Which nerve innervates the deltoid muscle?

The deltoid muscle, which is responsible for shoulder abduction, flexion, and extension, is primarily innervated by the axillary nerve. This nerve originates from the brachial plexus and specifically arises from the C5 and C6 spinal nerve roots, which correspond to the levels of the upper limb. The axillary nerve travels around the surgical neck of the humerus and provides motor innervation to the deltoid muscle as well as sensory innervation to the skin overlying the lateral aspect of the shoulder.

The axillary nerve's role is crucial because, without proper innervation, the deltoid muscle would not function effectively, leading to impaired shoulder movements. This highlights the anatomical significance of the axillary nerve in relation to the deltoid muscle and shoulder girdle mechanics.