The mechanism of lateral side of the shoulder pain due to the shoulder pathology was not clarified. The purpose of this study was to evaluate the dorsal root ganglion (DRG) neurons innervating the glenohumeral joint (GHJ), subacromial bursa (SAB) and lateral side of the shoulder. In this manner, dichotomizing axons projecting to the GHJ and lateral side of the shoulder (LS), SAB and LS were also evaluated. Two kinds of neurotracers, dioctadecyl-tetramethyl-indocarbocyanine-perchlorate (DiI) and fluoro-gold (FG), were used to detect the GHJ, SAB and LS separately. In the first group, FG was injected into the GHJ and DiI were injected into LS. In the second group, FG was injected into SAB and DiI were injected into LS. Seven days later, DRGs were harvested between C1 and T1. The number of FG labeled-neurons, DiI-labeled neurons and double-labeled neurons were counted respectively. DiI-labeled neurons which innervate LS were distributed from C2 to C7. FG-labeled neurons which innervate GHJ were distributed from C2 to C7 and FG-labeled neurons which innervate SAB were distributed from C2 to C8. DRG neurons which double-labeled with FG (GHJ) and DiI (LS) were 9.4% of FG-labeled neurons and DRG neurons which double-labeled with FG (SAB) and DiI (LS) were 17.1% of FG-labeled neurons. Dichotomizing axons projecting to the GHJ and LS, and SAB and LS induced the referred lateral side of the shoulder pain due to the shoulder pathology. It is also indicated that lateral side of the shoulder pain is stronger when the pathology involved SAB.
