Course
The musculocutaneous nerve runs from the axilla obliquely down through the front of the arm, between its flexor muscles, to become a cutaneous nerve just above the elbow.
In the axilla
It leaves the lateral cord opposite the lower border of pectoralis minor, lateral to the axillary artery and to the lateral root of the median nerve. Together with the lateral cord and median nerve it forms the lateral limb of the M on an axilla dissection.
In the arm
The nerve pierces coracobrachialis a few centimetres below the tip of the coracoid process, then runs down and laterally between biceps brachii in front and brachialis behind. The entry point into coracobrachialis varies, and in some limbs the nerve passes medial to the muscle without piercing it.
At the elbow
Just above the elbow the nerve emerges at the lateral border of the biceps tendon, pierces the deep fascia and continues as the lateral cutaneous nerve of the forearm. That nerve passes deep to the cephalic vein in the cubital fossa and divides into anterior and posterior branches that reach the wrist.
Branches and supply
The musculocutaneous nerve is the motor nerve of the anterior compartment of the arm and the sensory nerve of the lateral forearm.
| Branch | Supplies |
|---|---|
| To coracobrachialis | Coracobrachialis, usually given off before or as the nerve enters the muscle |
| To biceps brachii | Both the long and short heads |
| To brachialis | Most of the muscle; the lateral part also receives twigs from the radial nerve |
| Articular | The elbow joint, through the branch to brachialis |
| Lateral cutaneous nerve of forearm | Skin of the lateral forearm, anterior and posterior, as far as the wrist and the base of the thenar eminence |
The nerve therefore controls the two main elbow flexors and the strongest supinator of the forearm, since biceps supinates powerfully when the elbow is flexed. Its cutaneous field matches the C6 dermatome closely.
Relations
The musculocutaneous nerve is the most lateral of the large nerves leaving the axilla and the only one that enters a muscle there.
- Axilla: lateral to the axillary artery and the lateral root of the median nerve; the conjoint origin of coracobrachialis and the short head of biceps lies just lateral.
- Arm: in the plane between biceps and brachialis, separate from the neurovascular bundle of the medial bicipital groove.
- Elbow: the lateral cutaneous nerve of the forearm lies lateral to the biceps tendon and deep to the cephalic vein.
- Communications: a branch often joins it to the median nerve in the arm, and in some limbs the median nerve carries part or all of its fibres.
Clinical relevance
Isolated musculocutaneous nerve injury is uncommon, because the nerve is deep and protected, but it is at risk during anterior shoulder surgery and in upper trunk lesions.
What an injury looks like
- Weak elbow flexion: flexion is weakened but not lost, because brachioradialis (radial nerve), pronator teres and the forearm flexors can still bend the elbow.
- Weak supination: supinator (radial nerve) still works, but supination against resistance with the elbow bent is weak.
- Absent biceps reflex: the reflex tests C5–C6 through this nerve.
- Numbness: over the lateral forearm.
Causes
The nerve can be stretched or cut when the conjoint tendon is retracted or transferred during anterior shoulder stabilisation, such as a Latarjet procedure, and during deltopectoral approaches. It is injured with the upper trunk in Erb's palsy, and less often by penetrating wounds, anterior dislocation or strenuous repetitive exercise that compresses it in coracobrachialis. The lateral cutaneous nerve of the forearm can be damaged at the elbow by venepuncture or cannulation of the cephalic vein, giving painful paraesthesia over the lateral forearm.