Learning Objectives

By the end of this lesson, students will be able to:

1. Identify the spinal root values of the axillary nerve.
2. Describe the anatomical course of the axillary nerve from the brachial plexus to the shoulder.
3. Explain the boundaries and contents of the quadrangular space.
4. List the motor and sensory functions of the axillary nerve.
5. Recognize the clinical features of axillary nerve injury and Erb’s palsy.

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The axillary nerve is a major peripheral nerve of the upper limb that plays a vital role in shoulder movement and sensation. It arises from the posterior cord of the brachial plexus and carries fibres from spinal nerve roots C5 and C6.

Overview

• Roots: C5–C6
• Motor: Deltoid, teres minor
• Sensory: Upper lateral cutaneous nerve of the arm (skin over the lower deltoid – the regimental badge area)

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Anatomical Course

The axillary nerve forms in the axilla as a continuation of the posterior cord. It lies posterior to the axillary artery and anterior to the subscapularis muscle.

It exits the axilla through the quadrangular space, accompanied by the posterior circumflex humeral vessels, then winds around the surgical neck of the humerus.

At this point, it divides into three branches:

• Posterior terminal branch: Deltoid (posterior part), teres minor, and skin over the inferior deltoid
• Anterior terminal branch: Deltoid (anterior part) and skin of the shoulder
• Articular branch: Glenohumeral joint

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Quadrangular Space

Boundaries:

• Superior – Teres minor
• Inferior – Teres major
• Lateral – Surgical neck of the humerus
• Medial – Long head of triceps
• Anterior – Subscapularis

Contents: Axillary nerve, posterior circumflex humeral artery, and vein

Compression in this space can cause deltoid weakness and sensory loss.

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Motor Functions

• Deltoid: Abduction of the shoulder
• Teres minor: External rotation and shoulder stabilization

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Sensory Functions

The axillary nerve supplies skin over the lower deltoid via the upper lateral cutaneous nerve of the arm. Loss of sensation in this region is a key sign of axillary nerve injury.

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Clinical Relevance

Axillary Nerve Injury

• Causes: surgical neck fracture, shoulder dislocation, surgery
• Motor: inability to abduct beyond 15°
• Sensory: loss over the regimental badge area

Erb’s Palsy

Damage to C5–C6 roots → affects axillary nerve
→ loss of shoulder abduction and external rotation

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Activity