Axillary nerve
Nervus axillaris
Also known as: circumflex nerve
A branch of the posterior cord (C5–C6). It winds around the surgical neck of the humerus through the quadrangular space to supply deltoid and teres minor, and the skin over the lower deltoid (the "regimental badge" area).
Overview
The axillary nerve arises from the posterior cord behind the axillary artery. It passes backward through the quadrangular space with the posterior circumflex humeral artery. The space is bounded by teres minor above, teres major below, the long head of triceps medially and the surgical neck of the humerus laterally. The nerve gives an articular branch to the shoulder joint and divides:
• the anterior branch winds around the surgical neck deep to deltoid and supplies most of the muscle;
• the posterior branch supplies teres minor and the posterior deltoid, then becomes the superior lateral cutaneous nerve of the arm, which supplies the skin over the lower half of deltoid (the regimental badge area).
Its close contact with the surgical neck and the inferior shoulder capsule explains why it is injured in proximal humeral fractures and shoulder dislocations.
- Location
Axilla → quadrangular space → around the surgical neck of the humerus deep to deltoid.
Anatomy
- Innervation
- Motor: deltoid and teres minor.
• Sensory: the shoulder joint and the skin over the lower deltoid (superior lateral cutaneous nerve of the arm).- Nerve roots
C5, C6 (posterior cord).
- Course
Posterior cord → anterior to subscapularis → below the shoulder joint capsule → quadrangular space (with the posterior circumflex humeral vessels) → anterior and posterior branches around the surgical neck of the humerus.
Function
- Abduction of the arm from about 15° to 90° (deltoid; supraspinatus starts abduction)
- Flexion and medial rotation (anterior deltoid); extension and lateral rotation (posterior deltoid)
- Lateral rotation of the humerus (teres minor)
- Stabilisation of the humeral head (teres minor, rotator cuff)
- Sensation of the regimental badge area
Clinical significance
Surgical neck fracture and shoulder dislocation
Fracture of the surgical neck of the humerus or anterior (inferior) dislocation of the glenohumeral joint stretches or compresses the nerve. Weakness of abduction and wasting of deltoid give a flattened shoulder contour, and there is numbness over the regimental badge area. Test sensation there before and after reduction of any dislocation. Improper crutch use and quadrilateral space syndrome are other causes.
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.