Innervation Of Joint Muscle And Bone

Joints are supplied by branches descending from main nerve trunks or their muscular, cutaneous, and periosteal branches. A typical joint nerve contains thick myelinated Ap- (group II), thinly myelinated A8- (group III), and a high proportion (^80 percent) of unmyelinated C- (group IV) fibers. The latter are either sensory afferents or sympathetic efferents (each ^50 percent).12 Ap fibers terminate as corpuscular endings of the Ruffini-, Golgi-, and Pacini-type in fibrous capsule, articular ligaments, menisci, and adjacent periosteum.1 Articular A8- and C-fibers terminate as noncorpuscular or free nerve endings in the fibrous capsule, adipose tissue, ligaments, menisci, and the periosteum.16 Using staining for nerve fibers and neuropeptides, endings were also identified in the synovial layer. The cartilage is not innervated.12

Muscle nerves contain axons from motoneurons, sensory neurons, and postganglionic sympathetic neurons. For example, in the nerve of cat gastrocnemius-soleus (GS) muscle, about one-third of the axons are myelinated (^60 percent of these are from motoneurons and ^40 percent are sensory) and two-thirds of the fibers are unmyelinated. In the latter group, ^50 percent of the units are sensory and ^50 percent are sympathetic efferent. Thick myelinated afferents terminate as organized endings (muscle spindles, tendon organs), whereas A8- and C-fibers terminate as free nerve endings. Most of these endings are located in the wall of arterioles in the muscle belly and in the surrounding connective tissue.7

A large proportion of articular and muscular sensory neurons are peptidergic. The major neuropeptides in joint and muscle nerves are substance P, calcitonin gene-related peptide (CGRP), and somatostatin. Neurokinin

A, galanin, enkephalins, and neuropeptide Y have also been localized in joint afferents. Neuropeptides influence the inflammatory process in the periphery and modify spinal processing of joint and muscle input. They may also act on the primary afferent neurons themselves (see below under Molecular mechanisms of peripheral sensi-tization). However, these neuropeptides are not specific for deep afferents.7,12

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