Histologic Analysis of Sensory and Motor Axons in Branches of the Human Brachial Plexus

Background: The topographic distribution through histologic analysis of motor and sensory axons within peripheral nerves at the brachial plexus level is not clearly defined, as there has previously been little need to appreciate this microanatomy. A desire to better understand the topography of fascicle groups developed with the introduction of targeted muscle reinnervation. Methods: Fourteen bilateral brachial plexus specimens from seven fresh human cadavers were harvested at the time of organ donation, and immunofluorescent staining of motor and sensory nerves with choline acetyltransferase and Neurofilament 200 was performed to determine whether a consistent somatotopic orientation exists at the brachial plexus level. Results: There was significant variability in the number of fascicles at the level of the brachial plexus. Qualitative analysis of choline acetyltransferase staining demonstrated that although motor axons tended to be grouped in clusters, there were high degrees of variability in somatotopic orientation across specimens. The radial nerve demonstrated the highest number of total myelinated axons, whereas the median nerve exhibited the greatest number of motor axons. The ulnar nerve contained only 13 percent motor axons, which was significantly lower than the median, radial, and musculocutaneous nerves. Conclusions: There was no consistent somatotopic organization of motor and sensory axons of the mixed major nerves of the arm just distal to the brachial plexus, but clustering of motor axons may facilitate the splitting of nerves into primarily “motor” and “sensory” fascicles.