Morphological significance of the medial brachial cutaneous nerve: An anatomical study of the brachial plexus in primates.

The medial brachial cutaneous nerve (MBC) originates from the medial cord of the brachial plexus and innervates the skin sensory in the medial posterior surface of the upper arm. Considering previous reports of the primate brachial plexus, the MBC appeared to be the sole branch in the brachial plexus that only some primates possess. However, the detailed descriptions and records regarding the morphology of the MBC and related nerves, their origins and distributions (dermatomes) in particular, were frequently lacked in the previous reports, and it remains unclear why the difference in the MBC appearance exists among primates. In this study, the brachial plexus and its branches were first re-evaluated and certainly identified in several primates, humans, chimpanzee, macaque monkey, lutung, tamarin, squirrel monkey, and spider monkey. The MBC was identified in humans, chimpanzee, spider monkey, and squirrel monkey. In the other species, the intercostobrachial nerve (ICB) originating from some of 1st to 3rd intercostal nerves developed and distributed instead of the MBC. According to the kinesiological and behavioral studies, the former species possessing MBC show high shoulder joint mobility associated with their locomotive patterns. We speculate that the MBC corresponds to transformed ICB; specifically, where it originates presumably transfers from the 1st and/or 2nd intercostal nerves to the brachial plexus, which allows it to reach the upper arm by coursing the shortest distance even if the forelimb is raised high. Therefore, MBC may embody phylogenetic morphogenesis of the nerve associated with the locomotive evolution and adaptation in primate forelimb.