Chapter 20 Motor Units of Extraocular Muscles: Recent Findings

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The eye movement motor control system is characterized by exquisite precision and coordinated synchrony as the eyes acquire, pursue, and fixate visual targets. It is logical to think that the brain stem motoneurons and the muscle fibers that they innervate exhibit comparable precision and predictability. Consequently, the manner in which the final common path extraocular motor units are activated and their contractile forces assemble to produce these quick, stable, and repetitive movements is a significant research and clinical problem. This chapter discusses some of the progress made toward understanding the mammalian extraocular motor units. The principal goals of the studies over the past ten years have been to (1) compare and contrast extraocular motor unit types to spinal cord innervated units, (2) examine how single motor units are distributed within a particular muscle, (3) describe the motor unit forces when driven by stimulation paradigms that attempt to mimic motoneuron firing patterns observed during the eye movements in alert, behaving animals, and (4) determine how single motor unit forces summate as they contract in unison. Most of the findings over this period have been collected from deeply anesthetized, in vivo preparations involving the lateral rectus muscle in adult cats and monkeys.

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