[Re-evaluation of the human brachialis muscle by fiber analysis of supply nerves].

The nerve supply to the human brachialis muscle has been investigated by many researchers with gross anatomical and/or microdissectional methods. It has been made clear that the brachialis muscle is innervated by two groups of nerve branches, that is, branches from the musculocutaneous nerve innervating most of the muscle and those from the radial nerve innervating the small inferolateral part. However, the constancy of the radial nerve supply is controversial. Furthermore, some researchers have reported a median nerve supply. On the other hand, the morphological meaning of the double nerve supply by the musculocutaneous nerve from the anterior division of the brachial plexus and by the radial nerve from the posterior division has been generally explained to be a result of the fusion of two muscular primordia, one from the ventral (flexor) premuscular mass and the other from the dorsal (extensor) premuscular mass. The inferolateral part of the brachialis muscle has been considered as a detached portion of the brachioradialis muscle. In contrast with this view, there also has been another view that the brachialis muscle has only one origin from the ventral premuscular mass. The branch from the radial nerve derives from the anterior division of the brachial plexus and uses the radial nerve only as a route to the brachialis muscle by unknown mechanisms. However, the latter view has shown no reliable evidence. Therefore, the purposes of this study are 1) to obtain reliable evidence concerning the anterior derivation of nerve fibers of the branch from the radial nerve (by teasing fibers), 2) to make clear the topographical relationship between the nerve fibers from the musculocutaneous nerve and those from the radial nerve in the brachial plexus (by teasing fibers) and in the brachialis muscle (by an investigation of intramuscular nerve supply) and 3) the topographical relationship between nerve fibers to the brachialis muscle and those to the brachioradialis muscle (by teasing fibers), and 4) to investigate the constancy or the incidence of the radial and median nerve supply. Materials used were sixteen human arms of ten cadavers from the 1996 student course of dissection at Iwate Medical University School of Medicine. Two groups of nerve branches innervating the brachialis muscle, one from the musculocutaneous and the other from the radial nerves, were dissected by a gross anatomical method. The best dissected three specimens were used for the fiber analysis (teasing fibers) of the branches to the brachialis and brachioradialis muscles from the branching points to the nerve roots of the brachial plexus and the investigation of the intramuscular nerve supply in the brachialis muscle. The fiber analysis and investigation of the intramuscular nerve supply were done using an operational microscope (Zeiss, 6-H). The results were as follows: 1) The radial nerve supply to the brachialis muscle was constantly observed (the incidence was 16/16), but no median nerve supply was seen. 2) The branch from the musculocutaneous nerve supplied most part of the brachialis muscle except for the small inferolateral part of the muscle supplied by the radial nerve branch. 3) Communications between the two nerve sources were observed in the brachialis muscle in all three cases. 4) Nerve fibers composing the branch from the musculocutaneous nerve derived from the anterior divisions of the ventral rami of the fifth and sixth cervical nerves. 5) Nerve fibers composing the branch from the radial nerve derived from the anterior divisions of the ventral rami of the sixth and seventh cervical nerves, and were situated in the same bundle of fibers with those from the musculocutaneous nerve at the sixth cervical nerve in the first and third cases (right side of the first and second bodies). 6) Nerve fibers composing the branch to the brachioradialis muscle ran in the same bundles with those to the brachialis muscle in the radial nerve, but were situated in the