Axillary Nerve Anatomy

The axillary nerve arises from the posterior cord of the brachial plexus, carrying nerve fibers from the C5 and C6 nerve roots. This anatomical relationship explains why upper trunk brachial plexus injuries can significantly impact deltoid function and shoulder stability.

Axillary Nerve Course

The Axillary nerve descends in the axilla posterior to the axillary artery and anterior to the subscapularis muscle, in this location, it can be injured during anterior shoulder dislocations or surgical approaches to the shoulder joint.

The most critical portion of the nerve’s course occurs as it passes anterior to the subscapularis muscle and inferior to the shoulder capsule, traveling from anterior to posterior through the quadrangular space. This quadrangular space is bounded by the teres minor superiorly, the teres major inferiorly, the long head of the triceps medially, and the surgical neck of the humerus laterally.

Terminal Branching Pattern

The axillary nerve typically divides into anterior and posterior branches within the quadrangular space in approximately 88% of individuals. This division pattern has important surgical implications, as partial nerve injuries may affect only one branch while sparing the other.

Anterior Branch

The anterior branch represents the larger of the two terminal divisions. It passes around the surgical neck of the humerus within the deep deltoid fascia, maintaining a consistent distance of approximately 5 to 7 cm distal to the acromion. This measurement is particularly relevant during deltoid-splitting approaches, as staying proximal to this landmark helps avoid iatrogenic nerve injury.

The anterior branch provides motor innervation to the anterior and middle portions of the deltoid muscle, which are essential for shoulder flexion, abduction, and the initial phases of arm elevation. Additionally, this branch gives rise to the superior lateral cutaneous nerve of the arm, which provides sensory innervation to the skin overlying the lateral aspect of the shoulder.

Posterior Branch

The posterior branch of the axillary nerve takes a more direct course to reach the posterior deltoid and teres minor muscles. It supplies motor innervation to the posterior deltoid, which is crucial for shoulder extension and external rotation, and to the teres minor, one of the four rotator cuff muscles responsible for external rotation and shoulder stability.

See Also: Rotator Cuff Muscles

Articular Branch

Often overlooked in basic anatomical descriptions, the articular branch arises from the main trunk of the axillary nerve and provides innervation to the glenohumeral joint capsule. This branch has clinical significance in shoulder pain patterns and may contribute to the complex pain experienced in shoulder pathology.

Motor Innervation

The axillary nerve’s motor function centers on two primary muscles:

Deltoid Muscle: The deltoid represents the primary target of axillary nerve innervation. This multipennate muscle consists of three distinct heads – anterior, middle, and posterior – each with specific functional roles. The anterior head assists in shoulder flexion and internal rotation, the middle head is the primary shoulder abductor, and the posterior head contributes to shoulder extension and external rotation. Loss of deltoid function results in the characteristic “flat shoulder” appearance and significant functional limitation.

Teres Minor: This rotator cuff muscle works in conjunction with the infraspinatus to provide external rotation of the shoulder. While the infraspinatus (innervated by the suprascapular nerve) can compensate for teres minor loss, the combination provides optimal external rotation strength and control.

Sensory Innervation

The sensory component of the axillary nerve is often underappreciated but clinically significant. The superior lateral cutaneous nerve of the arm provides sensation to the skin overlying the lateral shoulder, creating a “badge patch” area of numbness when the nerve is injured. This sensory loss can be particularly bothersome for patients and may persist even after motor function recovery.

Clinical Significance

Understanding axillary nerve anatomy is essential for several clinical scenarios. The nerve’s proximity to the surgical neck of the humerus makes it vulnerable during proximal humeral fractures and their surgical repair. Its passage through the quadrangular space creates a potential site for compression, particularly in overhead athletes or individuals with posterior shoulder tightness.

See Also: Axillary Nerve Injury

The axillary nerve is most commonly injured during orthopedic surgeries, shoulder dislocations, and rotator cuff tears.

The consistent anatomical landmarks associated with the axillary nerve, particularly the 5-7 cm distance from the acromion to the anterior branch, provide surgeons with reliable reference points during deltoid-splitting approaches and arthroscopic procedures. This knowledge helps minimize iatrogenic injury while maintaining adequate surgical exposure.

References & More

1. Orthobullets – Axillary Nerve
2. Okwumabua E, Thompson JH. Anatomy, Shoulder and Upper Limb, Axillary Nerve. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493212/