Musculocutaneous

Question 1

Nerve roots: .

Answer 1

C5-C6

Question 2

Motor functions:

Answer 2

Innervates the muscles in the anterior compartment of the arm – the coracobrachialis, biceps brachii and the brachialis

Question 3

Sensory functions:

Answer 3

Gives rise to the lateral cutaneous nerve of forearm, which innervates skin on the lateral surface of the forearm.

Question 4

Anatomical Course

Answer 4

The musculocutaneous nerve arises from the lateral cord of the brachial plexus, and therefore contains fibres from spinal roots C5, C6 and C7.

After originating from the brachial plexus, the musculocutaneous nerve leaves the axilla, and pierces the coracobrachialis muscle, near its point of insertion on the humerus.

It then passes down the arm, anterior to the brachialis muscle but deep to the biceps brachii, innervating them both.

The musculocutaneous nerve emerges laterally to the biceps tendon, and continues into the forearm as the lateral cutaneous nerve. It provides sensory innervation to the lateral aspect of the forearm

NB: The musculocutaneous nerve is well recognised to have a varied anatomical course. It can interact with the median nerve, adhering to the nerve and exchanging fibres.

Question 5

Motor Functions

Answer 5

The musculocutaneous nerve innervates the muscles in the anterior compartment of the arm – the biceps brachii, brachialis and coracobrachialis. These muscles flex the upper arm at the shoulder and the elbow. In addition the biceps brachii also performs supination of the forearm.

A good memory tool to help you remember these muscles is BBC.

Question 6

Sensory Functions

Answer 6

The musculocutaneous nerve gives rise to the lateral cutaneous nerve of forearm.

This nerve initially enters the deep forearm, but then pierces the deep fascia to become subcutaneous. In this region, it can be found in close proximity to the cephalic vein.

The lateral cutaneous nerve of forearm innervates the skin of the lateral aspect of the forearm.

Question 7

Clinical significance

Answer 7

Although rare, the musculocutaneous nerve can be affected through compression due to hypertrophy or entrapment between the biceps aponeurosis and brachialis fascia or it may be injured through stretch as occurs in dislocations and sometimes in surgery.

Isolated injury causes weakness of elbow flexion and supination of the forearm.

A discrete sensory disturbance is present on the radial side of the forearm. The biceps reflex is also affected.

The nerve is usually involved in an upper brachial plexus palsy.

Injury can occur before entering the coracobrachialis due to dislocation or apparently due to stretch due to throwing injury

Heavy backpacks can cause damage to the upper trunk of the brachial plexus – dysfunction can be severe and prolonged with similar injury as occurs with Erb’s palsy from breech deliveries. Early detection is important – the combination of time, avoidance of wearing a backpack, and strengthening of the shoulder muscles will probably be effective.