Efferent Visual System (Ocular Motor Pathways)
The anatomical, physiologic, and pathologic understanding of the ocular motor system has increased dramatically over the past decades due to results derived from primate model experiments, human electrophysiology testing, functional magnetic resonance imaging (fMRI) studies, and the clinical-pathologic-radiologic correlations of disorders in patients with documented eye movement abnormalities.
The ultimate purpose of the ocular motor system is to establish clear, stable, and binocular vision. Two basic human eye movements help perform these tasks:
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gaze shift
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gaze stabilization
These 2 movements can be further divided into 6 functional systems or classes (see the section Supranuclear Ocular Motor Systems: Function, Anatomy, Clinical Testing, and Disorders of Eye Movements in Chapter 8):
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ocular fixation system
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vestibular-ocular system
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optokinetic system
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saccadic system
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smooth-pursuit system
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vergence system
Each system appears to be under the control of—and modulated by—different regions of the brain (cortex) and brainstem, with considerable anatomical and functional overlap. This section provides an overview of the ocular motor system, with a detailed discussion of particularly clinically relevant structures. Interested readers can find a comprehensive description of the ocular motor system in the textbook The Neurology of Eye Movements by Leigh and Zee. To facilitate learning, the discussion in this section follows a top-to-bottom approach:
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cortical control of eye movements, including frontal eye fields (FEFs), supplementary eye fields (SEFs), and posterior parietal cortex (PPC)
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subcortical centers, including basal ganglia (BG), thalamus, and SC
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brainstem or premotor coordination of conjugate eye movements, including coordination by the vestibular-ocular system and cerebellum
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ocular motor CNs (CNs III, IV, and VI)
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extraocular muscles (EOMs)
Leigh RJ, Zee DS. The Neurology of Eye Movements. 5th ed. Contemporary Neurology Series. New York: Oxford University Press; 2015.
Cortical Input
The efferent visual system spans a large segment of the central nervous system; many areas of the brain generate eye movements (Fig 1-23). Saccadic and smooth-pursuit eye movements were once thought to be derived from distinct supranuclear pathways; however, it now appears that these systems overlap considerably. In addition, there are 2 major visual pathways: one for the movement of images (magnocellular: M cells) and the other for discrimination of images (parvocellular: P cells).
Saccadic system
The cortical, or supranuclear, input for generating saccadic eye movements is divided into 2 parallel and interconnected descending pathways: (1) visually reflexive (parietal lobe) movements and (2) memory-guided and volitional (frontal lobe) movements. Visually guided movements require afferent system information either from the primary visual system and cortex or from the accessory afferent system. The visually guided (to seen or remembered targets) and volitional saccadic supranuclear input comes largely from the FEFs, or Brodmann area 8. Cortical cells discharge prior to all voluntary and visually guided contralateral saccades. SEFs—located on the dorsomedial surface of the superior frontal gyrus—receive input from the FEFs and are responsible for programming saccades, particularly as part of learned behavior. In general, these cortical fibers project to the following structures in an organized framework:
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subcortical structures: BG, thalamus, and SC
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brainstem neural network or premotor neurons: several types of pontine neurons
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brainstem saccade generators: PPRF and rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF)
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motoneurons of the ocular motor CNs
Few cortical fibers project directly to the PPRF and riMLF. Cortical projections to the SC also arise from the PPC for visually guided reflexive saccades.
Smooth-pursuit system
The smooth-pursuit system for visual targets originates in area V5—the human homologue of the medial temporal (MT) visual area in the rhesus monkey—where it receives input from the primary visual system, both from the cortex (striate and extrastriate areas) and likely from magnocellular input directly from the LGN (Fig 1-24). The medial superior temporal (MST) area is also involved in generating pursuit signals in response to moving stimuli. The area appears to be supplied with information about head movement as well as eye movement commands (efference copy) and thus generates pursuit movements to follow a target while the head is moving. Target recognition and selection probably receive additional input through the reciprocal connections to the PPC. Information from the MT and MST projects via the posterior portion of the internal capsule to the dorsolateral pontine nuclei (DLPN) and lateral pontine nuclei. From these pontine nuclei, projections are sent to the cerebellum (paraflocculus and dorsal vermis), with outflow signals to the vestibular nuclei. The smooth-pursuit system is a double decussating pathway (the decussation occurs in both the pons and cerebellum); therefore, it can be simplistically considered an ipsilateral system.
For information on clinical disorders of the pursuit function, see Chapter 8 in this volume.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.