Figure 13-1. The supranuclear pathways subserving horizontal gaze to the left. The pathway originates in the right frontal cortex, descends in the internal capsule, decussates at the level of the rostral pons, and descends to synapse in the left pontine paramedian reticular formation (PPRF). Further connections with the ipsilateral sixth nerve nucleus and contralateral medial longitudinal fasciculus are also indicated. The right MLF (green line) is labeled between the abducens and oculomotor nuclei and the vestibular nuclei (VN) are shown on the right. LR, lateral rectus; MLF, medial longitudinal fasciculus; MR, medial rectus.
Figure 13-2. Pathways for the control of vertical eye movements. The main structures are the interstitial nucleus of Cajal (INC), the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), and the subnuclei of the third nerve, all located in the dorsal midbrain. Voluntary vertical movements are initiated by the simultaneous activity of both frontal cortical eye fields. The riMLF serves as the generator of vertical saccades and the INC acts tonically to hold eccentric vertical gaze. The INC and riMLF connect with their contralateral nuclei via the posterior commissure, where fibers are subject to damage. Projections for upgaze cross through the commissure before descending to innervate the third nerve nucleus, while those for downgaze may travel directly to the third nerve, thus accounting for the frequency of selective upgaze palsies (see text). The MLF carries signals from the vestibular nuclei, mainly ipsilaterally, to stabilize the eyes in the vertical plane (VOR) and maintain tonic vertical position.
Figure 13-3. Topographic arrangement of oculomotor fascicular fibers in the mesencephalon. CCN, central caudal nucleus; IO, inferior oblique; IR, inferior rectus; LP, levator palpebrae; MR, medial rectus; P, pupil; SR, superior rectus. (From Ksiazek SM, Slamovits TL, Rosen CE, et al: Fascicular arrangement in partial oculomotor paresis. Am J Ophthalmol 118: 97, 1994. With permission.)
Cerebral aqueductCorticospinal tractCorticospinal tractSubstantia nigraOculomotor nucleus (III N.)Red nucleusIII N.fibersVI N.fibersVII N.fibersFacial nucleusVII N.4th ventricleAbducens nucleus(VI N.)Inferior cerebellarpeduncleSuperior vestibularnucleusIII NerveNodulusAB
Figure 13-4. A. Midbrain in horizontal section, indicating the effects of lesions at different points along the intramedullary course of the third-nerve fibers. A lesion at the level of oculomotor nucleus results in homolateral third-nerve paralysis and homolateral anesthesia of the cornea. A lesion at the level of red nucleus results in homolateral third-nerve paralysis and contralateral ataxic tremor (Benedikt and Claude syndromes). A lesion near the point of exit of third-nerve fibers results in homolateral third-nerve paralysis and crossed corticospinal tract signs (Weber syndrome; see Table 44-2). B. Brainstem at the level of the sixth-nerve nuclei, indicating effects of lesions at different loci. A lesion at the level of the nucleus results in homolateral sixthand seventh-nerve paralyses with varying degrees of nystagmus and weakness of conjugate gaze to the homolateral side. A lesion at the level of corticospinal tract results in homolateral sixth-nerve paralysis and crossed hemiplegia (Millard-Gubler syndrome).