The ultrastructure of sensory nerve endings in the human knee joint capsule was studied. Three types of nerve endings were found: free nerve endings (FNE), Ruffini corpuscles and Pacini corpuscles. In the joint capsule, FNE are located below the synovial layer and within the fibrous layer near blood vessels. These nerve terminals derive from myelinated A delta-fibres or from unmyelinated C-fibres. Their structure is almost identical to FNE in human hairy and non-hairy skin. Ruffini corpuscles are present within the fibrous layer and the ligaments of the capsule in three variations: small Ruffini corpuscles without a capsule, small with a connective tissue capsule, and large Ruffini corpuscles with an incomplete perineural capsule. Their afferent axons are myelinated and measure 3-5 micron in diameter. Inside the corpuscle, nerve terminals are anchored in the connective tissue belonging to the fibrous layer or to the ligaments respectively. The presence of an incomplete perineural capsule depends on the structure of the surrounding connective tissue. In ligaments with collagenous fibrils oriented in a parallel fashion, the perineural capsule is well-developed and the Ruffini corpuscle resembles a Golgi tendon organ; in areas where the fibrils show no predominant orientation, Ruffini corpuscles lack a capsule. Small Pacini corpuscles are situated within the fibrous layer near the capsular insertion at the meniscus articularis or at the periost. They consist of one or several inner cores and a perineural capsule of 1-2 layers. Larger Pacini corpuscles with one or several inner cores and a perineural capsule consisting of 20-30 layers are found on the outer surface of the fibrous layer. The ultrastructure of these nerve endings is compared with the ultrastructure of articular receptors of various animals and with the ultrastructure of sensory nerve endings in the skin of several mammalian species including man.
