axillarynervus

The axillary nerve, also known as the circumflex humeral nerve, is a mixed peripheral nerve originating from the posterior cord of the brachial plexus. It is primarily a motor nerve, supplying the deltoid and teres minor muscles. These muscles are crucial for shoulder abduction (lifting the arm away from the body) and external rotation. The axillary nerve also carries sensory information from a small patch of skin over the deltoid muscle, a region known as the "regimental badge" area.

The nerve arises from the posterior cord of the brachial plexus, typically at the C5 and C6 spinal nerve roots. It then travels posteriorly, passing through the quadrangular space, a narrow anatomical region bounded by the teres minor superiorly, teres major inferiorly, the long head of the triceps laterally, and the humerus medially. Following its passage through this space, it winds around the surgical neck of the humerus before innervating its target muscles and skin.

Injury to the axillary nerve can result from various mechanisms, including direct trauma to the shoulder, fractures of the surgical neck of the humerus, dislocations of the shoulder joint, or prolonged compression. Common symptoms of axillary nerve injury include weakness or paralysis of the deltoid and teres minor muscles, leading to difficulty with arm abduction and external rotation. Sensory loss in the regimental badge area may also be present. A characteristic sign of deltoid paralysis is an inability to initiate abduction of the arm from a fully adducted position.