Illustrated Encyclopedia of Human Anatomic Variation: Opus III: Nervous System: Plexuses

Musculocutaneous Nerve

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed

A number of variations in the course and distribution of the musculocutaneous nerve have been reported. Instead of piercing the coracobrachialis muscle, the nerve may adhere to the median nerve for some distance down the arm and then, either as a single trunk or as several branches, pass between the biceps and brachialis muscles to supply all three muscles (about 22% of arms).

Sometimes only a part of the nerve follows this course; this part then rejoins the main trunk after it transits through and supplies coracobrachialis.

In some cases, instead of the whole trunk of the nerve piercing coracobrachialis, only its muscular branch or only its cutaneous branch pierces the muscle.

The musculocutaneous nerve may be accompanied by fibers from the median nerve as its transits coracobrachialis; a communicating branch passes from the musculocutaneous to the median nerve. Instead of penetrating coracobrachialis, the nerve may pass behind it or between it and the short head of the biceps muscle. Occasionally, the nerve perforates, not only coracobrachialis, but also the brachialis or the short or long head of the biceps muscles.

Very rarely the lateral cord may pierce coracobrachialis and then divide into the musculocutaneous and lateral branch of the median nerve.

Branches to the following structures have been reported: brachioradialis, pronator teres, the skin of the dorsum of the hand over and adjacent to the first metacarpal bone, the dorsum of the thumb (the radial nerve being absent), both sides of the ring finger, and the radial side of the little finger.

The musculocutaneous nerve may receive a root from the seventh cervical nerve.

Anastomoses with the median nerve usually occur in the lower third of the arm, and may form an ansa; the frequency of anastomoses in this region of the arm is 8% of cases.

The musculocutaneous nerve may be doubled, unusually short, or absent.

Riche-Cannieu anastomoses (between the median and ulnar nerves in the palm of the hand) may exist in 70% of individuals. Martin-Gruber (median-ulnar nerves) anastomoses in the forearm have a reported frequency of 15%. It should be remembered that the "anastomoses" occur between the median, ulnar, radial, and musculocutaneous nerves, as well as between other nerves. These are errors in the pathway (course) of some, inappropriately placed nerve fibers. In order for these nerve fibers to get to their proper end-point, the bundle of nerves fibers leave the inappropriate trunk and join the proper nerve trunk. Both Martin-Gruber and Riche-Cannieu "anastomoses" are examples of nature's correction of nerve fibers located in inappropriate nerve pathways. Certainly a mystery, these "anastomoses". On a broader and larger scale, the brachial plexus and sacral plexuses are other examples of neuronal sorting.

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Variety of Anastomoses between the Musculocutaneous and Median Nerves

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