Learning Objectives
By the end of this topic, you should be able to:
- Identify the root values of the musculocutaneous nerve.
- Describe its anatomical course from the brachial plexus to the forearm.
- List the motor and sensory functions of the nerve.
- Recognize the clinical features of musculocutaneous nerve injury.
Introduction
The musculocutaneous nerve is a major peripheral nerve of the upper limb. It supplies the muscles of the anterior compartment of the arm and provides sensory innervation to the lateral forearm.
In this section, we will examine the origin, course, functions, and clinical relevance of the musculocutaneous nerve.
Overview
- Roots: C5–C7
- Motor: Coracobrachialis, biceps brachii, brachialis
- Sensory: Lateral cutaneous nerve of the forearm → lateral forearm skin
Anatomical Course
The musculocutaneous nerve is the terminal branch of the lateral cord of the brachial plexus (C5–C7). It emerges at the inferior border of the pectoralis minor in the axilla.
It then:
- Pierces the coracobrachialis muscle (supplying it).
- Descends in the anterior compartment of the arm, lying deep to the biceps brachii and superficial to the brachialis.
- Innervates biceps brachii and brachialis, and gives small articular branches to the shoulder and elbow.
- Pierces the deep fascia lateral to the biceps tendon and continues as the lateral cutaneous nerve of the forearm.
Anatomical Variations
The musculocutaneous nerve may:
- Communicate with the median nerve
- Pass under the coracobrachialis instead of through it
- Occasionally, pass through the biceps brachii
Motor Functions
The musculocutaneous nerve innervates the anterior arm muscles:
- Biceps brachii – elbow flexion, forearm supination
- Brachialis – elbow flexion
- Coracobrachialis – shoulder flexion
Mnemonic: BBC
(Biceps, Brachialis, Coracobrachialis)
Sensory Functions
The nerve continues as the lateral cutaneous nerve of the forearm, which:
- Becomes subcutaneous near the cephalic vein
- Supplies the anterolateral forearm
Clinical Relevance: Musculocutaneous Nerve Injury
Injury is uncommon due to the deep location, but may occur due to:
- Penetrating trauma to the axilla
- Iatrogenic injury during shoulder surgery
Motor Deficits
- Weak elbow flexion
- Weak shoulder flexion
- Weak forearm supination
(Still possible via pectoralis major and brachioradialis)
Sensory Deficit
- Loss of sensation over the lateral forearm
