Axillary Nerve

The Axillary Nerve is a major peripheral nerve of the upper limb that originates from the brachial plexus.

• Type: mixed sensory and motor nerve.
  
• Origin: it emerges from the posterior cord of the brachial plexus.
  
• Course:it passes through the quadrangular space with the posterior circumflex humeral artery.
• Offers: a motor supply to deltoid and teres minor; a sensory supply to the skin overlying deltoid; and an articular branch to the shoulder joint.

Origin

The axillary nerve (C5, C6) arises from the posterior cord of the brachial plexus near the lower border of the subscapularis. It passes backwards on subscapularis through the quadrangular space along with the posterior circumflex humeral arteryruns. Thoroughly, here it is related to the medial aspect of the surgical neck of the humerus instantly inferior to the capsule of the shoulder joint.

Branches

The nerve splits into branches across the shoulder joint, and then works laterally to divide into the anterior and posterior divisions/branches, deep to deltoid.

Anterior Branch

The anterior branch goes on horizontally between the deltoid and surgical neck of the humerus with posterior circumflex humeral vessels. It supplies deltoid and transmits a few branches through it to innervate the overlying skin.

Posterior Branch

The posterior branch supplies teres minor and posterior part of the deltoid. It then proceeds over the posterior border of the deltoid as upper lateral cutaneous nerve of the arm and supplies the skin over the lower half of the deltoid. The nerve to teres minor has a ‘pseudoganglion’

The nerve to teres minor enters the muscle on its inferior surface. The posterior branch often supplies the posterior aspect of deltoid, generally via a separate branch from the main stem, but occasionally from the superior lateral cutaneous nerve of the arm. However, the posterior part of deltoid has a more constant supply from the anterior branch of the axillary nerve, which should be considered when performing a posterior deltoid-splitting approach to the shoulder.

The upper lateral cutaneous nerve of the arm pierces the deep fascia at the medial border of the posterior aspect of deltoid, and supplies the skin over the lower part of deltoid and the upper part of the long head of triceps. The posterior branch is totally related to the inferior aspects of the glenoid and shoulder joint capsule. There is often an enhancement or pseudoganglion on the branch to teres minor. The axillary trunk supplies a branch to the shoulder joint below subscapularis.

Clinical Correlation

Injury of the axillary nerve: The axillary nerve is at chance of harm in inferior dislocation of the head of humerus from shoulder joint and in fractures of the surgical neck of the humerus because of its close relation to these structures. The damage of axillary nerve presents the following clinical features:

• Impaired abduction of the shoulder: due to paralysis of the deltoid and teres minor muscles.
• Loss of sensations over the lower half of the deltoid: due to involvement of the upper lateral cutaneous nerve of the arm.
• Loss of shoulder contour with prominence of greater tubercle of the humerus: due to losing of the deltoid muscle.