Axillary nerve

At a glance

English

Axillary nerve

Latin (TA2)

Nervus axillaris

System

Nerve

Region

Upper limb

Side

Paired, left and right

About the axillary nerve

The axillary nerve, also known as the circumflex nerve, is one of the two terminal branches of the plexus's posterior cord; the other is the radial nerve. Its fibres come from C5 and C6, which travel through the upper trunk and its posterior division before reaching the cord.

The nerve begins in the axilla posterior to the axillary artery, lying on subscapularis. It runs down to the lower edge of that muscle and then turns backwards, leaving the axilla through the quadrangular space accompanied by the posterior circumflex humeral vessels. This space is framed by teres minor above, teres major below, the long head of triceps medially and the humeral surgical neck laterally. Close to the joint it sends an articular twig into the shoulder below subscapularis.

Behind the humerus the nerve divides. The anterior (upper) branch curls forwards around the surgical neck beneath deltoid, with the circumflex vessels, as far as the front edge of the muscle; it supplies most of deltoid and sends a few twigs through it to the skin. The posterior (lower) branch supplies teres minor and the back of deltoid, then pierces the deep fascia as the superior lateral cutaneous nerve of the arm. This serves the skin covering the lower deltoid, the so-called regimental badge area, and over the long head of triceps. Dissection studies also find that triceps' long head often receives a small branch from the axillary nerve or the nearby posterior cord.

The nerve is at risk where it hugs the humerus. Anterior dislocation of the shoulder, fracture of the surgical neck, pressure from a badly fitted crutch and surgery around the shoulder can all damage it. Loss of deltoid and teres minor weakens abduction beyond the first few degrees and makes flexion, extension and rotation of the shoulder feeble. Over time deltoid wastes, flattening the rounded contour of the shoulder, and testing sensation over the badge area is a quick bedside check of the nerve.

Course and supply

Arises from

Posterior cord of the plexus (C5, C6) in the axilla, behind the axillary artery.

Ends in

Divides behind the humerus into an anterior branch, ending at the front of deltoid, and a posterior branch, ending as the superior lateral cutaneous nerve of the arm.

Supplies

Motor to deltoid and teres minor, and often the long head of triceps; sensory to the shoulder joint and the skin over the lower deltoid and upper back of the arm.

Blood supply

Accompanied through the quadrangular space by the posterior circumflex humeral vessels.