Folia Morphol. Vol. 63, No. 4, pp. 503–506 Copyright © 2004 Via Medica C A S E R E P O R T ISSN 0015–5659 www.fm.viamedica.pl Innervation of the coracobrachialis muscle by a branch from the lateral root of the median nerve Ilkan Tatar, Recep Brohi, Fikret Sen, Alikemal Tonak, Hamdi Celik Hacettepe University, Faculty of Medicine, Department of Anatomy, Ankara, Turkey [Received 23 July 2004; Revised 5 October 2004; Accepted 5 October 2004] Anomalies of the brachial plexus have previously been described in the litera- ture. The coracobrachialis muscle is typically innervated by the musculocutane- ous nerve. During a gross anatomy dissection we found that the coracobrachia- lis muscle was innervated by a branch from the lateral root of the median nerve. Knowledge of the anatomical variations of the peripheral nervous system is im- portant in interpreting unusual clinical presentations. This report will assist clini- cians and surgeons by pointing out anatomical anomalies associated with the musculocutaneous nerve, the median nerve and their branches to the anterior compartment muscles of the upper arm. Key words: brachial plexus, median nerve, musculocutaneous nerve, coracobrachialis muscle INTRODUCTION alis muscle was innervated by a branch of the lateral Anomalies of the brachial plexus and its terminal root of the median nerve. branches are not uncommon [1, 3, 4, 6, 7, 9, 10, 11, A review of the embryological development of 15, 17, 18]. The musculocutaneous nerve is a motor the brachial plexus is helpful in understanding the and sensory peripheral nerve that originates from origin of possible anatomical variations. The upper the lateral cord of the brachial plexus in the axilla. It limb buds are visible by day 26 or 27 in the develop- usually supplies the coracobrachialis, biceps brachii ing embryo. During the 5th week the peripheral nerves and the brachialis muscles in the anterior compart- grow from the brachial plexus into the mesenchyme ment of the arm and then continues as the lateral of the upper limb buds. The brachial plexus is a union cutaneous nerve of the forearm. The branch to the of the lower 4 cervical ventral rami and the first tho- coracobrachialis is given off from the nerve close to racic ventral ramus, with a small contribution from its origin, and in some instances as a separate fila- the ventral ramus of the 4th cervical spinal nerve. ment from the lateral cord of the plexus. It is de- These ventral rami are the roots of the plexus and rived from the 7th cervical nerve. The median nerve are variable in their mode of junction. The lateral cord typically does not innervate muscles in the upper arm. gives off the lateral pectoral nerve, the musculocuta- However, a branch to the pronator teres muscle can neous nerve and the lateral root of the median nerve, occasionally be observed [21]. whereas the medial cord gives off the medial pecto- In our case, the musculocutaneous nerve inner- ral nerve, the medial cutaneous nerve of the arm, the vated the biceps brachii and brachialis, but not the medial cutaneous nerve of the forearm, the ulnar coracobrachialis muscle. Instead, the coracobrachi- nerve and the medial root of the median nerve [12]. Address for correspondence: Ilkan Tatar MD, Hacettepe University Faculty of Medicine Department of Anatomy, 06100 Ankara, Turkey, tel: +90 312 305 21 14, fax:+90 312 310 71 69, e-mail: [email protected] 503 Folia Morphol., 2004, Vol. 63, No. 4 The musculocutaneous nerve has frequent varia- DISCUSSION tions and these were discussed in detail in earlier Variations in the brachial plexus have been well articles. It may be doubled, unusually short, or even described by many authors. Unilateral brachial plex- absent. The musculocutaneous nerve arises from the us anomalies were demonstrated in 4 of 71 cadav- lateral cord (90.5%), from the lateral and posterior ers by Sarsilmaz et al. [18]. Sargon et al. [16] de- cord (4%), from the median nerve (2%), as two sep- scribed an unusual formation of the median nerve arate bundles from the medial and lateral cords in the axilla. Basar et al. [2], Uzun and Seelig [19] (1.4%) or from the posterior cords (1.4%). Some fi- reported a connecting branch between the muscu- bres of the median nerve may run in the musculocu- locutaneous and median nerves. taneous nerve, leaving it to join their proper trunk. During the dissection of a 72 year-old female Less frequently the reverse occurs, the median nerve cadaver, Gumusburun et al. [8] observed a variation sending a branch to the musculocutaneous nerve. of the brachial plexus characterised by the absence A single puncture wound, therefore, could theoret- of the musculocutaneous nerve bilaterally. The var- ically impair the muscles innervated by both the iation in the musculocutaneous and median nerves musculocutaneous and the median nerves if there was classified into the following 5 types by Gumus- were complete or partial fusion of these nerves. burun et al. [8]. Type 1: there are no receiving fibres Nerves following an anomalous course are more sus- between the nerves as described in the classic text- ceptible to accidental injury [8]. book; Type 2: some fibres of the medial root of the Very rarely the whole lateral cord of the brachial median nerve merge with the musculocutaneous plexus pierces the coracobrachialis and only then nerve and, after some distance, leave to join the prop- does the cord divide into the musculocutaneous er trunk; Type 3: the lateral root of the median nerve nerve and the lateral root of the median nerve. runs into the musculocutaneous nerve and after some distance leaves it to join its proper trunk; CASE REPORT Type 4: the fibres of the musculocutaneous nerve Variation was detected in a 55-year-old male ca- unite with the lateral root of the median nerve and daver’s right axilla during a gross anatomy dissec- after some distance the musculocutaneous nerve tion during the autumn 2002 phase at the dissec- arises from the median nerve; Type 5: the musculo- tion room of the Anatomy Department of the Fac- cutaneous nerve is absent. ulty of Medicine, Hacettepe University. The coraco- Venieratos et al. [20] classified the sites of com- brachialis in our specimen was not innervated by munication between the musculocutaneous and the musculocutaneous nerve. Instead, the coraco- median nerves into 3 types. Type 1: the communica- brachialis was innervated by a branch from the la- tion is proximal to the point of entry of the muscu- teral root of the median nerve. This branch bifur- locutaneous nerve into coracobrachialis muscle; cated into an upper and lower division prior to pier- Type 2: the communication is distal to the muscle; cing the coracobrachialis muscle. The musculocu- Type 3: the musculocutaneous nerve (as well as the taneous nerve was found to course deep to the up- communicating branch) does not pierce the coraco- per division of the branch of the lateral root of the brachialis muscle. Choi et al. [5] classified the varia- median nerve. The branch from the lateral root of tions between the musculocutaneous and the medi- the median nerve originated 8 mm proximal to the an nerve into 3 patterns. Pattern 1: fusion of both junction of the lateral and medial roots of the me- nerves (19.2%); Pattern 2: the presence of one sup- dian nerve. The branch of the lateral root of the plementary branch between the two nerves (72.6%); median nerve measured 50 mm prior to bifurca- Pattern 3: two branches (6.8%). tion into upper and lower divisions. The musculo- There are 2 more articles closely related to our cutaneous nerve passed deep to the upper division case. In the dissection of 36 arms El-Naggar investi- 7 mm distal to the bifurcation. The upper division gated variations in the musculocutaneous nerve in of the branch of the lateral root of the median nerve the form of a lower origin from the lateral root of pierced the coracobrachialis muscle and measured the median nerve and a nerve with a short course, 32 mm in length. The lower division measured after which it united with the median nerve [6]. 15 mm in length. The brachial artery was not relat- During gross anatomy dissections of 60 em- ed to these small branches and coursed classically balmed cadavers by Gumusalan et al. the coraco- among the medial and lateral cords of the median brachialis muscle was found to be innervated by nerve (Fig. 1). a nerve branch arising from the lateral root of the 504 Ilkan Tatar et al., Coracobrachialis muscle innervation, median nerve Figure 1. 1 — lateral root of the median nerve (LRMN), 2 — musculocutaneous nerve, 3 — branch from the LRMN (bf-LRMN) to the co- racobrachialis muscle, 4 — upper division from the bf-LRMN to the coracobrachialis muscle, 5 — lower division from the bf-LRMN to the coracobrachialis muscle, 6 — the coracobrachialis muscle. median nerve (as in our case), but not from the [8, 13, 15]. Anatomical variants of the brachial plex- musculocutaneous nerve nor from the lateral cord us may be significant in the context of axillary trau- of the brachial plexus. Furthermore, the musculocu- ma. Theoretically, in a person with an anomaly of taneous nerve was found not to pierce the coraco- the brachial plexus that we have described injury to brachialis muscle and to course downwards medial the musculocutaneous nerve would not result in loss to it. [7]. In our case, in contrast, the musculocuta- of function of the coracobrachialis muscle. By the neous nerve pierced the coracobrachialis muscle. same token, injury to the lateral root could result in The absence of the musculocutaneous nerve and functional deficiency of the coracobrachialis.