Unit identification, sampling bias and technical issues in microneurographic recordings from muscle spindle afferents

Recordings have been made directly from human muscle spindle afferents for some 3 decades, and these have allowed assessment of fusimotor function in human subjects during normal motor acts and in patients with motor disturbances. However, inferences about fusimotor function are indirect, valid only if identification of the axon as of muscle spindle origin is secure and all extrafusal influences on the spindle have been controlled. As is discussed, the identification of spindle afferents and their classification into Group Ia and Group II are more problematic than in the cat, but these problems are probably relatively minor given the insight into normal function that can come from appropriately designed experiments in awake cooperative human subjects.