The Axillary Nerve
The axillary nerve is a major peripheral nerve of the upper limb. In this article, we shall look at the applied anatomy of the nerve – its anatomical course, motor functions and cutaneous innervation. We shall also consider the clinical correlations of damage to the axillary nerve.
Spinal roots: C5 and C6.
Sensory functions: Gives rise to superior lateral cutaneous nerve of arm, which innervates the skin over the lower deltoid ‘(‘regimental badge area’).
Motor functions: Innervates the teres minor and deltoid muscles.
Immediately after its formation, the axillary nerve lies posteriorly to the axillary artery and anteriorly to the subscapularis muscle. It descends to the inferior border of the subscapularis muscle, and then exits the axilla posteriorly via the quadrangular space. It is accompanied by the posterior circumflex humeral artery.
In the posterior scapular region, the axillary nerve terminates by dividing into two branches:
- Posterior terminal branch – Provides motor innervation to the teres minor muscle, and innervates the skin over the inferior part of the deltoid.
- Anterior terminal branch – Provides motor innervation to the deltoid muscle
The axillary nerve also provides articular branches to the shoulder joint itself.
The Quadrangular Space
The quadrangular space is a gap in the muscles of the posterior scapular region. It is a pathway for neurovascular structures to move from the axilla to the posterior shoulder and arm.
Its boundaries are:
- Superior – Subscapularis and teres minor.
- Inferior – Teres major.
- Laterally – Surgical neck of humerus.
- Medially – Long head of triceps brachii.
The axillary nerve and posterior circumflex humeral artery pass through the quadrangular space.
The axillary nerve innervates the teres minor and the deltoid muscles.
The teres minor is part of the rotator cuff muscles of the shoulder. This set of muscles acts to stabilise the glenohumeral joint. Acting individually, the teres minor externally rotates the upper limb. The muscle is innervated the posterior terminal branch of the axillary nerve.
The deltoid is situated at the superior aspect of the shoulder. It performs abduction of the upper limb at the glenohumeral joint. The muscle is innervated by the anterior terminal branch of the axillary nerve.
The sensory component of the axillary nerve is delivered via its posterior terminal branch.
After the posterior terminal branch of the axillary nerve has innervated the teres minor, it continues as the upper lateral cutaneous nerve of the arm. This nerve innervates the skin over the inferior portion of the deltoid (known as the ‘regimental badge area’).
In a patient with axillary nerve damage, sensation at the regimental badge area may be impaired or absent. The patient may also report paraesthesia (pins and needles) in the distribution of the axillary nerve.
Clinical Relevance: Injury to the Axillary Nerve
The axillary nerve is most commonly damaged by trauma to the shoulder or proximal humerus – such as a fracture of the humerus surgical neck.
- Motor functions: Paralysis of the deltoid and teres minor muscles. This renders the patient unable to abduct the affected limb.
- Sensory functions: The upper lateral cutaneous nerve of arm will be affected, resulting in loss of sensation over the regimental badge area.
- Characteristic clinical signs: In long standing cases, the paralysed deltoid muscle rapidly atrophies, and the greater tuberosity can be palpated in that area.