Sympathetic and sensory neural elements in the tendon of the long head of the biceps

Background: Although the tendon of the long head of the biceps is a well-known source of shoulder pain, the pathophysiological basis of this pain has yet to be explained. The aim of this study was to detect and characterize any nervous element of the tendon and to determine a possible explanation for pain originating from this structure. Methods: The nature of the neuronal innervation of the tendon of the long head of the biceps was studied immunohistochemically, in four tendons from different human cadavers, with use of neurofilament antibody 21-13, neurofilament-like antibody 3A10, calcitonin gene-related peptide, substance P, and tyrosine hydroxylase. Results: A large neuronal network, asymmetrically distributed along the length of the tendon with a higher degree of innervation at the tendon origin, was identified by the neurofilament and neurofilament-like antibodies 2H3 and 3A10. This innervation was found to be positive for calcitonin gene-related peptide and substance P, suggesting the presence of thinly myelinated or unmyelinated sensory neurons. It was also positive for tyrosine hydroxylase, suggesting a postganglionic sympathetic origin. Conclusions and Clinical Relevance: These findings demonstrate that the tendon of the long head of the biceps is innervated by a network of sensory sympathetic fibers, which may play a role in the pathogenesis of shoulder pain.