A sensory role for the cruciate ligaments : regulation of joint stability via reflexes onto the γ-muscle-spindle system

Abstract: Reflex effects evoked by graded electrical stimulation of the posterior articular nerves (PAN) of the ipsi- and contralateral knee joints were investigated using both micro-electrode recordings from 7 - motoneurones and recordings from single muscle muscle spindle afferents. Spindle afferent responses were also recorded using natural stimulation of different types of receptors, to elucidate if the articular reflexes onto the y -motoneurones were potent enough to significantly alter the muscle spindle afferent activity. Stretches of the ipsilateral posterior (PCL) and anterior (ACL) cruciate ligaments, pressure on the ipsi- and contralateral knee and ankle joint capsules, and passive flexion/extension movements of the joints in the contralateral hind limb were performed. The occurrance of different sensory endings in the ACL and PCL was examined using gold chloride staining for neuronal elements. All experiments were performed on chloralose anaesthetized cats.More than 90% of the static and dynamic y -motoneurones were responsive to electrical stimulation of the PAN. Most 7-cells responded to low intensity electrical stimulation. Excitatoiy reflex effects predominated on both static and dynamic posterior biceps-semitendinosus (PBSt) 7 -cells, while excitatory and inhibitory effects occurred with an about equal frequency on triceps-plantaris (GS) 7-cells. The fastest segmental route for excitatory PAN effects on hind limb 7-motoneurones seems to be di- or trisynaptic, while the path for inhibitory effects seems to be at least one synaps longer.Physiological stimulations of ipsi- and contralateral joint capsules and of ipsilateral cruciate ligaments were all found to evoke frequent and potent changes in spindle afferent responses from the GS and PBSt muscles. It was shown that these effects were due to reflexes onto dynamic and static fusimotor neurones caused by physiological activation of articular sensory endings. Both ipsi- and contralateral joint receptor stimulation evoked excitatory as well as inhibitory fusimotor effects. The highest responsiveness was found during stimula­tion of the cruciate ligaments, i.e. 58% for GS and 47% for PBSt primary spindle afferents to PCL stimula­tion, and 73% for GS and 55% for PBSt primary spindle afferents to ACL stimulation. Significant altera­tions in spindle afferent activity was encountered at very low traction forces applied to the cruciate ligaments (5-10 N). The low thresholds, the tonic character of the stimuli, and the fact that different types of sensory endings were demonstrated in the cruciate ligaments (i.e. Ruffini endings, Pacinian corpuscles, Golgi ten­don organ like endings and free nerve endings), indicate that the fusimotor effects observed were caused by activation of slowly adapting mechanoreceptors, most likely Ruffini endings and/or Golgi tendon organ like endings.The potent reflex effects on the muscle spindle afferents elicited by increased tension in the cruciate ligaments indicate that these ligaments may play a more important sensory role that hitherto believed, and it is suggested that they may be important in the regulation of the stiffness of muscles around the knee joint, and thereby for the joint stability. The possible clinical relevance and the mechanisms by which joint receptor afferents, via adjustment of the muscle stiffness, may control joint stability are discussed.