th fascicle that innervates the ipsilateral lateral rectus. Additionally, interneurons from the 6th nucleus decussate and ascend the contralateral medial longitudinal fasciculus (MLF) where they project to the medial rectus subnucleus to innervate the ipsilateral medial rectus. This ensures conjugate eye movements. Excitation by the paramedian pontine reticular formation (PPRF - contains the horizontal burst neurons) generates horizontal saccades. Once a saccade is made to an eccentric location, the orbital elastic forces pull the eyes back to the midline. The neural integrator is responsible for maintaining the tone required to keep the eyes in an eccentric position. The trochlear (4th) nucleus is located at the level of the inferior colliculus, and the nerve exits dorsally and decussates to innervate the contralateral superior oblique muscle - this is the basis for a contralesional fourth nerve palsy resulting from a lesion of the trochlear nucleus or fascicle within the midbrain. The oculomotor (3rd) nucleus is located at the level of the superior colliculus, and innervates the ipsilateral pupillary sphincter, levator palpebrae (via the central caudal nucleus [CCN] - sends projections to both LP muscles), inferior rectus, medial rectus, inferior oblique and contralateral superior rectus. Therefore, bilateral findings in a nuclear third nerve palsy can be seen and include: 1) bilateral ptosis (usually ipsilateral>contralateral) due to the single midline CCN, and 2) bilateral superior rectus weakness due to the decussating fibers that originate in the superior rectus subnuclei - e.g., a right nuclear third will damage the right SR subnucleus that sends its fibers to the left SR, and the fibers that decussated from the left SR subnucleus to the right side (destined for right superior rectus muscle) are also affected. While the PPRF contains the horizontal burst (saccade generating) neurons, the rostral interstitial MLF (riMLF) is responsible for torsional fast phases and vertical saccades. Because the riMLF projections to the depressors are ipsilateral whereas the projections to the elevators are bilateral, a unilateral riMLF lesion tends to affect downward more than upward saccades. In contrast, a lesion of the posterior commissure (PC) mainly affects upward movements (saccades, pursuit, vestibulo-ocular reflex), and features of the pretectal or Parinaud's syndrome are usually seen. While the NPH contains the horizontal gaze-holding machinery, the interstitial nucleus of Cajal (INC) is responsible for vertical and torsional gaze-holding.