Anatomy and histology of joint innervation

Joint diseases are mostly accompanied with hyperalgesia and/or pain in the joint region. As about 60% of the knee joint innervation may be classified as nociceptive, the structure of joint afferents became a matter of interest. The function of a sensory nerve fibre may be defined by its location, its morphology and its biochemical composition. This review is therefore focused on the composition of articular nerves, the morphology of sensory endings, their content of mitochondria and neuropeptides, and the plasticity of neuropeptide synthesis during joint inflammation. As the knee joint of the cat belongs to the best examined articular tissue concerning the sensory function of its afferent innervation, most of the morphological data have also been determined in this model. The histological studies revealed a dense afferent innervation of all parts of joints. Most of the afferents terminate in non-corpuscular endings. Although the ultrastructure of these endings has been examined in great detail, up to now no morphological and biochemical details can be used to discriminate between mechanosensitive and nociceptive afferents. Only the differences in the content of mitochondria in single nerve fibres may be related to different sensory functions. The content of different neuropeptides in joint afferents highlights an additional efferent function of a proportion of joint afferents. The dense innervation with peptidergic joint afferents indicates an important, but up to now mainly unknown role of neuropeptides in a normal joint. Pathological conditions like joint inflammations lead to an enhanced release of neuropeptides into the joint tissue and a specific upregulation of neuropeptide biosynthesis in articular afferents.