Roles of Proprioceptive Input in the Programming of Arm Trajectories

It has been known for more than 100 years that loss or impairment of sensation in our limbs may produce severe disorders of movement and that sensory input plays a critical role in controlling movement. Indeed, the skin, muscles, and ioints of our limbs are richly innervated by a variety of sensory receptors that convey proprioceptive information to all levels of the nervous system. What role this input plays in movement control has been a question of recurring interest but remains incompletely understood. In 1895, Mott and Sherring-ton demonstrated that surgical deafferentation of a monkeys limb produces evere disorders of movement and an unwillingness to use the limb in purposeful action. They therefore concluded that movement initia-tion requires the support of afferent information and proposed that coordinated movement results from the concatenation of reflex responses. Subsequently, how-ever, it was established that deafferentation does not abolish the capacity to make purposeful movements and that motor performance may be substantially pre-served when deafferentation is bilateral or when specific training procedures are used (Munk 1909; Knapp et al. 1963: Taub and Berman 1963; Polit and Bizzi 1979). This indicated that sensory input from the limbs is required neither to initiate movement nor to perform complex motor acts, as originally believed by Sherrington. Rather, the central nervous ystem makes use of motor programs to direct movements (Keele 1968). Nevertheless, the movements of deafferented monkeys have been repeatedly described as clumsy, inaccurate, and poorly coordinated (Munk 1909; Bos-sore 1974). Thus, although not necessary for the pro-duction of movement, sensory information, particularly from muscle receptors, clearly plays an important role in its control. To examine the role of proprioceptive input in the control of limb movement, several investigators