Axillary nerve

The axillary nerve, derived from the ventral rami of the C5 and C6 spinal nerves, is one of the terminal branches of the posterior cord of the brachial plexus.

Exiting the axilla via the quadrangular space–a region bordered by the teres minor superiorly, the surgical neck of the humerus laterally, the teres major inferiorly, and the long head of the triceps medially–the axillary nerve travels alongside the posterior humeral circumflex artery and vein.

Once through this space, the nerve splits into anterior and posterior branches for motor and sensory innervation. The axillary nerve innervates the deltoid and teres minor muscles.

1. Anterior branch

The anterior branch of the axillary nerve wraps around the surgical neck of the humerus to supply the anterior and middle fibers of the deltoid, which are responsible for abducting the arm beyond 15 degrees after initiation by the supraspinatus.

2. Posterior branch

The posterior branch innervates the posterior deltoid for shoulder extension and lateral rotation, as well as the teres minor, which stabilizes the glenohumeral joint and assists in lateral rotation.

Upper lateral cutaneous nerve of the arm

Additionally, the posterior branch of the axillary nerve gives rise to the upper lateral cutaneous nerve of the arm, which provides sensation to the "regimental badge area," located over the lateral deltoid region.

The axillary nerve's course and location, particularly its proximity to the surgical neck of the humerus, make it vulnerable to injuries such as fractures, inferior humeral dislocations, or trauma that overstretches the nerve, often leading to symptoms like muscle weakness, atrophy, or sensory deficits in its distribution area.

The trunk of the axillary nerve gives off an articular filament which enters the shoulder-joint below the Subscapularis.