Fusimotor discharge patterns during rhythmic movements.

Sensory information from muscle is a major factor in the control of posture and movement. The central nervous system can greatly vary this proprioceptive feedback via the fusimotor (gamma) system that innervates the muscle spindle, a length receptor. Despite 50 years of intensive research, the role of the fusimotor system still remains controversial. One of the major reasons for this state of affairs is, because of technical difficulties, the complete lack of direct recordings from classified gamma-motoneurones (that is, static or dynamic) in intact animals. However, such recordings have been achieved in reduced feline preparations during three types of rhythmic movement: respiration, jaw movements and locomotion. The recordings indicate that the patterns of discharge of static and dynamic fusimotor neurones can vary in different types of movement, or in different muscles during the same behaviour. Notwithstanding such variation, a generalization has emerged in which it is proposed that, for rhythmic movements, extrafusal muscle contraction is accompanied by coactivity in static and dynamic gamma-efferents. Such coactivity serves to optimize spindle afferent feedback for reflex contributions to muscle contraction.