Muscle spindles and the regulation of movement.

This paper presents the complexities of muscle spindle structure and function. Experimental evidence indicates that the contribution of muscle spindles to motor control depends upon the cooperation of two distinct systems, one dynamic and one static. The traditional idea that load compensation depends on the segmental stretch reflex action of muscle spindle afferents on alpha motoneurons is questioned and evidence is provided to support the direct dependence of load compensation upon supraspinal centers. Information from muscle spindle afferents allows the nervous system 1) to compare the actual muscle length and velocity of contraction to the desired length and velocity for a particular movement and 2) to compensate for any discrepancies that develop. Segmental pathways from muscle spindle afferents appear to function with Golgi tendon organs to regulate the stiffness or compliance of a muscle.