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Musculocutaneous nerve
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Origin
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The musculocutaneous nerve originates from the lateral cord of the brachial plexus (C5-C7) at the inferior border of the pectoralis minor muscle. The brachial plexus itself originates from the ventral rami of the C5-T1 nerve roots. The roots emerge between the anterior and middle scalene muscles, and coalesce and mingle to form an upper, middle, and lower trunk, which eventually give rise to a medial, lateral, and posterior cord (all associated closely with and surrounding the axillary artery).
The cords are named according to their relation with the axillary artery. Locating the nerve whilst dissecting is simply a case of identifying coracobrachialis, and looking for the nerve entering it.
Course and innervation
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The musculocutaneous nerve emerges as the terminal branch of the lateral cord of the brachial plexus, from the C5-C7 nerve roots. The first muscle it enters is the coracobrachialis and gives branches to this muscle before entering it. From here it runs in the flexor compartment superficial to the brachialis but deep to the biceps brachii muscle. As it descends it innervates both of these muscles.
After giving a small branch to the humerus and articular branches to the elbow joint it pierces deep fascia and emerges lateral to biceps brachii. It then continues as the lateral cutaneous nerve of the forearm.
Clinical notes
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Brachial plexus injury
Backpackers carry heavy weights while hiking. They can damage the nerve by compression of the upper trunks of the brachial plexus. Erb’s palsy may result. Read more about the brachial plexus injury here.
Stab wound and clinical consequences
This is the commonest reason for musculocutaneous nerve injury. The nerve is well protected within the axilla and also by musculature in the arm.
Imagine for a second that the musculocutaneous nerve has been severed at its roots so that it is no longer innervating the structures it normally does, i.e. the elbow flexors. Will the patient still be able to flex their elbow? The answer surprisingly is yes, but it will be weakened. This is because of a muscle called brachioradialis that originates from the lateral epicondylar eminence and lateral epicondyle and inserts onto the radial styloid process.
This muscle has a number of functions including supination, and wrist abduction, wrist extension, but also some elbow flexion. It is innervated by the radial nerve, so is not affected by musculocutaneous nerve injury.
Supination and flexion at the shoulder joint are also weakened but not lost due to the role of supinator and pectoralis major respectively. Sensation to the lateral part of the forearm would also be affected.
Variation
There have been case reports of the musculocutaneous nerve variations. These include its absence (very rare), it not entering the coracobrachialis muscle and the nerve originating abnormally. It is important for doctors and surgeons to be aware of anomalous innervation of the upper limb, so when presented with seemingly contradictory findings, a correct diagnosis can be reached.
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20 дек 2020
