The Axillary nerve arises from the posterior
cord of the brachial plexus. It exits the axilla via the quadrangular space.
Its anterior branch innervates the deltoid. Its posterior branch innervates
the teres minor and the posterior part of the deltoid.
Common causes of axillary nerve lesions are trauma and neuralgic amyotrophy. There is wasting and weakness of the deltoid, and a patch of sensory loss on the outer aspect of the arm. This can be differentiated from a C5 root lesion by assessing normal function of the suprascapular nerve.