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The continuing debate about CNS control of proprioception

Arthur Prochazka and Manuel Hulliger *

More axons are devoted to transmitting signals to and from muscle spindles than to activating the muscles themselves. This implies not only an important role for the sensory signals from muscle spindles, but also the need to adjust these signals at their source via fusimotor action. Most fusimotor axons are -motoneurons exclusively innervating spindles. A minority are -fibres (-motoneurons that innervate spindle intrafusal muscle fibres and skeletal muscle fibres). The way the CNS controls -motoneurons in real life has been debated ever since the first microneurographic recordings in humans (Hagbarth & Vallbo, 1968). These showed that spindle afferent firing was correlated to the electromyogram (EMG) activity of the parent muscles, suggesting - coactivation. Unfortunately, it has never been possible to record directly from the small -axons, so their activity has always been inferred from the behaviour of spindle afferents. Recordings from spindle afferents in awake monkeys and cats indicated not only the independence of control, but also task- and context-dependent activation, also known as 'fusimotor set' (Prochazka et al. 1985).