Neuroreceptors in the tissues of the knee joint.

Four types of receptors have been described in the articular tissues of the knee joint in humans and animals. The first three types are encapsulated; the fourth is unencapsulated: type I, Ruffini endings; type II, Pacinian corpuscles; type III, Golgi tendon organs; and type IV, free nerve endings. Ruffini endings, Pacinian corpuscles, and free nerve endings are most prevalent in the fibrous joint capsule; Golgi tendon organs are most common in the collateral and cruciate ligaments and the menisci. In the anterior and posterior cruciate ligaments (ACL, PCL), receptors are concentrated at the tibial and femoral attachments of the ligaments. In the menisci, neural elements penetrate the horns and the outer and middle thirds of the body. Ruffini mechanoreceptors are believed to contribute mainly to maintenance of muscle tone, Pacinian corpuscles and Golgi tendon organs are stimulated during movement, and free nerve endings are nociceptors. Thus, receptors of the knee joint are able to produce a discriminating afferent inflow to the central nervous system (CNS), thereby contributing to the protection and function of the joint through the musculature.