Anatomy of the oculomotor system

The sensory and motor control of eye muscles are considered in this chapter. Eye muscles differ from skeletal muscles in several ways. One is the absence of muscle spindles and Golgi tendon organs in the eye muscles of some species, and their poor development in others. Second, eye muscles have an inner 'global layer', and the outer 'orbital layer', each containing different types of muscle fiber. Third, eye muscles contain not only twitch muscle fibers with a single endplate zone (SIFs), but also nontwitch muscle fibers with multiple endplate zones (MIFs), which are otherwise absent from mammalian muscles. There are cuffs of nerve terminals, called palisade endings, around the myotendinous junctions of global layer MIFs. Palisade endings are unique to eye muscles, and have been found in all mammalian species investigated up to now. The function of palisade endings is uncertain, but it is possible that they are 'sensory receptors'. Motoneurons innervating the eye muscles lie in the oculomotor, trochlear and abducens motor nuclei, and are contacted by several relatively independent premotor networks, which generate different types of eye movements such as saccades, vestibulo-ocular reflexes, optokinetic responses, smooth pursuit convergence or gaze-holding. In each motor nucleus, the motoneurons can be divided into two distinct sets: the first set innervating SIF muscle fibers and receiving inputs from all oculomotor premotor networks, and the second set innervating the MIFs and receiving premotor afferents from the gaze holding, convergence or smooth pursuit premotor networks, but not from the saccadic and vestibulo-oculomotor networks. We suggest that the SIF motoneurons and muscles are more suited to driving eye movements, and the MIF motoneurons and muscles to setting the tonic tension in eye muscles. Furthermore the 'palisade ending-MIF unit' may be part of a sensory feedback system in eye muscles, which should be considered in association with the causes and treatment of strabismus.