Reflexes Evoked by Group II Afferent Fibers from Muscle Spindles

The purpose of this paper is to review research on the reflex action of group II afferent fibers originating in the muscle spindle. Although the reflex actions of the Ia afferent fibers from muscle spindles are well defined, the reflex action of the group II afferent fibers is not clearly understood. Experiments on animals whose spinal cord had been transected gave rise to the idea that group II afferent fibers facilitate flexor muscles and inhibit extensor muscles regardless of the origin of the fibers. This concept has been accepted by neurophysiologists and has even been incorporated into therapeutic exercise procedures by physical therapists. However, accumulating evidence is presented that challenges this classical concept. Physical therapists, when trying to incorporate the reflexes evoked by the group II afferent fibers into treatment, must become better aware of the complex reflex actions of which these fibers are capable.