Axillary Nerve

Origin

Terminal branch of the posterior cord, receiving fibres from C5 (and C6).

NB the other branch of the posterior cord is the radial nerve.

Course

Exits the axillary fossa posteriorly, passing through the quadrangular space (bounded superiorly by the subscapularis, head of humerus and teres minor; inferiorly by teres major; medially by the long head of the triceps and laterally by the coracobrachialis and the surgical neck of the humerus) with the posterior circumflex humeral artery; gives rise to the lateral brachial cutaneous nerve before winding round the surgical neck of the humerus, deep to the deltoid to supply it.

Innervations

Glenohumeral (shoulder) joint; teres minor and deltoid muscles

Skin over the superolateral arm (inferior to deltoid- badge patch)

Because of its relationship with the shoulder, the axillary nerve can be damaged if the shoulder is dislocated.  An axillary neuropathy would cause

1. Paralysis of the teres minor muscle and deltoid muscle , resulting in loss of abduction of arm (from 15-90 degrees), weak flexion, extension, and rotation of shoulder. Paralysis of deltoid & teres minor results in Flat shoulder deformity (the patient is unable to lie their shoulder flat on the bed).
2. Loss of sensation in the skin over a small part of the lateral upper arm (badge patch).

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