Origin of sympathetic and sensory innervation of the knee joint. A retrograde axonal tracing study in the rat.

The origin of sympathetic and sensory innervation of the knee joint was investigated by the intra-axonal transport method. After injection of horseradish peroxidase (HRP) or horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP) into the knee joint unilaterally in adult rats, labelled neurons were found ipsilaterally both in sympathetic and dorsal root ganglia at the lower thoracic and lumbosacral levels. A maximum of 1500 and 1000 peroxidase-positive cells were observed in the sympathetic and dorsal root ganglia, respectively; in the former they were mainly located at the levels of L2-L4, and in the latter mainly at L3-L5. The functional implications of the above observations are discussed in relation to pain mechanisms in joints and to the earlier hypothesis that a nervous component might be involved in the pathogenesis of chronic joint inflammation.