Morphology and relationships of the biceps brachii and brachialis with the musculocutaneous nerve

Introduction Major anatomical textbooks generally state that the biceps brachii muscle (BB) is composed of long and short heads, whereas the brachialis muscle (BR) consists of a single head. However, the numbers of heads comprising the BB and the BR are very variable. The purpose of this study was to investigate how the branching patterns of the musculocutaneous nerve (MC) influence the number of heads of the BB and the BR. Materials and methods Morphological examinations of the BB and MC were conducted using cadavers of 22 Japanese individuals, and morphological examinations of the BR and the MC were conducted in 9 of those 22 individuals. Results A three-headed BB was observed in 7 of the 22 specimens (31.8%). Most of these specimens showed a Type III branch pattern (after penetrating the long head or the short head, the MC innervated the supernumerary head or communicated with the main root again). The number of BR heads was categorized into three types: Type A, two heads (superficial and deep heads, 22.2%); Type B, three or four heads (two or three superficial heads and one deep head, 44.4%); and Type C, multiple heads (33.3%). Among these categories, branches of the MC in Type A specimens were most simple. Conclusion A supernumerary head of the BB seemed to be present if the MC penetrates it. The BR basically consists of superficial and deep heads, and the number of superficial heads is affected by branches of the MC.