Conventionally, the term ocular motor nuclei refers to the nuclei of the third, fourth, and sixth cranial nerves; the term oculomotor nucleus refers to the third nerve nucleus alone. Ultimately, all pathways that mediate saccadic, pursuit, vestibular, and optokinetic movements in the horizontal plane converge on the pontine tegmental center for horizontal gaze, the PPRF. The PPRF projects to the sixth nerve nucleus to command horizontal eye movement. However, it is understood from animal experiments that supranuclear neural signals that encode smooth pursuit, and vestibular and optokinetic movements may bypass the PPRF and project independently to the abducens nuclei (Hanson et al). Also required for horizontal versional movements are the nuclei prepositus hypoglossi and their commissure, the medial vestibular nuclei, and pathways in the pontine and tegmentum of the brainstem that interconnect the ocular motor nuclei (Fig. 13-1).
The fiber bundle connecting the ipsilateral sixth and third nuclei, and connecting both these nuclei with the vestibular nuclei, is the medial longitudinal fasciculus (MLF), lying in the medial tegmentum of the brainstem. The fibers of the MLF emanating from the sixth nerve nucleus cross in the pons and ascend to the contralateral medial rectus subnucleus of the third nerve. In this way, the abduction of one eye is yoked to adduction of the opposite one to produce conjugate horizontal gaze (Fig. 13-1).
The abducens nucleus contains two groups of neurons, each with distinctive morphologic and physiologic properties: (1) the intranuclear abducens motor neurons, which innervate the ipsilateral lateral rectus muscle, and (2) abducens internuclear neurons, which project via the contralateral MLF to the medial rectus neurons of the opposite oculomotor nucleus. Conjugate lateral gaze is accomplished by the simultaneous activation of the ipsilateral lateral rectus, and the contralateral medial rectus, again, the latter through fibers that run in the medial portion of the MLF. Interruption of the MLF results in a discrete impairment or loss of adduction of the eye ipsilateral to the lesion, a sign referred to as internuclear ophthalmoplegia, the details of which are discussed further on (Fig. 13-1).
Two other ascending pathways between the pontine centers and the mesencephalic reticular formation have been traced: one traverses the central tegmental tract and terminates in the pretectum, in the nucleus of the posterior commissure; the other is a bundle separate from the MLF that passes around the nuclei of Cajal and Darkschewitsch to the riMLF. These nuclei are involved more in vertical gaze and are described in the following text. In addition, each vestibular nucleus projects onto the abducens nucleus and the MLF of the opposite side to generate the slow phase of the VOR.
Control of voluntary eye movements depends upon a hierarchy of cell stations and parallel pathways that do not project directly to ocular motor nuclei but to adjacent premotor or burst neurons, which discharge at high frequencies immediately preceding a saccade (Leigh and Zee). The premotor or burst neurons for horizontal saccades lie within the PPRF and those for vertical saccades in the riMLF (see below). Yet a third class of neurons (omnipause cells), lying in the midline of the pons, is involved in the inhibition of unwanted saccadic discharges. Nonetheless, in clinical work, the circuit that comprises in sequence (1) frontal lobe eye fields, (2) contralateral pontine PPRF, (3) abducens nucleus, (4) MLF, and (5) opposite oculomotor nucleus makes understandable a number of highly characteristic defects of horizontal ocular motion, as detailed in the remainder of the chapter.