Anatomy

Anatomy of the n. axillaris

The n. axillaris arises from the posterior cord of the brachial plexus, mainly from the C5 and C6 nerve roots. From the axillary region, it courses posteriorly together with the posterior circumflex humeral artery and passes through the quadrangular space.

The quadrangular space is bordered by the m. teres minor superiorly, m. teres major inferiorly, the long head of the m. triceps brachii medially, and the surgical neck of the humerus laterally. After passing through this space, the nerve winds around the posterior aspect of the surgical neck of the humerus, deep to the m. deltoideus.

The n. axillaris innervates two important muscles:

m. deltoideus – the main muscle responsible for shoulder abduction, especially from approximately 15° to 90°. It also contributes to flexion, extension and rotation of the shoulder depending on the deltoid portion involved.

m. teres minor – part of the rotator cuff; it contributes to external rotation of the shoulder and helps stabilize the humeral head in the glenoid cavity.

The n. axillaris also gives rise to the superior lateral cutaneous nerve of the arm, which provides sensory innervation to the skin over the lateral shoulder region, commonly called the “regimental badge” area.

Clinically, the n. axillaris is important because it may be affected in anterior shoulder dislocation, fractures of the surgical neck of the humerus, or compression within the quadrangular space. Its injury can lead to weakness of shoulder abduction, deltoid atrophy, and sensory loss over the lateral shoulder.