The visual motor (efferent) system can be functionally divided into supranuclear/internuclear, nuclear/infranuclear, and orbit/extraocular muscle components. Supranuclear pathways originate in the cerebral cortex and are modulated by the cerebellum and vestibular apparatus to coordinate movement of both eyes with respect to the position and movement of the head and body. These areas and the functions they perform are supranuclear because they are higher up in the chain of command than the cranial nerve motor nuclei (for cranial nerves III, IV, and VI) that drive the extraocular muscles. Internuclear pathways provide direct connections between specific motor nuclei to coordinate binocular movement. Infranuclear pathways begin with the fascicles of cranial nerves III, IV, and VI and include the course of these cranial nerves to the extraocular muscles, delivering coordinated impulses to the muscles that move the eyes, eyelids, and pupil.
Disorders anywhere in the visual motor system can result in ocular misalignment (strabismus). The pattern of misalignment frequently suggests the site of the lesion, similar to the manner in which visual field defects identify the neuroanatomic location of afferent visual pathway lesions. Discussion of the ocular motor system begins with the techniques and tools used to examine the visual motor system (Chapter 7). The next three chapters work through the motor system in reverse order: extraocular muscles in the orbit and the neuromuscular junction (Chapter 8), cranial nerves III, IV, and VI (Chapter 9), and supranuclear control systems (Chapter 10). The pupil and facial nerve are also part of the efferent visual system and are discussed in Chapters 11 and 12, respectively. In each chapter, disorders and their signs and symptoms are discussed in the context of neuroanatomy.