FUSIMOTOR DISCHARGE PATTERNS DURING RHYTHMIC MOVEMENTS

Sensory information from muscle is a major factor in the control of po sture and movement. The central nervous system can greatly vary this p roprioceptive feedback via the fusimotor (gamma) system that innervate s the muscle spindle, a length receptor. Despite 50 years of intensive research, the role of the fusimotor system still remains controversia l. One of the major reasons for this state of affairs is, because of t echnical difficulties, the complete lack of direct recordings from cla ssified gamma-motoneurones (that is, static or dynamic) in intact anim als. However, such recordings have been achieved in reduced feline pre parations during three types of rhythmic movement: respiration, jaw mo vements and locomotion. The recordings indicate that the patterns of d ischarge of static and dynamic fusimotor neurones can vary in differen t types of movement, or in different muscles during the same behaviour . Notwithstanding such variation, a generalization has emerged in whic h it is proposed that, for rhythmic movements, extrafusal muscle contr action is accompanied by coactivity in static and dynamic gamma-effere nts. Such coactivity serves to optimize spindle afferent feedback for reflex contributions to muscle contraction.