Spinal origins of the nerve branches innervating the coracobrachialis muscle: clinical implications

The coracobrachialis muscle (Cb M) receives nerve branches from the musculocutaneous nerve (Mc N). Many textbooks describe that the Cb M is innervated by the C5, C6, and C7 spinal nerves. The present study was performed to identify the spinal nerve composition of the nerve branches innervating the Cb M and to evaluate the number of myelinated axons of the nerve branches. Two types of spinal nerve compositions were observed: type I, composed of both C6 and C7 nerves, comprised 88.9% (40/45) of the sides; and type II, composed of only C7 nerves, comprised 11.1% (5/45) of the sides. Of the type I branches, 245.4 ± 26.3 (mean ± SD, range 201–267) of the myelinated nerve fibers were derived from C6 and 520.0 ± 41.8 (range 469–567) of the fibers were derived from C7. Of the type II branches, 836.4 ± 125.5 (range 709–991) of the nerve fibers were derived from C7. Imbroglio Modometer version 2.0 was used to count the number of fibers innervating the Cb M. The main component of these nerve branches was the C7 spinal nerve. The C5 spinal nerve, did not contribute to innervating the Cb M in the present study. The number of branches varied from one to four, with C7 being concentrated in the first and second branches. Therefore, these two branches may be suitable for the neurotization procedure as a donor nerve at the upper trunk (C5 and C6) lesion of the brachial plexus.