To the Editor:
I greatly enjoyed reading the article by Hu et al. (2) on the innervation of the brachialis muscle. Their electrophysiological analysis of the brachial plexus nerve roots confirms the long-established notion that the brachialis muscle has dual innervation from the radial and musculocutaneous nerves. However, the authors state that “most anatomy textbooks describe the nerve supply of brachialis as coming solely from the musculocutaneous nerve” (2, p 910). I do not agree with this statement, as most of my anatomy textbooks clearly discuss the dual origin of the brachialis nerve supply (1,3–7).
Studying the embryological development of the upper limb provides insight into the reasons for this dual innervation of the brachialis, in addition to an explanation for other anatomic oddities, such as the course of the ulnar nerve (4). The fetal flexor and extensor compartments of the arm are not identical to the adult compartments. The fetal preaxial and postaxial borders of the limb define the flexor and extensor compartments. In the fetus, the lateral part of the brachialis muscle is in the extensor compartment with the radial nerve, whereas the medial muscle resides in the flexor compartment with the musculocutaneous nerve (Fig. C1). The muscular innervation is determined at the fetal stage. As the limb develops, with rotation such that the preaxial border becomes lateral, the brachialis becomes a flexor compartment muscle, bringing with it the radial nerve supply to its lateral part (7).
FIGURE C1. Schematic...Image Tools
The fetal compartmentalization does not correspond to the adult medial and lateral intermuscular septum. In the fetus, the ulnar nerve is in the flexor compartment. In the adult, the ulnar nerve at the elbow is in the extensor compartment. Appreciation of this development should assist with a comprehensive understanding of such anatomic peculiarities.
1. Haymaker W, Woodhall B: Peripheral Nerve Injuries, Principles of Diagnosis. Philadelphia, W.B. Saunders, 1953, ed 2, pp 237–239.
2. Hu SN, Zhou WJ, Wang H, Chen L, Zhu Y, Gu YD, Xu JG: Origination of the brachialis branch of the musculocutaneous nerve: An electrophysiological study. Neurosurgery 62:908–912, 2008.
3. Kline DG, Hudson AR, Kim DH: Atlas of Peripheral Nerve Surgery. Philadelphia, W.B. Saunders, 2001, p 254.
4. McMinn RMH: Last's Anatomy. Edinburgh, Churchill Livingstone, 1990, ed 8, pp 22–24.
5. Netter FH: Nervous system, in The Ciba Collection of Medical Illustrations. Summit, Ciba-Geigy Corp., 1991, vol 1, p 117.
6. Sunderland S: Nerves and Nerve Injuries. Edinburgh, Churchill Livingstone, 1972, p 901.
7. Williams PL, Warwick R: Gray's Anatomy. Edinburgh, Churchill Livingstone, 1980, ed 36, pp 153, 572.