Musculocutaneous nerve

Authors

Affiliations

Nathaniel Yomogida, B.S., SPT

Doctor of Physical Therapy

B.S. in Kinesiology

Chloë Kerstein, B.S., SPT

Doctor of Physical Therapy

B.A. in Neuroscience

Brachial Plexus¹

The musculocutaneous nerve (Fig. 3-10) is the terminal branch of the lateral cord, which in turn is derived from the anterior division of the upper and middle trunks of the fifth through seventh cervical nerve roots.1

Anatomy

Origin

The musculocutaneous nerve arises from the terminal branch of the lateral cord which comes from anterior divisions of the Superior trunk and middle trunk and thus is supplied by nerve roots C5, C6, C7¹.

Musculocutaneous nerve¹

Path

After arising from the lateral cord of the brachial plexus at the level of coracoid process and proceeds caudally and laterally¹. As it travels caudally and laterally down the arm, the musculocutaneous nerve gives branches to the coracobrachialis, before penetrating this muscle 3–8 cm below the coracoid process¹. The nerve then runs through and supplies, the biceps brachii and partially innervates brachialis muscles, before emerging between the biceps brachii and the brachioradialis muscles 2–5 cm above the elbow¹. After emerging between the biceps and brachioradialis (2-5cm above the elbow), the musculocutaneous nerve becomes the Lateral Cutaneous (antebrachial) Nerve of the Forearm which supplies the anterior-lateral aspect of the forearm¹.

Musculocutaneous and Axillary Nerves¹

Innervation

Motor innervation

Table 1: Muscles innervated by Musculocutaneous n.

Muscle	Origin	Insertion	Innervation	Action
Biceps brachii Long head	Supraglenoid tubercle of scapula Adjacent rim of Glenoid Labrum	Radial tuberosity Bicipital aponeurosis	Musculocutaneous n. C5 - C6	Elbow: Flexion, Supination GHJ: Flexion, Stabilization of humeral head during deltoid contraction, Abduction, IR
Biceps brachii Short head	Coracoid process	Radial tuberosity Bicipital aponeurosis	Musculocutaneous n. C5 - C6	Elbow: Flexion, Supination GHJ: Flexion, Stabilization of humeral head during deltoid contraction, Abduction, IR
Brachialis	Distal half of anterior Humerus	Ulnar tuberosity	Musculocutaneous n. Radial n. C5 - C7	Elbow: Flexion
Coracobrachialis	Coracoid process	Humerus (in line with crest of lesser tubercle)	Musculocutaneous n. C5 - C7	GHJ: Flexion, Adduction, IR

Cutaneous Innervation

The Musculocutaneous nerve becomes the

Dysfunction

• Coracobrachialis entrapment¹

Lesion

• Muscular impact Coracobrachialis, biceps brachii, and brachialis
• Impact: Weak elbow flexion with forearm supinated¹

Etiology

Atraumatic

Cases of atraumatic isolated musculocutaneous neuropathies are rare¹. Reported cases include poor positioning while under anesthesia, neoplastic structures impinging the musculocutaneous nerve, and strenuous upper extremity exercise without apparent underlying disease¹. Some theorize that exercise-related cases are caused by entrapment of the nerve within the coracobrachialis and/or traction between a proximal fixation point at the coracobrachialis and a distal fixation point at the deep fascia at the elbow¹.
Injury to this nerve can also result from demanding physical work involving shoulder flexion and repetitive elbow flexion with a pronated forearm.35 Although rare, an isolated lesion to the musculocutaneous nerve can result in weakness of the biceps, coracobrachialis, and brachialis (Table 3-3). These muscles help stabilize the elbow and the G-H joint and maintain the static position of the arm.

Clinical Presentation

The typical clinical presentation includes ▶ reports or evidence of muscle wasting and sensory changes to the lateral side of the forearm; ▶ weakness of the biceps, brachialis, and coracobrachialis; ▶ diminished biceps reflex; ▶ decreased sensation at the lateral forearm; and ▶ positive electromyogram (EMG) study.

Examination

Intervention

Conservative Intervention

Conservative intervention as recommended by Dutton incldues cessation of aggravating activities and a gradual return to activity with resolution of symptoms¹.

References

1.

Dutton M. Dutton’s Orthopaedic Examination, Evaluation, and Intervention. 5th ed. McGraw Hill Education; 2020.