Activity in receptor afferents from the anterior cruciate ligament evokes reflex effects on fusimotor neurones.

Responses from 2-4 muscle spindle afferents from triceps surae and/or posterior biceps and semitendinosus muscles were simultaneously recorded in cats anaesthetized with alpha-chloralose. It was demonstrated that stretch of the anterior cruciate ligament (ACL) of the ipsilateral knee causes changes in dynamic and/or static sensitivity of primary and secondary spindle afferents to sinusoidal stretching. The changes were due to reflex actions of stretch/tension-sensitive receptor afferents from ACL on dynamic and static fusimotor neurones. The findings support the hypothesis that ACL-afferents contribute to the regulation of muscular stiffness around the knee, and thereby also to dynamic knee joint stability.